v2, 36 partsPromisor remotes and external ODB support

Makefile, external-odb.c, external-odb.h, odb-helper.c, odb-helper.h

diff --git a/Makefile b/Makefile
index a1d8775adb..11dd620155 100644
--- a/Makefile
+++ b/Makefile
@@ -808,6 +808,7 @@ LIB_OBJS += ewah/ewah_bitmap.o
 LIB_OBJS += ewah/ewah_io.o
 LIB_OBJS += ewah/ewah_rlw.o
 LIB_OBJS += exec_cmd.o
+LIB_OBJS += external-odb.o
 LIB_OBJS += fetch-object.o
 LIB_OBJS += fetch-pack.o
 LIB_OBJS += fsck.o
@@ -843,6 +844,7 @@ LIB_OBJS += notes-cache.o
 LIB_OBJS += notes-merge.o
 LIB_OBJS += notes-utils.o
 LIB_OBJS += object.o
+LIB_OBJS += odb-helper.o
 LIB_OBJS += oidmap.o
 LIB_OBJS += oidset.o
 LIB_OBJS += packfile.o
diff --git a/external-odb.c b/external-odb.c
new file mode 100644
index 0000000000..f3ea491333
--- /dev/null
+++ b/external-odb.c
@@ -0,0 +1,72 @@
+#include "cache.h"
+#include "external-odb.h"
+#include "odb-helper.h"
+#include "config.h"
+
+static struct odb_helper *helpers;
+static struct odb_helper **helpers_tail = &helpers;
+
+static struct odb_helper *find_or_create_helper(const char *name, int len)
+{
+	struct odb_helper *o;
+
+	for (o = helpers; o; o = o->next)
+		if (!strncmp(o->name, name, len) && !o->name[len])
+			return o;
+
+	o = odb_helper_new(name, len);
+	*helpers_tail = o;
+	helpers_tail = &o->next;
+
+	return o;
+}
+
+static int external_odb_config(const char *var, const char *value, void *data)
+{
+	struct odb_helper *o;
+	const char *name;
+	int namelen;
+	const char *subkey;
+
+	if (parse_config_key(var, "odb", &name, &namelen, &subkey) < 0)
+		return 0;
+
+	o = find_or_create_helper(name, namelen);
+
+	if (!strcmp(subkey, "promisorremote"))
+		return git_config_string(&o->dealer, var, value);
+
+	return 0;
+}
+
+static void external_odb_init(void)
+{
+	static int initialized;
+
+	if (initialized)
+		return;
+	initialized = 1;
+
+	git_config(external_odb_config, NULL);
+}
+
+const char *external_odb_root(void)
+{
+	static const char *root;
+	if (!root)
+		root = git_pathdup("objects/external");
+	return root;
+}
+
+int external_odb_has_object(const unsigned char *sha1)
+{
+	struct odb_helper *o;
+
+	external_odb_init();
+
+	for (o = helpers; o; o = o->next)
+		if (odb_helper_has_object(o, sha1))
+			return 1;
+	return 0;
+}
+
diff --git a/external-odb.h b/external-odb.h
new file mode 100644
index 0000000000..ae2b228792
--- /dev/null
+++ b/external-odb.h
@@ -0,0 +1,7 @@
+#ifndef EXTERNAL_ODB_H
+#define EXTERNAL_ODB_H
+
+extern const char *external_odb_root(void);
+extern int external_odb_has_object(const unsigned char *sha1);
+
+#endif /* EXTERNAL_ODB_H */
diff --git a/odb-helper.c b/odb-helper.c
new file mode 100644
index 0000000000..67196e6317
--- /dev/null
+++ b/odb-helper.c
@@ -0,0 +1,54 @@
+#include "cache.h"
+#include "object.h"
+#include "argv-array.h"
+#include "odb-helper.h"
+#include "run-command.h"
+#include "sha1-lookup.h"
+
+struct odb_helper *odb_helper_new(const char *name, int namelen)
+{
+	struct odb_helper *o;
+
+	o = xcalloc(1, sizeof(*o));
+	o->name = xmemdupz(name, namelen);
+
+	return o;
+}
+
+struct odb_helper_cmd {
+	struct argv_array argv;
+	struct child_process child;
+};
+
+static void odb_helper_load_have(struct odb_helper *o)
+{
+	if (o->have_valid)
+		return;
+	o->have_valid = 1;
+
+	/* TODO */
+}
+
+static const unsigned char *have_sha1_access(size_t index, void *table)
+{
+	struct odb_helper_object *have = table;
+	return have[index].oid.hash;
+}
+
+static struct odb_helper_object *odb_helper_lookup(struct odb_helper *o,
+						   const unsigned char *sha1)
+{
+	int idx;
+
+	odb_helper_load_have(o);
+	idx = sha1_pos(sha1, o->have, o->have_nr, have_sha1_access);
+	if (idx < 0)
+		return NULL;
+	return &o->have[idx];
+}
+
+int odb_helper_has_object(struct odb_helper *o, const unsigned char *sha1)
+{
+	return !!odb_helper_lookup(o, sha1);
+}
+
diff --git a/odb-helper.h b/odb-helper.h
new file mode 100644
index 0000000000..e3c9c1cfe4
--- /dev/null
+++ b/odb-helper.h
@@ -0,0 +1,24 @@
+#ifndef ODB_HELPER_H
+#define ODB_HELPER_H
+
+struct odb_helper {
+	const char *name;
+	const char *dealer;
+
+	struct odb_helper_object {
+		struct object_id oid;
+		unsigned long size;
+		enum object_type type;
+	} *have;
+	int have_nr;
+	int have_alloc;
+	int have_valid;
+
+	struct odb_helper *next;
+};
+
+extern struct odb_helper *odb_helper_new(const char *name, int namelen);
+extern int odb_helper_has_object(struct odb_helper *o,
+				 const unsigned char *sha1);
+
+#endif /* ODB_HELPER_H */

cache.h, sha1_file.c

diff --git a/cache.h b/cache.h
index d06932ed0b..2ac7d63e5c 100644
--- a/cache.h
+++ b/cache.h
@@ -1583,6 +1583,7 @@ extern void prepare_alt_odb(void);
 extern char *compute_alternate_path(const char *path, struct strbuf *err);
 typedef int alt_odb_fn(struct alternate_object_database *, void *);
 extern int foreach_alt_odb(alt_odb_fn, void*);
+extern void prepare_external_alt_odb(void);
 
 /*
  * Allocate a "struct alternate_object_database" but do _not_ actually
diff --git a/sha1_file.c b/sha1_file.c
index 1b94f39c4c..3f00fc716e 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -29,6 +29,7 @@
 #include "quote.h"
 #include "packfile.h"
 #include "fetch-object.h"
+#include "external-odb.h"
 
 const unsigned char null_sha1[GIT_MAX_RAWSZ];
 const struct object_id null_oid;
@@ -663,6 +664,21 @@ int foreach_alt_odb(alt_odb_fn fn, void *cb)
 	return r;
 }
 
+void prepare_external_alt_odb(void)
+{
+	static int linked_external;
+	const char *path;
+
+	if (linked_external)
+		return;
+
+	path = external_odb_root();
+	if (!access(path, F_OK)) {
+		link_alt_odb_entry(path, NULL, 0, "");
+		linked_external = 1;
+	}
+}
+
 void prepare_alt_odb(void)
 {
 	const char *alt;
@@ -676,6 +692,7 @@ void prepare_alt_odb(void)
 	link_alt_odb_entries(alt, PATH_SEP, NULL, 0);
 
 	read_info_alternates(get_object_directory(), 0);
+	prepare_external_alt_odb();
 }
 
 /* Returns 1 if we have successfully freshened the file, 0 otherwise. */

cache.h, sha1_file.c

diff --git a/cache.h b/cache.h
index 4aa0129cf8..e6fd9b97ec 100644
--- a/cache.h
+++ b/cache.h
@@ -951,6 +951,12 @@ extern void check_repository_format(void);
  */
 extern void sha1_file_name(struct strbuf *buf, const unsigned char *sha1);
 
+/*
+ * Like sha1_file_name, but put in `buf` the filename within a
+ * specific alternate object directory.
+ */
+extern void sha1_file_name_alt(struct strbuf *buf, const char *objdir, const unsigned char *sha1);
+
 /*
  * Return an abbreviated sha1 unique within this repository's object database.
  * The result will be at least `len` characters long, and will be NUL
@@ -1247,6 +1253,8 @@ extern int parse_sha1_header(const char *hdr, unsigned long *sizep);
 
 extern int check_sha1_signature(const unsigned char *sha1, void *buf, unsigned long size, const char *type);
 
+extern int create_object_tmpfile(struct strbuf *tmp, const char *filename);
+extern void close_sha1_file(int fd);
 extern int finalize_object_file(const char *tmpfile, const char *filename);
 
 /*
diff --git a/sha1_file.c b/sha1_file.c
index f2e078e840..c460eb2658 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -320,13 +320,18 @@ static void fill_sha1_path(struct strbuf *buf, const unsigned char *sha1)
 	}
 }
 
-void sha1_file_name(struct strbuf *buf, const unsigned char *sha1)
+void sha1_file_name_alt(struct strbuf *buf, const char *objdir, const unsigned char *sha1)
 {
-	strbuf_addstr(buf, get_object_directory());
+	strbuf_addstr(buf, objdir);
 	strbuf_addch(buf, '/');
 	fill_sha1_path(buf, sha1);
 }
 
+void sha1_file_name(struct strbuf *buf, const unsigned char *sha1)
+{
+	return sha1_file_name_alt(buf, get_object_directory(), sha1);
+}
+
 struct strbuf *alt_scratch_buf(struct alternate_object_database *alt)
 {
 	strbuf_setlen(&alt->scratch, alt->base_len);
@@ -905,6 +910,25 @@ static int stat_sha1_file(const unsigned char *sha1, struct stat *st,
 	return -1;
 }
 
+static int open_sha1_file_alt(const unsigned char *sha1, const char **path)
+{
+	struct alternate_object_database *alt;
+	int most_interesting_errno = errno;
+
+	prepare_alt_odb();
+	for (alt = alt_odb_list; alt; alt = alt->next) {
+		int fd;
+		*path = alt_sha1_path(alt, sha1);
+		fd = git_open(*path);
+		if (fd >= 0)
+			return fd;
+		if (most_interesting_errno == ENOENT)
+			most_interesting_errno = errno;
+	}
+	errno = most_interesting_errno;
+	return -1;
+}
+
 /*
  * Like stat_sha1_file(), but actually open the object and return the
  * descriptor. See the caveats on the "path" parameter above.
@@ -912,8 +936,6 @@ static int stat_sha1_file(const unsigned char *sha1, struct stat *st,
 static int open_sha1_file(const unsigned char *sha1, const char **path)
 {
 	int fd;
-	struct alternate_object_database *alt;
-	int most_interesting_errno;
 	static struct strbuf buf = STRBUF_INIT;
 
 	strbuf_reset(&buf);
@@ -923,19 +945,8 @@ static int open_sha1_file(const unsigned char *sha1, const char **path)
 	fd = git_open(*path);
 	if (fd >= 0)
 		return fd;
-	most_interesting_errno = errno;
 
-	prepare_alt_odb();
-	for (alt = alt_odb_list; alt; alt = alt->next) {
-		*path = alt_sha1_path(alt, sha1);
-		fd = git_open(*path);
-		if (fd >= 0)
-			return fd;
-		if (most_interesting_errno == ENOENT)
-			most_interesting_errno = errno;
-	}
-	errno = most_interesting_errno;
-	return -1;
+	return open_sha1_file_alt(sha1, path);
 }
 
 /*
@@ -1537,7 +1548,7 @@ int hash_object_file(const void *buf, unsigned long len, const char *type,
 }
 
 /* Finalize a file on disk, and close it. */
-static void close_sha1_file(int fd)
+void close_sha1_file(int fd)
 {
 	if (fsync_object_files)
 		fsync_or_die(fd, "sha1 file");
@@ -1561,7 +1572,7 @@ static inline int directory_size(const char *filename)
  * We want to avoid cross-directory filename renames, because those
  * can have problems on various filesystems (FAT, NFS, Coda).
  */
-static int create_tmpfile(struct strbuf *tmp, const char *filename)
+int create_object_tmpfile(struct strbuf *tmp, const char *filename)
 {
 	int fd, dirlen = directory_size(filename);
 
@@ -1605,7 +1616,7 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
 	strbuf_reset(&filename);
 	sha1_file_name(&filename, oid->hash);
 
-	fd = create_tmpfile(&tmp_file, filename.buf);
+	fd = create_object_tmpfile(&tmp_file, filename.buf);
 	if (fd < 0) {
 		if (errno == EACCES)
 			return error("insufficient permission for adding an object to repository database %s", get_object_directory());

external-odb.c, external-odb.h, odb-helper.c, odb-helper.h

diff --git a/external-odb.c b/external-odb.c
index d26e63d8b1..5d0afb9762 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -76,3 +76,18 @@ int external_odb_has_object(const unsigned char *sha1)
 			return 1;
 	return 0;
 }
+
+int external_odb_get_direct(const unsigned char *sha1)
+{
+	struct odb_helper *o;
+
+	external_odb_init();
+
+	for (o = helpers; o; o = o->next) {
+		if (odb_helper_get_direct(o, sha1) < 0)
+			continue;
+		return 0;
+	}
+
+	return -1;
+}
diff --git a/external-odb.h b/external-odb.h
index 9a3c2f01b3..fd6708163e 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -4,5 +4,6 @@
 extern int has_external_odb(void);
 extern const char *external_odb_root(void);
 extern int external_odb_has_object(const unsigned char *sha1);
+extern int external_odb_get_direct(const unsigned char *sha1);
 
 #endif /* EXTERNAL_ODB_H */
diff --git a/odb-helper.c b/odb-helper.c
index 67196e6317..c6127ce81f 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -4,6 +4,7 @@
 #include "odb-helper.h"
 #include "run-command.h"
 #include "sha1-lookup.h"
+#include "fetch-object.h"
 
 struct odb_helper *odb_helper_new(const char *name, int namelen)
 {
@@ -52,3 +53,15 @@ int odb_helper_has_object(struct odb_helper *o, const unsigned char *sha1)
 	return !!odb_helper_lookup(o, sha1);
 }
 
+int odb_helper_get_direct(struct odb_helper *o,
+			  const unsigned char *sha1)
+{
+	int res = 0;
+	uint64_t start = getnanotime();
+
+	fetch_object(o->dealer, sha1);
+
+	trace_performance_since(start, "odb_helper_get_direct");
+
+	return res;
+}
diff --git a/odb-helper.h b/odb-helper.h
index e3c9c1cfe4..57b14fe814 100644
--- a/odb-helper.h
+++ b/odb-helper.h
@@ -20,5 +20,6 @@ struct odb_helper {
 extern struct odb_helper *odb_helper_new(const char *name, int namelen);
 extern int odb_helper_has_object(struct odb_helper *o,
 				 const unsigned char *sha1);
-
+extern int odb_helper_get_direct(struct odb_helper *o,
+				 const unsigned char *sha1);
 #endif /* ODB_HELPER_H */

external-odb.c, external-odb.h, odb-helper.h

diff --git a/external-odb.c b/external-odb.c
index 5d0afb9762..9c77180d6c 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -57,6 +57,19 @@ int has_external_odb(void)
 	return !!helpers;
 }
 
+struct odb_helper *find_odb_helper(const char *dealer, enum odb_helper_type type)
+{
+	struct odb_helper *o;
+
+	external_odb_init();
+
+	for (o = helpers; o; o = o->next)
+		if (!strcmp(o->dealer, dealer) && o->type == type)
+			return o;
+
+	return NULL;
+}
+
 const char *external_odb_root(void)
 {
 	static const char *root;
diff --git a/external-odb.h b/external-odb.h
index fd6708163e..27c3d39c1b 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -1,7 +1,17 @@
 #ifndef EXTERNAL_ODB_H
 #define EXTERNAL_ODB_H
 
+enum odb_helper_type {
+	ODB_HELPER_NONE = 0,
+	ODB_HELPER_GIT_REMOTE,
+	ODB_HELPER_SCRIPT_CMD,
+	ODB_HELPER_SUBPROCESS_CMD,
+	OBJ_HELPER_MAX
+};
+
 extern int has_external_odb(void);
+extern struct odb_helper *find_odb_helper(const char *dealer,
+					  enum odb_helper_type type);
 extern const char *external_odb_root(void);
 extern int external_odb_has_object(const unsigned char *sha1);
 extern int external_odb_get_direct(const unsigned char *sha1);
diff --git a/odb-helper.h b/odb-helper.h
index 57b14fe814..d4b968693f 100644
--- a/odb-helper.h
+++ b/odb-helper.h
@@ -1,9 +1,12 @@
 #ifndef ODB_HELPER_H
 #define ODB_HELPER_H
 
+#include "external-odb.h"
+
 struct odb_helper {
 	const char *name;
 	const char *dealer;
+	enum odb_helper_type type;
 
 	struct odb_helper_object {
 		struct object_id oid;

external-odb.c, external-odb.h

diff --git a/external-odb.c b/external-odb.c
index 390958dbfe..d26e63d8b1 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -50,6 +50,13 @@ static void external_odb_init(void)
 	git_config(external_odb_config, NULL);
 }
 
+int has_external_odb(void)
+{
+	external_odb_init();
+
+	return !!helpers;
+}
+
 const char *external_odb_root(void)
 {
 	static const char *root;
diff --git a/external-odb.h b/external-odb.h
index ae2b228792..9a3c2f01b3 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -1,6 +1,7 @@
 #ifndef EXTERNAL_ODB_H
 #define EXTERNAL_ODB_H
 
+extern int has_external_odb(void);
 extern const char *external_odb_root(void);
 extern int external_odb_has_object(const unsigned char *sha1);
 

cache.h, environment.c, external-odb.c, sha1_file.c

diff --git a/cache.h b/cache.h
index 2ac7d63e5c..4aa0129cf8 100644
--- a/cache.h
+++ b/cache.h
@@ -413,6 +413,7 @@ static inline enum object_type object_type(unsigned int mode)
 #define CEILING_DIRECTORIES_ENVIRONMENT "GIT_CEILING_DIRECTORIES"
 #define NO_REPLACE_OBJECTS_ENVIRONMENT "GIT_NO_REPLACE_OBJECTS"
 #define GIT_REPLACE_REF_BASE_ENVIRONMENT "GIT_REPLACE_REF_BASE"
+#define NO_EXTERNAL_ODB_ENVIRONMENT "GIT_NO_EXTERNAL_ODB"
 #define GITATTRIBUTES_FILE ".gitattributes"
 #define INFOATTRIBUTES_FILE "info/attributes"
 #define ATTRIBUTE_MACRO_PREFIX "[attr]"
@@ -799,6 +800,14 @@ void reset_shared_repository(void);
 extern int check_replace_refs;
 extern char *git_replace_ref_base;
 
+/*
+ * Do external odbs need to be used this run?  This variable is
+ * initialized to true unless $GIT_NO_EXTERNAL_ODB is set, but it
+ * maybe set to false by some commands that do not want external
+ * odbs to be active.
+ */
+extern int use_external_odb;
+
 extern int fsync_object_files;
 extern int core_preload_index;
 extern int core_apply_sparse_checkout;
diff --git a/environment.c b/environment.c
index d6dd64662c..92f682cb9b 100644
--- a/environment.c
+++ b/environment.c
@@ -50,6 +50,7 @@ const char *excludes_file;
 enum auto_crlf auto_crlf = AUTO_CRLF_FALSE;
 int check_replace_refs = 1;
 char *git_replace_ref_base;
+int use_external_odb = 1;
 enum eol core_eol = EOL_UNSET;
 int global_conv_flags_eol = CONV_EOL_RNDTRP_WARN;
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
@@ -119,6 +120,7 @@ const char * const local_repo_env[] = {
 	INDEX_ENVIRONMENT,
 	NO_REPLACE_OBJECTS_ENVIRONMENT,
 	GIT_REPLACE_REF_BASE_ENVIRONMENT,
+	NO_EXTERNAL_ODB_ENVIRONMENT,
 	GIT_PREFIX_ENVIRONMENT,
 	GIT_SUPER_PREFIX_ENVIRONMENT,
 	GIT_SHALLOW_FILE_ENVIRONMENT,
@@ -158,6 +160,8 @@ void setup_git_env(void)
 	free(git_replace_ref_base);
 	git_replace_ref_base = xstrdup(replace_ref_base ? replace_ref_base
 							  : "refs/replace/");
+	if (getenv(NO_EXTERNAL_ODB_ENVIRONMENT))
+		use_external_odb = 0;
 	free(git_namespace);
 	git_namespace = expand_namespace(getenv(GIT_NAMESPACE_ENVIRONMENT));
 	shallow_file = getenv(GIT_SHALLOW_FILE_ENVIRONMENT);
diff --git a/external-odb.c b/external-odb.c
index f3ea491333..390958dbfe 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -43,7 +43,7 @@ static void external_odb_init(void)
 {
 	static int initialized;
 
-	if (initialized)
+	if (initialized || !use_external_odb)
 		return;
 	initialized = 1;
 
@@ -69,4 +69,3 @@ int external_odb_has_object(const unsigned char *sha1)
 			return 1;
 	return 0;
 }
-
diff --git a/sha1_file.c b/sha1_file.c
index 3f00fc716e..f2e078e840 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -669,6 +669,9 @@ void prepare_external_alt_odb(void)
 	static int linked_external;
 	const char *path;
 
+	if (!use_external_odb)
+		return;
+
 	if (linked_external)
 		return;
 

external-odb.c, external-odb.h, odb-helper.c, odb-helper.h

diff --git a/external-odb.c b/external-odb.c
index ae6bc017fe..8be92277d2 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -182,3 +182,20 @@ int external_odb_get_direct(const unsigned char *sha1)
 
 	return -1;
 }
+
+int external_odb_get_many_direct(const struct oid_array *to_get)
+{
+	struct odb_helper *o;
+
+	external_odb_init();
+
+	for (o = helpers; o; o = o->next) {
+		if (!(o->supported_capabilities & ODB_HELPER_CAP_GET_DIRECT))
+			continue;
+		if (odb_helper_get_many_direct(o, to_get) < 0)
+			continue;
+		return 0;
+	}
+
+	return -1;
+}
diff --git a/external-odb.h b/external-odb.h
index c5ac071995..a4eda95b09 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -17,5 +17,6 @@ extern const char *external_odb_root(void);
 extern int external_odb_has_object(const unsigned char *sha1);
 extern int external_odb_get_object(const unsigned char *sha1);
 extern int external_odb_get_direct(const unsigned char *sha1);
+extern int external_odb_get_many_direct(const struct oid_array *to_get);
 
 #endif /* EXTERNAL_ODB_H */
diff --git a/odb-helper.c b/odb-helper.c
index a99f4a1299..7e5eab94cf 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -345,3 +345,19 @@ int odb_helper_get_direct(struct odb_helper *o,
 
 	return res;
 }
+
+int odb_helper_get_many_direct(struct odb_helper *o,
+			       const struct oid_array *to_get)
+{
+	int res = 0;
+	uint64_t start;
+
+	start = getnanotime();
+
+	if (o->type == ODB_HELPER_GIT_REMOTE)
+		fetch_objects(o->dealer, to_get);
+
+	trace_performance_since(start, "odb_helper_get_many_direct");
+
+	return res;
+}
diff --git a/odb-helper.h b/odb-helper.h
index 7720684744..2c529af6af 100644
--- a/odb-helper.h
+++ b/odb-helper.h
@@ -39,5 +39,7 @@ extern int odb_helper_get_object(struct odb_helper *o,
 				 int fd);
 extern int odb_helper_get_direct(struct odb_helper *o,
 				 const unsigned char *sha1);
+extern int odb_helper_get_many_direct(struct odb_helper *o,
+				      const struct oid_array *to_get);
 
 #endif /* ODB_HELPER_H */

diff --git a/t/t0500-external-odb.sh b/t/t0500-external-odb.sh
index 977fea852d..4ccca1e965 100755
--- a/t/t0500-external-odb.sh
+++ b/t/t0500-external-odb.sh
@@ -7,10 +7,15 @@ test_description='basic tests for external object databases'
 ALT_SOURCE="$PWD/alt-repo/.git"
 export ALT_SOURCE
 write_script odb-helper <<\EOF
+die() {
+	printf >&2 "%s\n" "$@"
+	exit 1
+}
 GIT_DIR=$ALT_SOURCE; export GIT_DIR
 case "$1" in
 init)
 	echo "capability=get_git_obj"
+	echo "capability=put_raw_obj"
 	echo "capability=have"
 	;;
 have)
@@ -20,6 +25,16 @@ have)
 get_git_obj)
 	cat "$GIT_DIR"/objects/$(echo $2 | sed 's#..#&/#')
 	;;
+put_raw_obj)
+	sha1="$2"
+	size="$3"
+	kind="$4"
+	written=$(git hash-object -w -t "$kind" --stdin)
+	test "$written" = "$sha1" || die "bad sha1 passed '$sha1' vs written '$written'"
+	;;
+*)
+	die "unknown command '$1'"
+	;;
 esac
 EOF
 HELPER="\"$PWD\"/odb-helper"
@@ -45,4 +60,13 @@ test_expect_success 'helper can retrieve alt objects' '
 	test_cmp expect actual
 '
 
+test_expect_success 'helper can add objects to alt repo' '
+	hash=$(echo "Hello odb!" | git hash-object -w -t blob --stdin) &&
+	test -f .git/objects/$(echo $hash | sed "s#..#&/#") &&
+	size=$(git cat-file -s "$hash") &&
+	git cat-file blob "$hash" | ./odb-helper put_raw_obj "$hash" "$size" blob &&
+	alt_size=$(git -C alt-repo cat-file -s "$hash") &&
+	test "$size" -eq "$alt_size"
+'
+
 test_done

diff --git a/external-odb.c b/external-odb.c
index 806dc8e699..de2ae70db1 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -210,6 +210,10 @@ int external_odb_put_object(const void *buf, size_t len,
 
 	external_odb_init();
 
+	/* For now accept only blobs */
+	if (strcmp(type, "blob"))
+		return 1;
+
 	for (o = helpers; o; o = o->next) {
 		int r = odb_helper_put_object(o, buf, len, type, sha1);
 		if (r <= 0)

external-odb.c, external-odb.h, odb-helper.c, odb-helper.h, sha1_file.c

diff --git a/external-odb.c b/external-odb.c
index 192b09e843..806dc8e699 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -202,3 +202,18 @@ int external_odb_get_many_direct(const struct oid_array *to_get)
 
 	return -1;
 }
+
+int external_odb_put_object(const void *buf, size_t len,
+			    const char *type, unsigned char *sha1)
+{
+	struct odb_helper *o;
+
+	external_odb_init();
+
+	for (o = helpers; o; o = o->next) {
+		int r = odb_helper_put_object(o, buf, len, type, sha1);
+		if (r <= 0)
+			return r;
+	}
+	return 1;
+}
diff --git a/external-odb.h b/external-odb.h
index a4eda95b09..60da8ed3d4 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -18,5 +18,7 @@ extern int external_odb_has_object(const unsigned char *sha1);
 extern int external_odb_get_object(const unsigned char *sha1);
 extern int external_odb_get_direct(const unsigned char *sha1);
 extern int external_odb_get_many_direct(const struct oid_array *to_get);
+extern int external_odb_put_object(const void *buf, size_t len,
+				   const char *type, unsigned char *sha1);
 
 #endif /* EXTERNAL_ODB_H */
diff --git a/odb-helper.c b/odb-helper.c
index 30cfe689de..2734af793b 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -72,9 +72,10 @@ static void prepare_helper_command(struct argv_array *argv, const char *cmd,
 	strbuf_release(&buf);
 }
 
-__attribute__((format (printf,3,4)))
+__attribute__((format (printf,4,5)))
 static int odb_helper_start(struct odb_helper *o,
 			    struct odb_helper_cmd *cmd,
+			    int use_stdin,
 			    const char *fmt, ...)
 {
 	va_list ap;
@@ -91,7 +92,10 @@ static int odb_helper_start(struct odb_helper *o,
 
 	cmd->child.argv = cmd->argv.argv;
 	cmd->child.use_shell = 1;
-	cmd->child.no_stdin = 1;
+	if (use_stdin)
+		cmd->child.in = -1;
+	else
+		cmd->child.no_stdin = 1;
 	cmd->child.out = -1;
 
 	if (start_command(&cmd->child) < 0) {
@@ -124,7 +128,10 @@ int odb_helper_init(struct odb_helper *o)
 		return 0;
 	o->initialized = 1;
 
-	if (odb_helper_start(o, &cmd, "init") < 0)
+	if (o->type != ODB_HELPER_SCRIPT_CMD)
+		return 0;
+
+	if (odb_helper_start(o, &cmd, 0, "init") < 0)
 		return -1;
 
 	fh = xfdopen(cmd.child.out, "r");
@@ -180,7 +187,7 @@ static void have_object_script(struct odb_helper *o)
 	FILE *fh;
 	struct strbuf line = STRBUF_INIT;
 
-	if (odb_helper_start(o, &cmd, "have") < 0)
+	if (odb_helper_start(o, &cmd, 0, "have") < 0)
 		return;
 
 	fh = xfdopen(cmd.child.out, "r");
@@ -245,7 +252,7 @@ int odb_helper_get_object(struct odb_helper *o, const unsigned char *sha1,
 	if (!obj)
 		return -1;
 
-	if (odb_helper_start(o, &cmd, "get_git_obj %s", sha1_to_hex(sha1)) < 0)
+	if (odb_helper_start(o, &cmd, 0, "get_git_obj %s", sha1_to_hex(sha1)) < 0)
 		return -1;
 
 	memset(&stream, 0, sizeof(stream));
@@ -348,7 +355,7 @@ int odb_helper_get_direct(struct odb_helper *o,
 		if (!obj)
 			return -1;
 
-		if (odb_helper_start(o, &cmd, "get_direct %s", sha1_to_hex(sha1)) < 0)
+		if (odb_helper_start(o, &cmd, 0, "get_direct %s", sha1_to_hex(sha1)) < 0)
 			return -1;
 
 		if (odb_helper_finish(o, &cmd))
@@ -375,3 +382,35 @@ int odb_helper_get_many_direct(struct odb_helper *o,
 
 	return res;
 }
+
+int odb_helper_put_object(struct odb_helper *o,
+			  const void *buf, size_t len,
+			  const char *type, unsigned char *sha1)
+{
+	struct odb_helper_cmd cmd;
+
+	if (o->type != ODB_HELPER_SCRIPT_CMD)
+		return 1;
+
+	if (odb_helper_start(o, &cmd, 1, "put_raw_obj %s %"PRIuMAX" %s",
+			     sha1_to_hex(sha1), (uintmax_t)len, type) < 0)
+		return -1;
+
+	do {
+		int w = xwrite(cmd.child.in, buf, len);
+		if (w < 0) {
+			error("unable to write to odb helper '%s': %s",
+			      o->name, strerror(errno));
+			close(cmd.child.in);
+			close(cmd.child.out);
+			odb_helper_finish(o, &cmd);
+			return -1;
+		}
+		len -= w;
+	} while (len > 0);
+
+	close(cmd.child.in);
+	close(cmd.child.out);
+	odb_helper_finish(o, &cmd);
+	return 0;
+}
diff --git a/odb-helper.h b/odb-helper.h
index 3ec2d8448f..b6a1498662 100644
--- a/odb-helper.h
+++ b/odb-helper.h
@@ -42,5 +42,8 @@ extern int odb_helper_get_direct(struct odb_helper *o,
 				 const unsigned char *sha1);
 extern int odb_helper_get_many_direct(struct odb_helper *o,
 				      const struct oid_array *to_get);
+extern int odb_helper_put_object(struct odb_helper *o,
+				 const void *buf, size_t len,
+				 const char *type, unsigned char *sha1);
 
 #endif /* ODB_HELPER_H */
diff --git a/sha1_file.c b/sha1_file.c
index 99adfceede..64679fae36 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1709,6 +1709,8 @@ int write_object_file(const void *buf, unsigned long len, const char *type,
 	 * it out into .git/objects/??/?{38} file.
 	 */
 	write_object_file_prepare(buf, len, type, oid, hdr, &hdrlen);
+	if (!external_odb_put_object(buf, len, type, oid->hash))
+		return 0;
 	if (freshen_packed_object(oid->hash) || freshen_loose_object(oid->hash))
 		return 0;
 	return write_loose_object(oid, hdr, hdrlen, buf, len, 0);

builtin/fetch.c, external-odb.c, list-objects-filter-options.c, list-objects-filter-options.h, odb-helper.h, t/t5616-partial-clone.sh

diff --git a/builtin/fetch.c b/builtin/fetch.c
index f4a138acc8..11c9561bb4 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1333,7 +1333,7 @@ static inline void fetch_one_setup_partial(struct remote *remote)
 	 * the config.
 	 */
 	if (!filter_options.choice)
-		partial_clone_get_default_filter_spec(&filter_options);
+		partial_clone_get_default_filter_spec(&filter_options, remote->name);
 	return;
 }
 
diff --git a/external-odb.c b/external-odb.c
index 8be92277d2..c3d90f39fb 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -47,6 +47,8 @@ static int external_odb_config(const char *var, const char *value, void *data)
 		o->type = ODB_HELPER_SUBPROCESS_CMD;
 		return git_config_string(&o->dealer, var, value);
 	}
+	if (!strcmp(subkey, "partialclonefilter"))
+		return git_config_string(&o->partial_clone_filter, var, value);
 
 	return 0;
 }
diff --git a/list-objects-filter-options.c b/list-objects-filter-options.c
index 12e52bf659..ff493b9163 100644
--- a/list-objects-filter-options.c
+++ b/list-objects-filter-options.c
@@ -7,6 +7,7 @@
 #include "list-objects-filter.h"
 #include "list-objects-filter-options.h"
 #include "external-odb.h"
+#include "odb-helper.h"
 
 /*
  * Parse value of the argument to the "filter" keyword.
@@ -116,6 +117,7 @@ void partial_clone_register(
 	const struct list_objects_filter_options *filter_options)
 {
 	char *cfg_name;
+	char *filter_name;
 
 	/* Check if it is already registered */
 	if (find_odb_helper(remote, ODB_HELPER_GIT_REMOTE))
@@ -131,25 +133,26 @@ void partial_clone_register(
 	/*
 	 * Record the initial filter-spec in the config as
 	 * the default for subsequent fetches from this remote.
-	 *
-	 * TODO: move core.partialclonefilter into odb.<name>
 	 */
-	core_partial_clone_filter_default =
-		xstrdup(filter_options->filter_spec);
-	git_config_set("core.partialclonefilter",
-		       core_partial_clone_filter_default);
+	filter_name = xstrfmt("odb.%s.partialclonefilter", remote);
+	git_config_set(filter_name, filter_options->filter_spec);
+	free(filter_name);
 
 	/* Make sure the config info are reset */
 	external_odb_reinit();
 }
 
 void partial_clone_get_default_filter_spec(
-	struct list_objects_filter_options *filter_options)
+	struct list_objects_filter_options *filter_options,
+	const char *remote)
 {
+	struct odb_helper *helper = find_odb_helper(remote, ODB_HELPER_GIT_REMOTE);
+
 	/*
 	 * Parse default value, but silently ignore it if it is invalid.
 	 */
-	gently_parse_list_objects_filter(filter_options,
-					 core_partial_clone_filter_default,
-					 NULL);
+	if (helper)
+		gently_parse_list_objects_filter(filter_options,
+						 helper->partial_clone_filter,
+						 NULL);
 }
diff --git a/list-objects-filter-options.h b/list-objects-filter-options.h
index 0000a61f82..12ceef3230 100644
--- a/list-objects-filter-options.h
+++ b/list-objects-filter-options.h
@@ -74,6 +74,7 @@ void partial_clone_register(
 	const char *remote,
 	const struct list_objects_filter_options *filter_options);
 void partial_clone_get_default_filter_spec(
-	struct list_objects_filter_options *filter_options);
+	struct list_objects_filter_options *filter_options,
+	const char *remote);
 
 #endif /* LIST_OBJECTS_FILTER_OPTIONS_H */
diff --git a/odb-helper.h b/odb-helper.h
index 2c529af6af..3ec2d8448f 100644
--- a/odb-helper.h
+++ b/odb-helper.h
@@ -17,6 +17,7 @@ struct odb_helper {
 	enum odb_helper_type type;
 	unsigned int supported_capabilities;
 	int initialized;
+	const char *partial_clone_filter;
 
 	struct odb_helper_object {
 		struct object_id oid;
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 5c84b0f95b..d9b8418275 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -41,7 +41,7 @@ test_expect_success 'do partial clone 1' '
 	test_cmp expect_1.oids observed.oids &&
 	test "$(git -C pc1 config --local core.repositoryformatversion)" = "1" &&
 	test "$(git -C pc1 config --local odb.origin.promisorRemote)" = "origin" &&
-	test "$(git -C pc1 config --local core.partialclonefilter)" = "blob:none"
+	test "$(git -C pc1 config --local odb.origin.partialclonefilter)" = "blob:none"
 '
 
 # checkout master to force dynamic object fetch of blobs at HEAD.

builtin/cat-file.c, builtin/fetch.c, builtin/gc.c, builtin/repack.c, cache.h, connected.c, environment.c, list-objects-filter-options.c, packfile.c, setup.c, sha1_file.c, t/t0410-partial-clone.sh, t/t5500-fetch-pack.sh, t/t5601-clone.sh, t/t5616-partial-clone.sh, unpack-trees.c

diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index d90170f070..fb7b85502f 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -13,6 +13,7 @@
 #include "tree-walk.h"
 #include "sha1-array.h"
 #include "packfile.h"
+#include "external-odb.h"
 
 struct batch_options {
 	int enabled;
@@ -475,8 +476,8 @@ static int batch_objects(struct batch_options *opt)
 
 		for_each_loose_object(batch_loose_object, &sa, 0);
 		for_each_packed_object(batch_packed_object, &sa, 0);
-		if (repository_format_partial_clone)
-			warning("This repository has extensions.partialClone set. Some objects may not be loaded.");
+		if (has_external_odb())
+			warning("This repository uses an odb. Some objects may not be loaded.");
 
 		cb.opt = opt;
 		cb.expand = &data;
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 6d73656a48..f4a138acc8 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -20,6 +20,7 @@
 #include "utf8.h"
 #include "packfile.h"
 #include "list-objects-filter-options.h"
+#include "external-odb.h"
 
 static const char * const builtin_fetch_usage[] = {
 	N_("git fetch [<options>] [<repository> [<refspec>...]]"),
@@ -1303,7 +1304,7 @@ static inline void fetch_one_setup_partial(struct remote *remote)
 	 * If no prior partial clone/fetch and the current fetch DID NOT
 	 * request a partial-fetch, do a normal fetch.
 	 */
-	if (!repository_format_partial_clone && !filter_options.choice)
+	if (!has_external_odb() && !filter_options.choice)
 		return;
 
 	/*
@@ -1311,7 +1312,7 @@ static inline void fetch_one_setup_partial(struct remote *remote)
 	 * on this repo and remember the given filter-spec as the default
 	 * for subsequent fetches to this remote.
 	 */
-	if (!repository_format_partial_clone && filter_options.choice) {
+	if (!has_external_odb() && filter_options.choice) {
 		partial_clone_register(remote->name, &filter_options);
 		return;
 	}
@@ -1320,7 +1321,7 @@ static inline void fetch_one_setup_partial(struct remote *remote)
 	 * We are currently limited to only ONE promisor remote and only
 	 * allow partial-fetches from the promisor remote.
 	 */
-	if (strcmp(remote->name, repository_format_partial_clone)) {
+	if (!find_odb_helper(remote->name, ODB_HELPER_GIT_REMOTE)) {
 		if (filter_options.choice)
 			die(_("--filter can only be used with the remote configured in core.partialClone"));
 		return;
@@ -1456,7 +1457,7 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	if (depth || deepen_since || deepen_not.nr)
 		deepen = 1;
 
-	if (filter_options.choice && !repository_format_partial_clone)
+	if (filter_options.choice && !has_external_odb())
 		die("--filter can only be used when extensions.partialClone is set");
 
 	if (all) {
@@ -1490,7 +1491,7 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	}
 
 	if (remote) {
-		if (filter_options.choice || repository_format_partial_clone)
+		if (filter_options.choice || has_external_odb())
 			fetch_one_setup_partial(remote);
 		result = fetch_one(remote, argc, argv, prune_tags_ok);
 	} else {
diff --git a/builtin/gc.c b/builtin/gc.c
index f51e5a6500..ff6e9805b2 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -20,6 +20,7 @@
 #include "argv-array.h"
 #include "commit.h"
 #include "packfile.h"
+#include "external-odb.h"
 
 #define FAILED_RUN "failed to run %s"
 
@@ -461,7 +462,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 			argv_array_push(&prune, prune_expire);
 			if (quiet)
 				argv_array_push(&prune, "--no-progress");
-			if (repository_format_partial_clone)
+			if (has_external_odb())
 				argv_array_push(&prune,
 						"--exclude-promisor-objects");
 			if (run_command_v_opt(prune.argv, RUN_GIT_CMD))
diff --git a/builtin/repack.c b/builtin/repack.c
index 7bdb40142f..a6d9cfb92c 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -8,6 +8,7 @@
 #include "strbuf.h"
 #include "string-list.h"
 #include "argv-array.h"
+#include "external-odb.h"
 
 static int delta_base_offset = 1;
 static int pack_kept_objects = -1;
@@ -234,7 +235,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	argv_array_push(&cmd.args, "--all");
 	argv_array_push(&cmd.args, "--reflog");
 	argv_array_push(&cmd.args, "--indexed-objects");
-	if (repository_format_partial_clone)
+	if (has_external_odb())
 		argv_array_push(&cmd.args, "--exclude-promisor-objects");
 	if (window)
 		argv_array_pushf(&cmd.args, "--window=%s", window);
diff --git a/cache.h b/cache.h
index e6fd9b97ec..da967a5cdc 100644
--- a/cache.h
+++ b/cache.h
@@ -900,13 +900,11 @@ extern int grafts_replace_parents;
 #define GIT_REPO_VERSION 0
 #define GIT_REPO_VERSION_READ 1
 extern int repository_format_precious_objects;
-extern char *repository_format_partial_clone;
 extern const char *core_partial_clone_filter_default;
 
 struct repository_format {
 	int version;
 	int precious_objects;
-	char *partial_clone; /* value of extensions.partialclone */
 	int is_bare;
 	int hash_algo;
 	char *work_tree;
diff --git a/connected.c b/connected.c
index 91feb78815..a336c69a2d 100644
--- a/connected.c
+++ b/connected.c
@@ -4,6 +4,7 @@
 #include "connected.h"
 #include "transport.h"
 #include "packfile.h"
+#include "external-odb.h"
 
 /*
  * If we feed all the commits we want to verify to this command
@@ -56,7 +57,7 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 	argv_array_push(&rev_list.args,"rev-list");
 	argv_array_push(&rev_list.args, "--objects");
 	argv_array_push(&rev_list.args, "--stdin");
-	if (repository_format_partial_clone)
+	if (has_external_odb())
 		argv_array_push(&rev_list.args, "--exclude-promisor-objects");
 	argv_array_push(&rev_list.args, "--not");
 	argv_array_push(&rev_list.args, "--all");
diff --git a/environment.c b/environment.c
index 92f682cb9b..2ff9dc9218 100644
--- a/environment.c
+++ b/environment.c
@@ -27,7 +27,6 @@ int warn_ambiguous_refs = 1;
 int warn_on_object_refname_ambiguity = 1;
 int ref_paranoia = -1;
 int repository_format_precious_objects;
-char *repository_format_partial_clone;
 const char *core_partial_clone_filter_default;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;
diff --git a/list-objects-filter-options.c b/list-objects-filter-options.c
index 6a3cc985c4..12e52bf659 100644
--- a/list-objects-filter-options.c
+++ b/list-objects-filter-options.c
@@ -6,6 +6,7 @@
 #include "list-objects.h"
 #include "list-objects-filter.h"
 #include "list-objects-filter-options.h"
+#include "external-odb.h"
 
 /*
  * Parse value of the argument to the "filter" keyword.
@@ -114,30 +115,32 @@ void partial_clone_register(
 	const char *remote,
 	const struct list_objects_filter_options *filter_options)
 {
-	/*
-	 * Record the name of the partial clone remote in the
-	 * config and in the global variable -- the latter is
-	 * used throughout to indicate that partial clone is
-	 * enabled and to expect missing objects.
-	 */
-	if (repository_format_partial_clone &&
-	    *repository_format_partial_clone &&
-	    strcmp(remote, repository_format_partial_clone))
-		die(_("cannot change partial clone promisor remote"));
+	char *cfg_name;
+
+	/* Check if it is already registered */
+	if (find_odb_helper(remote, ODB_HELPER_GIT_REMOTE))
+		return;
 
 	git_config_set("core.repositoryformatversion", "1");
-	git_config_set("extensions.partialclone", remote);
 
-	repository_format_partial_clone = xstrdup(remote);
+	/* Add odb config for the remote */
+	cfg_name = xstrfmt("odb.%s.promisorRemote", remote);
+	git_config_set(cfg_name, remote);
+	free(cfg_name);
 
 	/*
 	 * Record the initial filter-spec in the config as
 	 * the default for subsequent fetches from this remote.
+	 *
+	 * TODO: move core.partialclonefilter into odb.<name>
 	 */
 	core_partial_clone_filter_default =
 		xstrdup(filter_options->filter_spec);
 	git_config_set("core.partialclonefilter",
 		       core_partial_clone_filter_default);
+
+	/* Make sure the config info are reset */
+	external_odb_reinit();
 }
 
 void partial_clone_get_default_filter_spec(
diff --git a/packfile.c b/packfile.c
index 7c1a2519fc..3e7c0213f9 100644
--- a/packfile.c
+++ b/packfile.c
@@ -13,6 +13,7 @@
 #include "tag.h"
 #include "tree-walk.h"
 #include "tree.h"
+#include "external-odb.h"
 
 char *odb_pack_name(struct strbuf *buf,
 		    const unsigned char *sha1,
@@ -1941,7 +1942,7 @@ int is_promisor_object(const struct object_id *oid)
 	static int promisor_objects_prepared;
 
 	if (!promisor_objects_prepared) {
-		if (repository_format_partial_clone) {
+		if (has_external_odb()) {
 			for_each_packed_object(add_promisor_object,
 					       &promisor_objects,
 					       FOR_EACH_OBJECT_PROMISOR_ONLY);
diff --git a/setup.c b/setup.c
index 7287779642..16e7a6bb4c 100644
--- a/setup.c
+++ b/setup.c
@@ -422,11 +422,7 @@ static int check_repo_format(const char *var, const char *value, void *vdata)
 			;
 		else if (!strcmp(ext, "preciousobjects"))
 			data->precious_objects = git_config_bool(var, value);
-		else if (!strcmp(ext, "partialclone")) {
-			if (!value)
-				return config_error_nonbool(var);
-			data->partial_clone = xstrdup(value);
-		} else
+		else
 			string_list_append(&data->unknown_extensions, ext);
 	} else if (strcmp(var, "core.bare") == 0) {
 		data->is_bare = git_config_bool(var, value);
@@ -468,7 +464,6 @@ static int check_repository_format_gently(const char *gitdir, struct repository_
 	}
 
 	repository_format_precious_objects = candidate->precious_objects;
-	repository_format_partial_clone = candidate->partial_clone;
 	string_list_clear(&candidate->unknown_extensions, 0);
 	if (!has_common) {
 		if (candidate->is_bare != -1) {
diff --git a/sha1_file.c b/sha1_file.c
index 847883f261..99adfceede 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1313,13 +1313,9 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi,
 			break;
 
 		/* Check if it is a missing object */
-		if (fetch_if_missing && repository_format_partial_clone &&
+		if (fetch_if_missing && has_external_odb() &&
 		    !already_retried) {
-			/*
-			 * TODO Investigate haveing fetch_object() return
-			 * TODO error/success and stopping the music here.
-			 */
-			fetch_object(repository_format_partial_clone, real);
+			external_odb_get_direct(real);
 			already_retried = 1;
 			continue;
 		}
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index cc18b75c03..6af4712da8 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -43,7 +43,7 @@ test_expect_success 'missing reflog object, but promised by a commit, passes fsc
 
 	# But with the extension, it succeeds
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo fsck
 '
 
@@ -66,7 +66,7 @@ test_expect_success 'missing reflog object, but promised by a tag, passes fsck'
 	printf "$T\n" | pack_as_from_promisor &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo fsck
 '
 
@@ -84,7 +84,7 @@ test_expect_success 'missing reflog object alone fails fsck, even with extension
 	delete_object repo "$A" &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	test_must_fail git -C repo fsck
 '
 
@@ -100,7 +100,7 @@ test_expect_success 'missing ref object, but promised, passes fsck' '
 	promise_and_delete "$A" &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo fsck
 '
 
@@ -123,7 +123,7 @@ test_expect_success 'missing object, but promised, passes fsck' '
 	promise_and_delete "$AT" &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo fsck
 '
 
@@ -136,7 +136,7 @@ test_expect_success 'missing CLI object, but promised, passes fsck' '
 	promise_and_delete "$A" &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo fsck "$A"
 '
 
@@ -151,7 +151,7 @@ test_expect_success 'fetching of missing objects' '
 	rm -rf repo/.git/objects/* &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "origin" &&
+	git -C repo config odb.magic.promisorRemote "origin" &&
 	git -C repo cat-file -p "$HASH" &&
 
 	# Ensure that the .promisor file is written, and check that its
@@ -172,7 +172,7 @@ test_expect_success 'rev-list stops traversal at missing and promised commit' '
 	promise_and_delete "$FOO" &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo rev-list --exclude-promisor-objects --objects bar >out &&
 	grep $(git -C repo rev-parse bar) out &&
 	! grep $FOO out
@@ -197,7 +197,7 @@ test_expect_success 'rev-list stops traversal at missing and promised tree' '
 	promise_and_delete "$TREE2" &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo rev-list --exclude-promisor-objects --objects HEAD >out &&
 	grep $(git -C repo rev-parse foo) out &&
 	! grep $TREE out &&
@@ -216,7 +216,7 @@ test_expect_success 'rev-list stops traversal at missing and promised blob' '
 	promise_and_delete "$BLOB" &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo rev-list --exclude-promisor-objects --objects HEAD >out &&
 	grep $(git -C repo rev-parse HEAD) out &&
 	! grep $BLOB out
@@ -235,7 +235,7 @@ test_expect_success 'rev-list stops traversal at promisor commit, tree, and blob
 	printf "%s\n%s\n%s\n" $COMMIT $TREE $BLOB | pack_as_from_promisor &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo rev-list --exclude-promisor-objects --objects HEAD >out &&
 	! grep $COMMIT out &&
 	! grep $TREE out &&
@@ -259,7 +259,7 @@ test_expect_success 'rev-list accepts missing and promised objects on command li
 	promise_and_delete $BLOB &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo rev-list --exclude-promisor-objects --objects "$COMMIT" "$TREE" "$BLOB"
 '
 
@@ -272,7 +272,7 @@ test_expect_success 'gc does not repack promisor objects' '
 	HASH=$(printf "$TREE_HASH\n" | pack_as_from_promisor) &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo gc &&
 
 	# Ensure that the promisor packfile still exists, and remove it
@@ -296,7 +296,7 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
 	HASH=$(promise_and_delete $TREE_HASH) &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "arbitrary string" &&
+	git -C repo config odb.magic.promisorRemote "arbitrary string" &&
 	git -C repo gc &&
 
 	# Ensure that the promisor packfile still exists, and remove it
@@ -327,7 +327,7 @@ test_expect_success 'fetching of missing objects from an HTTP server' '
 	rm -rf repo/.git/objects/* &&
 
 	git -C repo config core.repositoryformatversion 1 &&
-	git -C repo config extensions.partialclone "origin" &&
+	git -C repo config odb.magic.promisorRemote "origin" &&
 	git -C repo cat-file -p "$HASH" &&
 
 	# Ensure that the .promisor file is written, and check that its
diff --git a/t/t5500-fetch-pack.sh b/t/t5500-fetch-pack.sh
index 0680dec808..3bb20b0566 100755
--- a/t/t5500-fetch-pack.sh
+++ b/t/t5500-fetch-pack.sh
@@ -792,14 +792,14 @@ fetch_filter_blob_limit_zero () {
 	test_config -C "$SERVER" uploadpack.allowfilter 1 &&
 
 	git clone "$URL" client &&
-	test_config -C client extensions.partialclone origin &&
+	test_config -C client odb.magic.promisorRemote origin &&
 
 	test_commit -C "$SERVER" two &&
 
 	git -C client fetch --filter=blob:limit=0 origin HEAD:somewhere &&
 
 	# Ensure that commit is fetched, but blob is not
-	test_config -C client extensions.partialclone "arbitrary string" &&
+	test_config -C client odb.magic.promisorRemote "arbitrary string" &&
 	git -C client cat-file -e $(git -C "$SERVER" rev-parse two) &&
 	test_must_fail git -C client cat-file -e $(git hash-object "$SERVER/two.t")
 }
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index 0b62037744..022c65a7ae 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -647,7 +647,7 @@ partial_clone () {
 	git -C client fsck &&
 
 	# Ensure that unneeded blobs are not inadvertently fetched.
-	test_config -C client extensions.partialclone "not a remote" &&
+	test_config -C client odb.origin.promisorRemote "not a remote" &&
 	test_must_fail git -C client cat-file -e "$HASH1" &&
 
 	# But this blob was fetched, because clone performs an initial checkout
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 29d8631184..5c84b0f95b 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -40,7 +40,7 @@ test_expect_success 'do partial clone 1' '
 		| sort >observed.oids &&
 	test_cmp expect_1.oids observed.oids &&
 	test "$(git -C pc1 config --local core.repositoryformatversion)" = "1" &&
-	test "$(git -C pc1 config --local extensions.partialclone)" = "origin" &&
+	test "$(git -C pc1 config --local odb.origin.promisorRemote)" = "origin" &&
 	test "$(git -C pc1 config --local core.partialclonefilter)" = "blob:none"
 '
 
diff --git a/unpack-trees.c b/unpack-trees.c
index d5685891a5..38124e233d 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -16,6 +16,7 @@
 #include "submodule-config.h"
 #include "fsmonitor.h"
 #include "fetch-object.h"
+#include "external-odb.h"
 
 /*
  * Error messages expected by scripts out of plumbing commands such as
@@ -371,7 +372,7 @@ static int check_updates(struct unpack_trees_options *o)
 		load_gitmodules_file(index, &state);
 
 	enable_delayed_checkout(&state);
-	if (repository_format_partial_clone && o->update && !o->dry_run) {
+	if (has_external_odb() && o->update && !o->dry_run) {
 		/*
 		 * Prefetch the objects that are to be checked out in the loop
 		 * below.
@@ -388,8 +389,7 @@ static int check_updates(struct unpack_trees_options *o)
 			}
 		}
 		if (to_fetch.nr)
-			fetch_objects(repository_format_partial_clone,
-				      &to_fetch);
+			external_odb_get_many_direct(&to_fetch);
 		fetch_if_missing = fetch_if_missing_store;
 	}
 	for (i = 0; i < index->cache_nr; i++) {

t/lib-httpd.sh, t/lib-httpd/upload.sh

diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index 2e659a8ee2..d80b004549 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -132,6 +132,7 @@ prepare_httpd() {
 	cp "$TEST_PATH"/passwd "$HTTPD_ROOT_PATH"
 	install_script broken-smart-http.sh
 	install_script error.sh
+	install_script upload.sh
 
 	ln -s "$LIB_HTTPD_MODULE_PATH" "$HTTPD_ROOT_PATH/modules"
 
diff --git a/t/lib-httpd/upload.sh b/t/lib-httpd/upload.sh
new file mode 100644
index 0000000000..64d3f31c31
--- /dev/null
+++ b/t/lib-httpd/upload.sh
@@ -0,0 +1,45 @@
+#!/bin/sh
+
+# In part from http://codereview.stackexchange.com/questions/79549/bash-cgi-upload-file
+
+FILES_DIR="www/files"
+
+OLDIFS="$IFS"
+IFS='&'
+set -- $QUERY_STRING
+IFS="$OLDIFS"
+
+while test $# -gt 0
+do
+	key=${1%%=*}
+	val=${1#*=}
+
+	case "$key" in
+	"sha1") sha1="$val" ;;
+	"type") type="$val" ;;
+	"size") size="$val" ;;
+	"delete") delete=1 ;;
+	*) echo >&2 "unknown key '$key'" ;;
+	esac
+
+	shift
+done
+
+case "$REQUEST_METHOD" in
+POST)
+	if test "$delete" = "1"
+	then
+		rm -f "$FILES_DIR/$sha1-$size-$type"
+	else
+		mkdir -p "$FILES_DIR"
+		cat >"$FILES_DIR/$sha1-$size-$type"
+	fi
+
+	echo 'Status: 204 No Content'
+	echo
+	;;
+
+*)
+	echo 'Status: 405 Method Not Allowed'
+	echo
+esac

diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index 435a37465a..2e659a8ee2 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -171,12 +171,14 @@ prepare_httpd() {
 }
 
 start_httpd() {
+	APACHE_CONF_FILE=${1-apache.conf}
+
 	prepare_httpd >&3 2>&4
 
 	trap 'code=$?; stop_httpd; (exit $code); die' EXIT
 
 	"$LIB_HTTPD_PATH" -d "$HTTPD_ROOT_PATH" \
-		-f "$TEST_PATH/apache.conf" $HTTPD_PARA \
+		-f "$TEST_PATH/$APACHE_CONF_FILE" $HTTPD_PARA \
 		-c "Listen 127.0.0.1:$LIB_HTTPD_PORT" -k start \
 		>&3 2>&4
 	if test $? -ne 0
@@ -191,7 +193,7 @@ stop_httpd() {
 	trap 'die' EXIT
 
 	"$LIB_HTTPD_PATH" -d "$HTTPD_ROOT_PATH" \
-		-f "$TEST_PATH/apache.conf" $HTTPD_PARA -k stop
+		-f "$TEST_PATH/$APACHE_CONF_FILE" $HTTPD_PARA -k stop
 }
 
 test_http_push_nonff () {

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index e9d3cfb9e3..59e12151e8 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -28,6 +28,7 @@
 #include "argv-array.h"
 #include "list.h"
 #include "packfile.h"
+#include "external-odb.h"
 
 static const char *pack_usage[] = {
 	N_("git pack-objects --stdout [<options>...] [< <ref-list> | < <object-list>]"),
@@ -1026,6 +1027,9 @@ static int want_object_in_pack(const struct object_id *oid,
 			return want;
 	}
 
+	if (external_odb_has_object(oid->hash))
+		return 0;
+
 	list_for_each(pos, &packed_git_mru) {
 		struct packed_git *p = list_entry(pos, struct packed_git, mru);
 		off_t offset;

diff --git a/t/t0520-transfer-http-e-odb.sh b/t/t0520-transfer-http-e-odb.sh
new file mode 100755
index 0000000000..f84fe950ec
--- /dev/null
+++ b/t/t0520-transfer-http-e-odb.sh
@@ -0,0 +1,142 @@
+#!/bin/sh
+
+test_description='tests for transfering external objects to an HTTPD server'
+
+. ./test-lib.sh
+
+# If we don't specify a port, the current test number will be used
+# which will not work as it is less than 1024, so it can only be used by root.
+LIB_HTTPD_PORT=$(expr ${this_test#t} + 12000)
+
+. "$TEST_DIRECTORY"/lib-httpd.sh
+
+start_httpd apache-e-odb.conf
+
+# odb helper script must see this
+export HTTPD_URL
+
+write_script odb-http-helper <<\EOF
+die() {
+	printf >&2 "%s\n" "$@"
+	exit 1
+}
+echo >&2 "odb-http-helper args:" "$@"
+case "$1" in
+init)
+	echo "capability=get_raw_obj"
+	echo "capability=put_raw_obj"
+	echo "capability=have"
+	;;
+have)
+	list_url="$HTTPD_URL/list/"
+	curl "$list_url" ||
+	die "curl '$list_url' failed"
+	;;
+get_raw_obj)
+	get_url="$HTTPD_URL/list/?sha1=$2"
+	curl "$get_url" ||
+	die "curl '$get_url' failed"
+	;;
+put_raw_obj)
+	sha1="$2"
+	size="$3"
+	kind="$4"
+	upload_url="$HTTPD_URL/upload/?sha1=$sha1&size=$size&type=$kind"
+	curl --data-binary @- --include "$upload_url" >out ||
+	die "curl '$upload_url' failed"
+	ref_hash=$(echo "$sha1 $size $kind" | GIT_NO_EXTERNAL_ODB=1 git hash-object -w -t blob --stdin) || exit
+	git update-ref refs/odbs/magic/"$sha1" "$ref_hash"
+	;;
+*)
+	die "unknown command '$1'"
+	;;
+esac
+EOF
+HELPER="\"$PWD\"/odb-http-helper"
+
+test_expect_success 'setup repo with a root commit and the helper' '
+	test_commit zero &&
+	git config odb.magic.scriptCommand "$HELPER"
+'
+
+test_expect_success 'setup another repo from the first one' '
+	git init other-repo &&
+	(cd other-repo &&
+	 git remote add origin .. &&
+	 git pull origin master &&
+	 git checkout master &&
+	 git log)
+'
+
+UPLOADFILENAME="hello_apache_upload.txt"
+
+UPLOAD_URL="$HTTPD_URL/upload/?sha1=$UPLOADFILENAME&size=123&type=blob"
+
+test_expect_success 'can upload a file' '
+	echo "Hello Apache World!" >hello_to_send.txt &&
+	echo "How are you?" >>hello_to_send.txt &&
+	curl --data-binary @hello_to_send.txt --include "$UPLOAD_URL" >out_upload
+'
+
+LIST_URL="$HTTPD_URL/list/"
+
+test_expect_success 'can list uploaded files' '
+	curl --include "$LIST_URL" >out_list &&
+	grep "$UPLOADFILENAME" out_list
+'
+
+test_expect_success 'can delete uploaded files' '
+	curl --data "delete" --include "$UPLOAD_URL&delete=1" >out_delete &&
+	curl --include "$LIST_URL" >out_list2 &&
+	! grep "$UPLOADFILENAME" out_list2
+'
+
+FILES_DIR="httpd/www/files"
+
+test_expect_success 'new blobs are transfered to the http server' '
+	test_commit one &&
+	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
+	echo "$hash1-4-blob" >expected &&
+	ls "$FILES_DIR" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'blobs can be retrieved from the http server' '
+	git cat-file blob "$hash1" &&
+	git log -p >expected
+'
+
+test_expect_success 'update other repo from the first one' '
+	(cd other-repo &&
+	 git fetch origin "refs/odbs/magic/*:refs/odbs/magic/*" &&
+	 test_must_fail git cat-file blob "$hash1" &&
+	 git config odb.magic.scriptCommand "$HELPER" &&
+	 git cat-file blob "$hash1" &&
+	 git pull origin master)
+'
+
+test_expect_success 'local clone from the first repo' '
+	mkdir my-clone &&
+	(cd my-clone &&
+	 git clone .. . &&
+	 git cat-file blob "$hash1")
+'
+
+test_expect_success 'no-local clone from the first repo fails' '
+	mkdir my-other-clone &&
+	(cd my-other-clone &&
+	 test_must_fail git clone --no-local .. .) &&
+	rm -rf my-other-clone
+'
+
+test_expect_success 'no-local clone from the first repo with helper succeeds' '
+	mkdir my-other-clone &&
+	(cd my-other-clone &&
+	 git clone -c odb.magic.scriptCommand="$HELPER" \
+		--no-local .. .) &&
+	rm -rf my-other-clone
+'
+
+stop_httpd
+
+test_done

t/t0560-read-object-git.sh, t/t0560/read-object-git

diff --git a/t/t0560-read-object-git.sh b/t/t0560-read-object-git.sh
new file mode 100755
index 0000000000..4e8fc04dfd
--- /dev/null
+++ b/t/t0560-read-object-git.sh
@@ -0,0 +1,28 @@
+#!/bin/sh
+
+test_description='tests for long running read-object process passing git objects'
+
+. ./test-lib.sh
+
+PATH="$PATH:$TEST_DIRECTORY/t0560"
+
+test_expect_success 'setup host repo with a root commit' '
+	test_commit zero &&
+	hash1=$(git ls-tree HEAD | grep zero.t | cut -f1 | cut -d\  -f3)
+'
+
+HELPER="read-object-git"
+
+test_expect_success 'blobs can be retrieved from the host repo' '
+	git init guest-repo &&
+	(cd guest-repo &&
+	 git config odb.magic.subprocessCommand "$HELPER" &&
+	 git cat-file blob "$hash1" >/dev/null)
+'
+
+test_expect_success 'invalid blobs generate errors' '
+	cd guest-repo &&
+	test_must_fail git cat-file blob "invalid"
+'
+
+test_done
diff --git a/t/t0560/read-object-git b/t/t0560/read-object-git
new file mode 100755
index 0000000000..4b3ca0948b
--- /dev/null
+++ b/t/t0560/read-object-git
@@ -0,0 +1,78 @@
+#!/usr/bin/perl
+#
+# Example implementation for the Git read-object protocol version 1
+# See Documentation/technical/read-object-protocol.txt
+#
+# Allows you to test the ability for blobs to be pulled from a host git repo
+# "on demand."  Called when git needs a blob it couldn't find locally due to
+# a lazy clone that only cloned the commits and trees.
+#
+# A lazy clone can be simulated via the following commands from the host repo
+# you wish to create a lazy clone of:
+#
+# cd /host_repo
+# git rev-parse HEAD
+# git init /guest_repo
+# git cat-file --batch-check --batch-all-objects | grep -v 'blob' |
+#	cut -d' ' -f1 | git pack-objects /e/guest_repo/.git/objects/pack/noblobs
+# cd /guest_repo
+# git config core.virtualizeobjects true
+# git reset --hard <sha from rev-parse call above>
+#
+# Please note, this sample is a minimal skeleton. No proper error handling 
+# was implemented.
+#
+
+use 5.008;
+use lib (split(/:/, $ENV{GITPERLLIB}));
+use strict;
+use warnings;
+use Git::Packet;
+
+#
+# Point $DIR to the folder where your host git repo is located so we can pull
+# missing objects from it
+#
+my $DIR = "../.git/";
+
+packet_initialize("git-read-object", 1);
+
+my %remote_caps = packet_read_and_check_capabilities("get_git_obj");
+packet_check_and_write_capabilities(\%remote_caps, "get_git_obj");
+
+while (1) {
+	my ($res, $command) = packet_txt_read();
+
+	if ( $res == -1 ) {
+		exit 0;
+	}
+
+	$command =~ s/^command=//;
+
+	if ( $command eq "init" ) {
+		packet_bin_read();
+
+		packet_txt_write("status=success");
+		packet_flush();
+	} elsif ( $command eq "get_git_obj" ) {
+		my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
+		packet_bin_read();
+
+		my $path = $sha1;
+		$path =~ s{..}{$&/};
+		$path = $DIR . "/objects/" . $path;
+
+		my $contents = do {
+		    local $/;
+		    open my $fh, $path or die "Can't open '$path': $!";
+		    <$fh>
+		};
+
+		packet_bin_write($contents);
+		packet_flush();
+		packet_txt_write("status=success");
+		packet_flush();
+	} else {
+		die "bad command '$command'";
+	}
+}

diff --git a/Documentation/technical/external-odb.txt b/Documentation/technical/external-odb.txt
new file mode 100644
index 0000000000..58ec8a8145
--- /dev/null
+++ b/Documentation/technical/external-odb.txt
@@ -0,0 +1,342 @@
+External ODBs
+^^^^^^^^^^^^^
+
+The External ODB mechanism makes it possible for Git objects, only
+blobs for now though, to be stored in an "external object database"
+(External ODB).
+
+An External ODB can be any object store as long as there is an helper
+program called an "odb helper" that can communicate with Git to
+transfer objects to/from the external odb and to retrieve information
+about available objects in the external odb.
+
+Purpose
+=======
+
+The purpose of this mechanism is to make possible to handle Git
+objects, especially blobs, in much more flexible ways.
+
+Currently Git can store its objects only in the form of loose objects
+in separate files or packed objects in a pack file. These existing
+object stores cannot be easily optimized for many different kind of
+contents.
+
+So the current stores are not flexible enough for some important use
+cases like handling really big binary files or handling a really big
+number of files that are fetched only as needed. And it is not
+realistic to expect that Git could fully natively handle many of such
+use cases. Git would need to natively implement different internal
+stores which would be a huge burden and which could lead to
+re-implement things like HTTP servers, Docker registries or artifact
+stores that already exist outside Git.
+
+Furthermore many improvements that are dependent on specific setups
+could be implemented in the way Git objects are managed if it was
+possible to customize how the Git objects are handled. For example a
+restartable clone using the bundle mechanism has often been requested,
+but implementing that would go against the current strict rules under
+which the Git objects are currently handled.
+
+What Git needs is a mechanism to make it possible to customize in a
+lot of different ways how the Git objects are handled. Though this
+mechanism should try as much as possible to avoid interfering with the
+usual way in which Git handle its objects.
+
+Helpers
+=======
+
+ODB helpers are commands that have to be registered using either the
+"odb.<odbname>.subprocessCommand" or the "odb.<odbname>.scriptCommand"
+config variables.
+
+Registering such a command tells Git that an external odb called
+<odbname> exists and that the registered command should be used to
+communicate with it.
+
+The communication happens through instructions that are sent by Git
+and that the commands should answer. If it makes sense, Git can send
+the same instruction to many commands in the order in which they are
+configured.
+
+There are 2 kinds of commands. Commands registered using the
+"odb.<odbname>.subprocessCommand" config variable are called "process
+commands" and the associated mode is called "process mode". Commands
+registered using the "odb.<odbname>.scriptCommand" config variables
+are called "script commands" and the associated mode is called "script
+mode".
+
+Early on git commands send an 'init' instruction to the registered
+commands. A capability negociation will take place during this
+request/response exchange which will let Git and the helpers know how
+they can further collaborate. The attribute system can also be used to
+tell Git which objects should be handled by which helper.
+
+Process Mode
+============
+
+In process mode the command is started as a single process invocation
+that should last for the entire life of the single Git command that
+started it.
+
+A packet format (pkt-line, see technical/protocol-common.txt) based
+protocol over standard input and standard output is used for
+communication between Git and the helper command.
+
+After the process command is started, Git sends a welcome message
+("git-read-object-client"), a list of supported protocol version
+numbers, and a flush packet. Git expects to read a welcome response
+message ("git-read-object-server"), exactly one protocol version
+number from the previously sent list, and a flush packet. All further
+communication will be based on the selected version.
+
+The remaining protocol description below documents "version=1". Please
+note that "version=42" in the example below does not exist and is only
+there to illustrate how the protocol would look with more than one
+version.
+
+After the version negotiation Git sends a list of all capabilities
+that it supports and a flush packet. Git expects to read a list of
+desired capabilities, which must be a subset of the supported
+capabilities list, and a flush packet as response:
+
+------------------------
+packet: git> git-read-object-client
+packet: git> version=1
+packet: git> version=42
+packet: git> 0000
+packet: git< git-read-object-server
+packet: git< version=1
+packet: git< 0000
+packet: git> capability=get_raw_obj
+packet: git> capability=have
+packet: git> capability=put_raw_obj
+packet: git> capability=not-yet-invented
+packet: git> 0000
+packet: git< capability=get_raw_obj
+packet: git< 0000
+------------------------
+
+Afterwards Git sends a list of "key=value" pairs terminated with a
+flush packet. The list will contain at least the instruction (based on
+the supported capabilities) and the arguments for the
+instruction. Please note, that the process must not send any response
+before it received the final flush packet.
+
+In general any response from the helper should end with a status
+packet. See the documentation of the 'get_*' instructions below for
+examples of status packets.
+
+After the helper has processed an instruction, it is expected to wait
+for the next "key=value" list containing another instruction.
+
+On exit Git will close the pipe to the helper. The helper is then
+expected to detect EOF and exit gracefully on its own. Git will wait
+until the process has stopped.
+
+Script Mode
+===========
+
+In this mode Git launches the script command each time it wants to
+communicates with the helper. There is no welcome message and no
+protocol version in this mode.
+
+The instruction and associated arguments are passed as arguments when
+launching the script command and if needed further information is
+passed between Git and the command through stdin and stdout.
+
+Capabilities/Instructions
+=========================
+
+The following instructions are currently supported by Git:
+
+- init
+- get_git_obj
+- get_raw_obj
+- get_direct
+- put_raw_obj
+- have
+
+The plan is to also support 'put_git_obj' and 'put_direct' soon, for
+consistency with the 'get_*' instructions.
+
+ - 'init'
+
+All the process and script commands must accept the 'init'
+instruction. It should be the first instruction sent to a command. It
+should not be advertised in the capability exchange. Any argument
+should be ignored.
+
+In process mode, after receiving the 'init' instruction and a flush
+packet, the helper should just send a status packet and then a flush
+packet. See the 'get_*' instructions below for examples of status
+packets.
+
+In script mode the command should print on stdout the capabilities
+that it supports if any. This is the only time in script mode when a
+capability exchange happens.
+
+For example a script command could use the following shell code
+snippet to handle the 'init' instruction:
+
+------------------------
+case "$1" in
+init)
+	echo "capability=get_git_obj"
+	echo "capability=put_raw_obj"
+	echo "capability=have"
+	;;
+------------------------
+
+ - 'get_git_obj <sha1>' and 'get_raw_obj <sha1>'
+
+These instructions should have a hexadecimal <sha1> argument to tell
+which object the helper should send to git.
+
+In process mode the sha1 argument should be followed by a flush packet
+like this:
+
+------------------------
+packet: git> command=get_git_obj
+packet: git> sha1=0a214a649e1b3d5011e14a3dc227753f2bd2be05
+packet: git> 0000
+------------------------
+
+After reading that the helper should send the requested object to Git in a
+packet series followed by a flush packet. If the helper does not experience
+problems then the helper must send a "success" status like the following:
+
+------------------------
+packet: git< status=success
+packet: git< 0000
+------------------------
+
+In case the helper cannot or does not want to send the requested
+object as well as any other object for the lifetime of the Git
+process, then it is expected to respond with an "abort" status at any
+point in the protocol:
+
+------------------------
+packet: git< status=abort
+packet: git< 0000
+------------------------
+
+Git neither stops nor restarts the helper in case a
+"notfound"/"error"/"abort" status is set. An "error" status means a
+possibly more transient error than an abort. The helper should also
+send a "notfound" error in case of a "get_*" instruction, which means
+that the requested object cannot be found.
+
+If the helper dies during the communication or does not adhere to the
+protocol then Git will stop and restart it with the next instruction.
+
+In script mode the helper should just send the requested object to Git
+by writing it to stdout and should then exit. The exit code should
+signal to Git if a problem occured or not.
+
+The only difference between 'get_git_obj' and 'get_raw_obj' is that in
+case of 'get_git_obj' the requested object should be sent as a Git
+object (that is in the same format as loose object files). In case of
+'get_raw_obj' the object should be sent in its raw format (that is the
+same output as `git cat-file <type> <sha1>`).
+
+ - 'get_direct <sha1>'
+
+This instruction is similar as the other 'get_*' instructions except
+that no object should be sent from the helper to Git. Instead the
+helper should directly write the requested object into a loose object
+file in the ".git/objects" directory.
+
+After the helper has sent the "status=success" packet and the
+following flush packet in process mode, or after it has exited in the
+script mode, Git will lookup again for the requested sha1 in its loose
+object files and pack files.
+
+ - 'put_raw_obj <sha1> <size> <type>'
+
+This instruction should be following by three arguments to tell which
+object the helper will receive from git: <sha1>, <size> and
+<type>. The hexadecimal <sha1> argument describes the object that will
+be sent from Git to the helper. The <type> is the object type ("blob",
+"tree", "commit" or "tag") of this object. The <size> is the size in
+bytes of the (decompressed) object content.
+
+In process mode the last argument (the type) should be followed by a
+flush packet.
+
+After reading that the helper should read the announced object from
+Git in a packet series followed by a flush packet.
+
+If the helper does not experience problems when receiving and storing
+or processing the object, then the helper must send a "success" status
+as described for the 'get_*' instructions.
+
+In script mode the helper should just receive the announced object
+from its standard input. After receiving and processing the object,
+the helper should exit and its exit code should signal to Git if a
+problem occured or not.
+
+- 'have'
+
+In process mode this instruction should be followed by a flush
+packet. After receiving this packet the helper should send the sha1,
+size and type, in this order, of all the objects it can provide to Git
+(through a 'get_*' instruction). There should be a space character
+between the sha1 and the size and between the size and the type, and
+then a new line character after the type.
+
+If many packets are needed to send back all this information, the
+split between packets should be made after the new line characters.
+
+If the helper does not experience problems, then it must then send a
+"success" status as described for the 'get_*' instructions.
+
+In script mode the helper should send to its standard output the sha1,
+size and type, in this order of all the objects it can provide to
+Git. There should also be a space character between the sha1 and the
+size and between the size and the type, and then a new line character
+after the type.
+
+After sending this, the script helper should exit and its exit code
+should signal to Git if a problem occured or not.
+
+Order of instructions
+=====================
+
+For get_*_object instructions the regular code to find objects is
+called before the odb helpers.
+
+For put_*_object instructions the regular code to store the objects is
+called after the odb helpers.
+
+For now this order is not configurable.
+
+Object caching
+==============
+
+If a helper returns the object data as requested by get_git_obj or
+get_raw_obj, then Git will itself store the object locally in its
+regular object store, so it is redundant for the helper to also store
+or try to store the object in the regular object store.
+
+Yeah, this seems to defeat the goal of enabling specialized object
+handlers to handle large or other "unusual" objects that git normally
+doesn't deal well with. So in the long run there should be a way to
+make this configurable.
+
+Selecting objects
+=================
+
+To select objects that should be handled by an external odb, one can
+use the git attributes system. For now this will only work with blobs
+and this will only work along with the 'put_raw_obj' instruction.
+
+For example if one has an external odb called "magic" and has
+registered an associated a process command helper that supports the
+'put_raw_obj' instruction, then one can tell Git that all the .jpg
+files should be handled by the "magic" odb using a .gitattributes file
+can that contains:
+
+------------------------
+*.jpg           odb=magic
+------------------------
+

external-odb.c, odb-helper.c, odb-helper.h, t/t0500-external-odb.sh

diff --git a/external-odb.c b/external-odb.c
index 9f797d66f4..ae6bc017fe 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -35,6 +35,8 @@ static int external_odb_config(const char *var, const char *value, void *data)
 
 	if (!strcmp(subkey, "promisorremote")) {
 		o->type = ODB_HELPER_GIT_REMOTE;
+		o->supported_capabilities |= ODB_HELPER_CAP_HAVE;
+		o->supported_capabilities |= ODB_HELPER_CAP_GET_DIRECT;
 		return git_config_string(&o->dealer, var, value);
 	}
 	if (!strcmp(subkey, "scriptcommand")) {
@@ -52,12 +54,16 @@ static int external_odb_config(const char *var, const char *value, void *data)
 static void external_odb_do_init(int force)
 {
 	static int initialized;
+	struct odb_helper *o;
 
 	if ((!force && initialized) || !use_external_odb)
 		return;
 	initialized = 1;
 
 	git_config(external_odb_config, NULL);
+
+	for (o = helpers; o; o = o->next)
+		odb_helper_init(o);
 }
 
 static inline void external_odb_init(void)
@@ -104,9 +110,12 @@ int external_odb_has_object(const unsigned char *sha1)
 
 	external_odb_init();
 
-	for (o = helpers; o; o = o->next)
+	for (o = helpers; o; o = o->next) {
+		if (!(o->supported_capabilities & ODB_HELPER_CAP_HAVE))
+			return 1;
 		if (odb_helper_has_object(o, sha1))
 			return 1;
+	}
 	return 0;
 }
 
@@ -164,6 +173,8 @@ int external_odb_get_direct(const unsigned char *sha1)
 	external_odb_init();
 
 	for (o = helpers; o; o = o->next) {
+		if (!(o->supported_capabilities & ODB_HELPER_CAP_GET_DIRECT))
+			continue;
 		if (odb_helper_get_direct(o, sha1) < 0)
 			continue;
 		return 0;
diff --git a/odb-helper.c b/odb-helper.c
index bf3c1ad8ac..a99f4a1299 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -6,6 +6,40 @@
 #include "sha1-lookup.h"
 #include "fetch-object.h"
 
+static void parse_capabilities(char *cap_buf,
+			       unsigned int *supported_capabilities,
+			       const char *process_name)
+{
+	struct string_list cap_list = STRING_LIST_INIT_NODUP;
+
+	string_list_split_in_place(&cap_list, cap_buf, '=', 1);
+
+	if (cap_list.nr == 2 && !strcmp(cap_list.items[0].string, "capability")) {
+		const char *cap_name = cap_list.items[1].string;
+
+		if (!strcmp(cap_name, "get_git_obj")) {
+			*supported_capabilities |= ODB_HELPER_CAP_GET_GIT_OBJ;
+		} else if (!strcmp(cap_name, "get_raw_obj")) {
+			*supported_capabilities |= ODB_HELPER_CAP_GET_RAW_OBJ;
+		} else if (!strcmp(cap_name, "get_direct")) {
+			*supported_capabilities |= ODB_HELPER_CAP_GET_DIRECT;
+		} else if (!strcmp(cap_name, "put_git_obj")) {
+			*supported_capabilities |= ODB_HELPER_CAP_PUT_GIT_OBJ;
+		} else if (!strcmp(cap_name, "put_raw_obj")) {
+			*supported_capabilities |= ODB_HELPER_CAP_PUT_RAW_OBJ;
+		} else if (!strcmp(cap_name, "put_direct")) {
+			*supported_capabilities |= ODB_HELPER_CAP_PUT_DIRECT;
+		} else if (!strcmp(cap_name, "have")) {
+			*supported_capabilities |= ODB_HELPER_CAP_HAVE;
+		} else {
+			warning("external process '%s' requested unsupported read-object capability '%s'",
+				process_name, cap_name);
+		}
+	}
+
+	string_list_clear(&cap_list, 0);
+}
+
 struct odb_helper *odb_helper_new(const char *name, int namelen)
 {
 	struct odb_helper *o;
@@ -80,6 +114,30 @@ static int odb_helper_finish(struct odb_helper *o,
 	return 0;
 }
 
+int odb_helper_init(struct odb_helper *o)
+{
+	struct odb_helper_cmd cmd;
+	FILE *fh;
+	struct strbuf line = STRBUF_INIT;
+
+	if (o->initialized)
+		return 0;
+	o->initialized = 1;
+
+	if (odb_helper_start(o, &cmd, "init") < 0)
+		return -1;
+
+	fh = xfdopen(cmd.child.out, "r");
+	while (strbuf_getline(&line, fh) != EOF)
+		parse_capabilities(line.buf, &o->supported_capabilities, o->name);
+
+	strbuf_release(&line);
+	fclose(fh);
+	odb_helper_finish(o, &cmd);
+
+	return 0;
+}
+
 static int parse_object_line(struct odb_helper_object *o, const char *line)
 {
 	char *end;
diff --git a/odb-helper.h b/odb-helper.h
index 819182f76a..7720684744 100644
--- a/odb-helper.h
+++ b/odb-helper.h
@@ -3,10 +3,20 @@
 
 #include "external-odb.h"
 
+#define ODB_HELPER_CAP_GET_GIT_OBJ    (1u<<0)
+#define ODB_HELPER_CAP_GET_RAW_OBJ    (1u<<1)
+#define ODB_HELPER_CAP_GET_DIRECT     (1u<<2)
+#define ODB_HELPER_CAP_PUT_GIT_OBJ    (1u<<3)
+#define ODB_HELPER_CAP_PUT_RAW_OBJ    (1u<<4)
+#define ODB_HELPER_CAP_PUT_DIRECT     (1u<<5)
+#define ODB_HELPER_CAP_HAVE           (1u<<6)
+
 struct odb_helper {
 	const char *name;
 	const char *dealer;
 	enum odb_helper_type type;
+	unsigned int supported_capabilities;
+	int initialized;
 
 	struct odb_helper_object {
 		struct object_id oid;
@@ -21,6 +31,7 @@ struct odb_helper {
 };
 
 extern struct odb_helper *odb_helper_new(const char *name, int namelen);
+extern int odb_helper_init(struct odb_helper *o);
 extern int odb_helper_has_object(struct odb_helper *o,
 				 const unsigned char *sha1);
 extern int odb_helper_get_object(struct odb_helper *o,
diff --git a/t/t0500-external-odb.sh b/t/t0500-external-odb.sh
index eaa3688a31..977fea852d 100755
--- a/t/t0500-external-odb.sh
+++ b/t/t0500-external-odb.sh
@@ -9,6 +9,10 @@ export ALT_SOURCE
 write_script odb-helper <<\EOF
 GIT_DIR=$ALT_SOURCE; export GIT_DIR
 case "$1" in
+init)
+	echo "capability=get_git_obj"
+	echo "capability=have"
+	;;
 have)
 	git cat-file --batch-check --batch-all-objects |
 	awk '{print $1 " " $3 " " $2}'

t/t0580-read-object-have-http-e-odb.sh, t/t0580/read-object-plain-have

diff --git a/t/t0580-read-object-have-http-e-odb.sh b/t/t0580-read-object-have-http-e-odb.sh
new file mode 100755
index 0000000000..9cbfb18d28
--- /dev/null
+++ b/t/t0580-read-object-have-http-e-odb.sh
@@ -0,0 +1,109 @@
+#!/bin/sh
+
+test_description='tests for read-object process with "have" cap and plain objects'
+
+. ./test-lib.sh
+
+# If we don't specify a port, the current test number will be used
+# which will not work as it is less than 1024, so it can only be used by root.
+LIB_HTTPD_PORT=$(expr ${this_test#t} + 12000)
+
+. "$TEST_DIRECTORY"/lib-httpd.sh
+
+start_httpd apache-e-odb.conf
+
+PATH="$PATH:$TEST_DIRECTORY/t0580"
+
+# odb helper script must see this
+export HTTPD_URL
+
+HELPER="read-object-plain-have"
+
+test_expect_success 'setup repo with a root commit' '
+	test_commit zero
+'
+
+test_expect_success 'setup another repo from the first one' '
+	git init other-repo &&
+	(cd other-repo &&
+	 git remote add origin .. &&
+	 git pull origin master &&
+	 git checkout master &&
+	 git log)
+'
+
+test_expect_success 'setup the helper in the root repo' '
+	git config odb.magic.subprocessCommand "$HELPER"
+'
+
+UPLOADFILENAME="hello_apache_upload.txt"
+
+UPLOAD_URL="$HTTPD_URL/upload/?sha1=$UPLOADFILENAME&size=123&type=blob"
+
+test_expect_success 'can upload a file' '
+	echo "Hello Apache World!" >hello_to_send.txt &&
+	echo "How are you?" >>hello_to_send.txt &&
+	curl --data-binary @hello_to_send.txt --include "$UPLOAD_URL" >out_upload
+'
+
+LIST_URL="$HTTPD_URL/list/"
+
+test_expect_success 'can list uploaded files' '
+	curl --include "$LIST_URL" >out_list &&
+	grep "$UPLOADFILENAME" out_list
+'
+
+test_expect_success 'can delete uploaded files' '
+	curl --data "delete" --include "$UPLOAD_URL&delete=1" >out_delete &&
+	curl --include "$LIST_URL" >out_list2 &&
+	! grep "$UPLOADFILENAME" out_list2
+'
+
+FILES_DIR="httpd/www/files"
+
+test_expect_success 'new blobs are transfered to the http server' '
+	test_commit one &&
+	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
+	echo "$hash1-4-blob" >expected &&
+	ls "$FILES_DIR" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'blobs can be retrieved from the http server' '
+	git cat-file blob "$hash1" &&
+	git log -p >expected
+'
+
+test_expect_success 'update other repo from the first one' '
+	(cd other-repo &&
+	 git fetch origin "refs/odbs/magic/*:refs/odbs/magic/*" &&
+	 test_must_fail git cat-file blob "$hash1" &&
+	 git config odb.magic.subprocessCommand "$HELPER" &&
+	 git cat-file blob "$hash1" &&
+	 git pull origin master)
+'
+
+test_expect_success 'local clone from the first repo' '
+	mkdir my-clone &&
+	(cd my-clone &&
+	 git clone .. . &&
+	 git cat-file blob "$hash1")
+'
+
+test_expect_success 'no-local clone from the first repo fails' '
+	mkdir my-other-clone &&
+	(cd my-other-clone &&
+	 test_must_fail git clone --no-local .. .) &&
+	rm -rf my-other-clone
+'
+
+test_expect_success 'no-local clone from the first repo with helper succeeds' '
+	mkdir my-other-clone &&
+	(cd my-other-clone &&
+	 git clone -c odb.magic.subprocessCommand="$HELPER" --no-local .. .) &&
+	rm -rf my-other-clone
+'
+
+stop_httpd
+
+test_done
diff --git a/t/t0580/read-object-plain-have b/t/t0580/read-object-plain-have
new file mode 100755
index 0000000000..f230cbd5eb
--- /dev/null
+++ b/t/t0580/read-object-plain-have
@@ -0,0 +1,103 @@
+#!/usr/bin/perl
+#
+
+use 5.008;
+use lib (split(/:/, $ENV{GITPERLLIB}));
+use strict;
+use warnings;
+use Git::Packet;
+use LWP::UserAgent;
+use HTTP::Request::Common;
+
+packet_initialize("git-read-object", 1);
+
+my %remote_caps = packet_read_and_check_capabilities("get_raw_obj", "put_raw_obj", "have");
+packet_check_and_write_capabilities(\%remote_caps, "get_raw_obj", "put_raw_obj", "have");
+
+my $http_url = $ENV{HTTPD_URL};
+
+while (1) {
+	my ($res, $command) = packet_txt_read();
+
+	if ( $res == -1 ) {
+		exit 0;
+	}
+
+	$command =~ s/^command=//;
+
+	if ( $command eq "init" ) {
+		packet_bin_read();
+
+		packet_txt_write("status=success");
+		packet_flush();
+	} elsif ( $command eq "have" ) {
+		# read the flush after the command
+		packet_bin_read();
+
+		my $have_url = $http_url . "/list/";
+
+		my $userAgent = LWP::UserAgent->new();
+		my $response = $userAgent->get( $have_url );
+
+		if ($response->is_error) {
+			packet_bin_write("");
+			packet_flush();
+			packet_txt_write("status=failure");
+		} else {
+			packet_bin_write($response->content);
+			packet_flush();
+			packet_txt_write("status=success");
+		}
+		packet_flush();
+	} elsif ( $command eq "get_raw_obj" ) {
+		my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
+		packet_bin_read();
+
+		my $get_url = $http_url . "/list/?sha1=" . $sha1;
+
+		my $userAgent = LWP::UserAgent->new();
+
+		my $response = $userAgent->get( $get_url );
+
+		if ($response->is_error) {
+			packet_txt_write("size=0");
+			packet_txt_write("kind=none");	    
+			packet_txt_write("status=notfound");
+		} else {
+			packet_txt_write("size=" . length($response->content));
+			packet_txt_write("kind=blob");
+			packet_bin_write($response->content);
+			packet_flush();
+			packet_txt_write("status=success");
+		}
+
+		packet_flush();
+	} elsif ( $command eq "put_raw_obj" ) {
+		my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
+		my ($size) = packet_txt_read() =~ /^size=([0-9]+)$/;
+		my ($kind) = packet_txt_read() =~ /^kind=(\w+)$/;
+
+		packet_bin_read();
+
+		# We must read the content we are sent and send it to the right url
+		my ($res, $buf) = packet_bin_read();
+		die "bad packet_bin_read res ($res)" unless ($res eq 0);
+		( packet_bin_read() eq ( 1, "" ) ) || die "bad send end";		
+
+		my $upload_url = $http_url . "/upload/?sha1=" . $sha1 . "&size=" . $size . "&type=blob";
+
+		my $userAgent = LWP::UserAgent->new();
+		my $request = POST $upload_url, Content_Type => 'multipart/form-data', Content => $buf;
+
+		my $response = $userAgent->request($request);
+
+		if ($response->is_error) {
+			packet_txt_write("status=failure");
+		} else {
+			packet_txt_write("status=success");
+		}
+		packet_flush();
+	} else {
+		die "bad command '$command'";
+	}
+}

diff --git a/odb-helper.c b/odb-helper.c
index 83f8cce3b1..5ec21e9d1f 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -649,6 +649,70 @@ static int odb_helper_object_cmp(const void *va, const void *vb)
 	return oidcmp(&a->oid, &b->oid);
 }
 
+static int send_have_packets(struct odb_helper *o,
+			     struct object_process *entry,
+			     struct strbuf *status)
+{
+	int packet_len;
+	int total_got = 0;
+	struct child_process *process = &entry->subprocess.process;
+	int err = packet_write_fmt_gently(process->in, "command=have\n");
+
+	if (err)
+		return err;
+
+	err = packet_flush_gently(process->in);
+	if (err)
+		return err;
+
+	for (;;) {
+		/* packet_read() writes a '\0' extra byte at the end */
+		char buf[LARGE_PACKET_DATA_MAX + 1];
+		char *p = buf;
+		int more;
+
+		packet_len = packet_read(process->out, NULL, NULL,
+			buf, LARGE_PACKET_DATA_MAX + 1,
+			PACKET_READ_GENTLE_ON_EOF);
+
+		if (packet_len <= 0)
+			break;
+
+		total_got += packet_len;
+
+		/* 'have' packets should end with '\n' or '\0' */
+		do {
+			char *eol = strchrnul(p, '\n');
+			more = (*eol == '\n');
+			*eol = '\0';
+			if (add_have_entry(o, p))
+				break;
+			p = eol + 1;
+		} while (more && *p);
+	}
+
+	if (packet_len < 0)
+		return packet_len;
+
+	return check_object_process_status(process->out, status);
+}
+
+static int have_object_process(struct odb_helper *o)
+{
+	int err;
+	struct object_process *entry;
+	struct strbuf status = STRBUF_INIT;
+
+	entry = launch_object_process(o, ODB_HELPER_CAP_HAVE);
+	if (!entry)
+		return -1;
+
+	err = send_have_packets(o, entry, &status);
+
+	return check_object_process_error(err, status.buf, entry, o->dealer,
+					  ODB_HELPER_CAP_HAVE);
+}
+
 static void have_object_script(struct odb_helper *o)
 {
 	struct odb_helper_cmd cmd;
@@ -670,12 +734,20 @@ static void have_object_script(struct odb_helper *o)
 
 static void odb_helper_load_have(struct odb_helper *o)
 {
+	uint64_t start;
+
 	if (o->have_valid)
 		return;
 	o->have_valid = 1;
 
+	start = getnanotime();
+
 	if (o->type == ODB_HELPER_SCRIPT_CMD)
 		have_object_script(o);
+	else if (o->type == ODB_HELPER_SUBPROCESS_CMD)
+		have_object_process(o);
+
+	trace_performance_since(start, "odb_helper_load_have");
 
 	qsort(o->have, o->have_nr, sizeof(*o->have), odb_helper_object_cmp);
 }
@@ -937,7 +1009,7 @@ int odb_helper_get_direct(struct odb_helper *o,
 		fetch_object(o->dealer, sha1);
 	else if (o->type == ODB_HELPER_SCRIPT_CMD)
 		res = get_direct_script(o, sha1);
-	else
+	else if (o->type == ODB_HELPER_SUBPROCESS_CMD)
 		res = get_object_process(o, sha1, -1);
 
 	trace_performance_since(start, "odb_helper_get_direct");

external-odb.c, external-odb.h, sha1_file.c, t/t0500-external-odb.sh, t/t0510-transfer-e-odb.sh, t/t0520-transfer-http-e-odb.sh, t/t0570-read-object-http-e-odb.sh, t/t0580-read-object-have-http-e-odb.sh

diff --git a/external-odb.c b/external-odb.c
index 5e5871c933..af2fc4a120 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -2,6 +2,7 @@
 #include "external-odb.h"
 #include "odb-helper.h"
 #include "config.h"
+#include "attr.h"
 
 static struct odb_helper *helpers;
 static struct odb_helper **helpers_tail = &helpers;
@@ -211,19 +212,37 @@ int external_odb_get_object(const unsigned char *sha1)
 	return external_odb_do_get_object(sha1);
 }
 
+static int has_odb_attrs(struct odb_helper *o, const char *path)
+{
+	static struct attr_check *check;
+
+	if (!check)
+		check = attr_check_initl("odb", NULL);
+
+	if (!git_check_attr(path, check)) {
+		const char *value = check->items[0].value;
+		return value ? !strcmp(o->name, value) : 0;
+	}
+	return 0;
+}
+
 int external_odb_put_object(const void *buf, size_t len,
-			    const char *type, unsigned char *sha1)
+			    const char *type, unsigned char *sha1,
+			    const char *path)
 {
 	struct odb_helper *o;
 
 	external_odb_init();
 
 	/* For now accept only blobs */
-	if (strcmp(type, "blob"))
+	if (!path || strcmp(type, "blob"))
 		return 1;
 
 	for (o = helpers; o; o = o->next) {
-		int r = odb_helper_put_object(o, buf, len, type, sha1);
+		int r;
+		if (!has_odb_attrs(o, path))
+			continue;
+		r = odb_helper_put_object(o, buf, len, type, sha1);
 		if (r <= 0)
 			return r;
 	}
diff --git a/external-odb.h b/external-odb.h
index 60da8ed3d4..a7f4d7eec0 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -19,6 +19,7 @@ extern int external_odb_get_object(const unsigned char *sha1);
 extern int external_odb_get_direct(const unsigned char *sha1);
 extern int external_odb_get_many_direct(const struct oid_array *to_get);
 extern int external_odb_put_object(const void *buf, size_t len,
-				   const char *type, unsigned char *sha1);
+				   const char *type, unsigned char *sha1,
+				   const char *path);
 
 #endif /* EXTERNAL_ODB_H */
diff --git a/sha1_file.c b/sha1_file.c
index 9ccdae35bc..b8e5af3cb8 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1708,8 +1708,9 @@ static int freshen_packed_object(const unsigned char *sha1)
 	return 1;
 }
 
-int write_object_file(const void *buf, unsigned long len, const char *type,
-		      struct object_id *oid)
+static int write_object_file_with_path(const void *buf, unsigned long len,
+				       const char *type, struct object_id *oid,
+				       const char *path)
 {
 	char hdr[32];
 	int hdrlen = sizeof(hdr);
@@ -1718,13 +1719,19 @@ int write_object_file(const void *buf, unsigned long len, const char *type,
 	 * it out into .git/objects/??/?{38} file.
 	 */
 	write_object_file_prepare(buf, len, type, oid, hdr, &hdrlen);
-	if (!external_odb_put_object(buf, len, type, oid->hash))
+	if (!external_odb_put_object(buf, len, type, oid->hash, path))
 		return 0;
 	if (freshen_packed_object(oid->hash) || freshen_loose_object(oid->hash))
 		return 0;
 	return write_loose_object(oid, hdr, hdrlen, buf, len, 0);
 }
 
+int write_object_file(const void *buf, unsigned long len,
+		      const char *type, struct object_id *oid)
+{
+	return write_object_file_with_path(buf, len, type, oid, NULL);
+}
+
 int hash_object_file_literally(const void *buf, unsigned long len,
 			       const char *type, struct object_id *oid,
 			       unsigned flags)
@@ -1846,7 +1853,8 @@ static int index_mem(struct object_id *oid, void *buf, size_t size,
 	}
 
 	if (write_object)
-		ret = write_object_file(buf, size, type_name(type), oid);
+		ret = write_object_file_with_path(buf, size, type_name(type),
+						  oid, path);
 	else
 		ret = hash_object_file(buf, size, type_name(type), oid);
 	if (re_allocated)
@@ -1868,8 +1876,8 @@ static int index_stream_convert_blob(struct object_id *oid, int fd,
 				 get_conv_flags(flags));
 
 	if (write_object)
-		ret = write_object_file(sbuf.buf, sbuf.len, type_name(OBJ_BLOB),
-					oid);
+		ret = write_object_file_with_path(sbuf.buf, sbuf.len, type_name(OBJ_BLOB),
+						  oid, path);
 	else
 		ret = hash_object_file(sbuf.buf, sbuf.len, type_name(OBJ_BLOB),
 				       oid);
diff --git a/t/t0500-external-odb.sh b/t/t0500-external-odb.sh
index f924de870f..944b7bb28f 100755
--- a/t/t0500-external-odb.sh
+++ b/t/t0500-external-odb.sh
@@ -71,6 +71,9 @@ test_expect_success 'helper can add objects to alt repo' '
 
 test_expect_success 'commit adds objects to alt repo' '
 	test_config odb.magic.scriptCommand "$HELPER" &&
+	echo "* odb=magic" >.gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git add .gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git commit -m "Add .gitattributes" &&
 	test_commit three &&
 	hash3=$(git ls-tree HEAD | grep three.t | cut -f1 | cut -d\  -f3) &&
 	content=$(git -C alt-repo show "$hash3") &&
diff --git a/t/t0510-transfer-e-odb.sh b/t/t0510-transfer-e-odb.sh
index 065ec7d759..fd3e37918c 100755
--- a/t/t0510-transfer-e-odb.sh
+++ b/t/t0510-transfer-e-odb.sh
@@ -111,6 +111,9 @@ test_expect_success 'setup other repo and its alternate repo' '
 '
 
 test_expect_success 'new blobs are put in first object store' '
+	echo "* odb=magic" >.gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git add .gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git commit -m "Add .gitattributes" &&
 	test_commit one &&
 	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
 	content=$(cd alt-repo1 && git show "$hash1") &&
diff --git a/t/t0520-transfer-http-e-odb.sh b/t/t0520-transfer-http-e-odb.sh
index f84fe950ec..d307af0457 100755
--- a/t/t0520-transfer-http-e-odb.sh
+++ b/t/t0520-transfer-http-e-odb.sh
@@ -94,6 +94,9 @@ test_expect_success 'can delete uploaded files' '
 FILES_DIR="httpd/www/files"
 
 test_expect_success 'new blobs are transfered to the http server' '
+	echo "* odb=magic" >.gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git add .gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git commit -m "Add .gitattributes" &&
 	test_commit one &&
 	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
 	echo "$hash1-4-blob" >expected &&
diff --git a/t/t0570-read-object-http-e-odb.sh b/t/t0570-read-object-http-e-odb.sh
index 1b2205e273..56980b700a 100755
--- a/t/t0570-read-object-http-e-odb.sh
+++ b/t/t0570-read-object-http-e-odb.sh
@@ -62,6 +62,9 @@ test_expect_success 'can delete uploaded files' '
 FILES_DIR="httpd/www/files"
 
 test_expect_success 'new blobs are transfered to the http server' '
+	echo "* odb=magic" >.gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git add .gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git commit -m "Add .gitattributes" &&
 	test_commit one &&
 	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
 	echo "$hash1-4-blob" >expected &&
diff --git a/t/t0580-read-object-have-http-e-odb.sh b/t/t0580-read-object-have-http-e-odb.sh
index 9cbfb18d28..fea0205e2e 100755
--- a/t/t0580-read-object-have-http-e-odb.sh
+++ b/t/t0580-read-object-have-http-e-odb.sh
@@ -62,6 +62,9 @@ test_expect_success 'can delete uploaded files' '
 FILES_DIR="httpd/www/files"
 
 test_expect_success 'new blobs are transfered to the http server' '
+	echo "* odb=magic" >.gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git add .gitattributes &&
+	GIT_NO_EXTERNAL_ODB=1 git commit -m "Add .gitattributes" &&
 	test_commit one &&
 	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
 	echo "$hash1-4-blob" >expected &&

external-odb.c, odb-helper.c

diff --git a/external-odb.c b/external-odb.c
index c3d90f39fb..192b09e843 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -138,7 +138,8 @@ int external_odb_get_object(const unsigned char *sha1)
 		int ret;
 		int fd;
 
-		if (!odb_helper_has_object(o, sha1))
+		if (!(o->supported_capabilities & ODB_HELPER_CAP_GET_RAW_OBJ) &&
+		    !(o->supported_capabilities & ODB_HELPER_CAP_GET_GIT_OBJ))
 			continue;
 
 		fd = create_object_tmpfile(&tmpfile, pathbuf.buf);
diff --git a/odb-helper.c b/odb-helper.c
index 7e5eab94cf..30cfe689de 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -336,14 +336,28 @@ int odb_helper_get_object(struct odb_helper *o, const unsigned char *sha1,
 int odb_helper_get_direct(struct odb_helper *o,
 			  const unsigned char *sha1)
 {
-	int res = 0;
 	uint64_t start = getnanotime();
 
-	fetch_object(o->dealer, sha1);
+	if (o->type == ODB_HELPER_GIT_REMOTE) {
+		fetch_object(o->dealer, sha1);
+	} else {
+		struct odb_helper_object *obj;
+		struct odb_helper_cmd cmd;
+
+		obj = odb_helper_lookup(o, sha1);
+		if (!obj)
+			return -1;
+
+		if (odb_helper_start(o, &cmd, "get_direct %s", sha1_to_hex(sha1)) < 0)
+			return -1;
+
+		if (odb_helper_finish(o, &cmd))
+			return -1;
+	}
 
 	trace_performance_since(start, "odb_helper_get_direct");
 
-	return res;
+	return 0;
 }
 
 int odb_helper_get_many_direct(struct odb_helper *o,

t/t0570-read-object-http-e-odb.sh, t/t0570/read-object-plain

diff --git a/t/t0570-read-object-http-e-odb.sh b/t/t0570-read-object-http-e-odb.sh
new file mode 100755
index 0000000000..1b2205e273
--- /dev/null
+++ b/t/t0570-read-object-http-e-odb.sh
@@ -0,0 +1,109 @@
+#!/bin/sh
+
+test_description='tests for read-object process passing plain objects to an HTTPD server'
+
+. ./test-lib.sh
+
+# If we don't specify a port, the current test number will be used
+# which will not work as it is less than 1024, so it can only be used by root.
+LIB_HTTPD_PORT=$(expr ${this_test#t} + 12000)
+
+. "$TEST_DIRECTORY"/lib-httpd.sh
+
+start_httpd apache-e-odb.conf
+
+PATH="$PATH:$TEST_DIRECTORY/t0570"
+
+# odb helper script must see this
+export HTTPD_URL
+
+HELPER="read-object-plain"
+
+test_expect_success 'setup repo with a root commit' '
+	test_commit zero
+'
+
+test_expect_success 'setup another repo from the first one' '
+	git init other-repo &&
+	(cd other-repo &&
+	 git remote add origin .. &&
+	 git pull origin master &&
+	 git checkout master &&
+	 git log)
+'
+
+test_expect_success 'setup the helper in the root repo' '
+	git config odb.magic.subprocessCommand "$HELPER"
+'
+
+UPLOADFILENAME="hello_apache_upload.txt"
+
+UPLOAD_URL="$HTTPD_URL/upload/?sha1=$UPLOADFILENAME&size=123&type=blob"
+
+test_expect_success 'can upload a file' '
+	echo "Hello Apache World!" >hello_to_send.txt &&
+	echo "How are you?" >>hello_to_send.txt &&
+	curl --data-binary @hello_to_send.txt --include "$UPLOAD_URL" >out_upload
+'
+
+LIST_URL="$HTTPD_URL/list/"
+
+test_expect_success 'can list uploaded files' '
+	curl --include "$LIST_URL" >out_list &&
+	grep "$UPLOADFILENAME" out_list
+'
+
+test_expect_success 'can delete uploaded files' '
+	curl --data "delete" --include "$UPLOAD_URL&delete=1" >out_delete &&
+	curl --include "$LIST_URL" >out_list2 &&
+	! grep "$UPLOADFILENAME" out_list2
+'
+
+FILES_DIR="httpd/www/files"
+
+test_expect_success 'new blobs are transfered to the http server' '
+	test_commit one &&
+	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
+	echo "$hash1-4-blob" >expected &&
+	ls "$FILES_DIR" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'blobs can be retrieved from the http server' '
+	git cat-file blob "$hash1" &&
+	git log -p >expected
+'
+
+test_expect_success 'update other repo from the first one' '
+	(cd other-repo &&
+	 git fetch origin "refs/odbs/magic/*:refs/odbs/magic/*" &&
+	 test_must_fail git cat-file blob "$hash1" &&
+	 git config odb.magic.subprocesscommand "$HELPER" &&
+	 git cat-file blob "$hash1" &&
+	 git pull origin master)
+'
+
+test_expect_success 'local clone from the first repo' '
+	mkdir my-clone &&
+	(cd my-clone &&
+	 git clone .. . &&
+	 git cat-file blob "$hash1")
+'
+
+test_expect_success 'no-local clone from the first repo fails' '
+	mkdir my-other-clone &&
+	(cd my-other-clone &&
+	 test_must_fail git clone --no-local .. .) &&
+	rm -rf my-other-clone
+'
+
+test_expect_success 'no-local clone from the first repo with helper succeeds' '
+	mkdir my-other-clone &&
+	(cd my-other-clone &&
+	 git clone -c odb.magic.subprocessCommand="$HELPER" --no-local .. .) &&
+	rm -rf my-other-clone
+'
+
+stop_httpd
+
+test_done
diff --git a/t/t0570/read-object-plain b/t/t0570/read-object-plain
new file mode 100755
index 0000000000..0766e16032
--- /dev/null
+++ b/t/t0570/read-object-plain
@@ -0,0 +1,83 @@
+#!/usr/bin/perl
+#
+
+use 5.008;
+use lib (split(/:/, $ENV{GITPERLLIB}));
+use strict;
+use warnings;
+use Git::Packet;
+use LWP::UserAgent;
+use HTTP::Request::Common;
+
+packet_initialize("git-read-object", 1);
+
+my %remote_caps = packet_read_and_check_capabilities("get_raw_obj", "put_raw_obj");
+packet_check_and_write_capabilities(\%remote_caps, "get_raw_obj", "put_raw_obj");
+
+my $http_url = $ENV{HTTPD_URL};
+
+while (1) {
+	my ($res, $command) = packet_txt_read();
+
+	if ( $res == -1 ) {
+		exit 0;
+	}
+
+	$command =~ s/^command=//;
+
+	if ( $command eq "init" ) {
+		packet_bin_read();
+
+		packet_txt_write("status=success");
+		packet_flush();
+	} elsif ( $command eq "get_raw_obj" ) {
+		my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
+		packet_bin_read();
+
+		my $get_url = $http_url . "/list/?sha1=" . $sha1;
+
+		my $userAgent = LWP::UserAgent->new();
+
+		my $response = $userAgent->get( $get_url );
+
+		if ($response->is_error) {
+		    packet_txt_write("size=0");
+		    packet_txt_write("kind=none");	    
+		    packet_txt_write("status=notfound");
+		} else {
+		    packet_txt_write("size=" . length($response->content));
+		    packet_txt_write("kind=blob");
+		    packet_bin_write($response->content);
+		    packet_flush();
+		    packet_txt_write("status=success");
+		}
+
+		packet_flush();
+	} elsif ( $command eq "put_raw_obj" ) {
+		my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
+		my ($size) = packet_txt_read() =~ /^size=([0-9]+)$/;
+		my ($kind) = packet_txt_read() =~ /^kind=(\w+)$/;
+		packet_bin_read();
+
+		# We must read the content we are sent and send it to the right url
+		my ($res, $buf) = packet_bin_read();
+		die "bad packet_bin_read res ($res)" unless ($res eq 0);
+		( packet_bin_read() eq ( 1, "" ) ) || die "bad send end";		
+
+		my $upload_url = $http_url . "/upload/?sha1=" . $sha1 . "&size=" . $size . "&type=blob";
+
+		my $userAgent = LWP::UserAgent->new();
+		my $request = POST $upload_url, Content_Type => 'multipart/form-data', Content => $buf;
+
+		my $response = $userAgent->request($request);
+
+		if ($response->is_error) {
+			packet_txt_write("status=failure");
+		} else {
+			packet_txt_write("status=success");
+		}
+		packet_flush();
+	} else {
+		die "bad command '$command'";
+	}
+}

diff --git a/odb-helper.c b/odb-helper.c
index cad3d1005c..83f8cce3b1 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -442,6 +442,58 @@ static int get_object_process(struct odb_helper *o, const unsigned char *sha1, i
 					  o->dealer, cur_cap);
 }
 
+static int send_put_packets(struct object_process *entry,
+			    const unsigned char *sha1,
+			    const void *buf,
+			    size_t len,
+			    struct strbuf *status)
+{
+	struct child_process *process = &entry->subprocess.process;
+	int err = packet_write_fmt_gently(process->in, "command=put_raw_obj\n");
+	if (err)
+		return err;
+
+	err = packet_write_fmt_gently(process->in, "sha1=%s\n", sha1_to_hex(sha1));
+	if (err)
+		return err;
+
+	err = packet_write_fmt_gently(process->in, "size=%"PRIuMAX"\n", len);
+	if (err)
+		return err;
+
+	err = packet_write_fmt_gently(process->in, "kind=blob\n");
+	if (err)
+		return err;
+
+	err = packet_flush_gently(process->in);
+	if (err)
+		return err;
+
+	err = write_packetized_from_buf(buf, len, process->in);
+	if (err)
+		return err;
+
+	return check_object_process_status(process->out, status);
+}
+
+static int put_object_process(struct odb_helper *o,
+			      const void *buf, size_t len,
+			      const char *type, unsigned char *sha1)
+{
+	int err;
+	struct object_process *entry;
+	struct strbuf status = STRBUF_INIT;
+
+	entry = launch_object_process(o, ODB_HELPER_CAP_PUT_RAW_OBJ);
+	if (!entry)
+		return -1;
+
+	err = send_put_packets(entry, sha1, buf, len, &status);
+
+	return check_object_process_error(err, status.buf, entry, o->dealer,
+					  ODB_HELPER_CAP_PUT_RAW_OBJ);
+}
+
 struct odb_helper *odb_helper_new(const char *name, int namelen)
 {
 	struct odb_helper *o;
@@ -938,15 +990,12 @@ int odb_helper_get_object(struct odb_helper *o,
 	return res;
 }
 
-int odb_helper_put_object(struct odb_helper *o,
-			  const void *buf, size_t len,
-			  const char *type, unsigned char *sha1)
+static int put_raw_object_script(struct odb_helper *o,
+				 const void *buf, size_t len,
+				 const char *type, unsigned char *sha1)
 {
 	struct odb_helper_cmd cmd;
 
-	if (o->type != ODB_HELPER_SCRIPT_CMD)
-		return 1;
-
 	if (odb_helper_start(o, &cmd, 1, "put_raw_obj %s %"PRIuMAX" %s",
 			     sha1_to_hex(sha1), (uintmax_t)len, type) < 0)
 		return -1;
@@ -969,3 +1018,20 @@ int odb_helper_put_object(struct odb_helper *o,
 	odb_helper_finish(o, &cmd);
 	return 0;
 }
+
+int odb_helper_put_object(struct odb_helper *o,
+			  const void *buf, size_t len,
+			  const char *type, unsigned char *sha1)
+{
+	int res = 1;
+	uint64_t start = getnanotime();
+
+	if (o->type == ODB_HELPER_SCRIPT_CMD)
+		res = put_raw_object_script(o, buf, len, type, sha1);
+	else if (o->type == ODB_HELPER_SUBPROCESS_CMD)
+		res = put_object_process(o, buf, len, type, sha1);
+
+	trace_performance_since(start, "odb_helper_put_object");
+
+	return res;
+}

t/t0550-read-object.sh, t/t0550/read-object

diff --git a/t/t0550-read-object.sh b/t/t0550-read-object.sh
new file mode 100755
index 0000000000..93c8eea3f9
--- /dev/null
+++ b/t/t0550-read-object.sh
@@ -0,0 +1,28 @@
+#!/bin/sh
+
+test_description='tests for long running read-object process'
+
+. ./test-lib.sh
+
+PATH="$PATH:$TEST_DIRECTORY/t0550"
+
+test_expect_success 'setup host repo with a root commit' '
+	test_commit zero &&
+	hash1=$(git ls-tree HEAD | grep zero.t | cut -f1 | cut -d\  -f3)
+'
+
+HELPER="read-object"
+
+test_expect_success 'blobs can be retrieved from the host repo' '
+	git init guest-repo &&
+	(cd guest-repo &&
+	 git config odb.magic.subprocessCommand "$HELPER" &&
+	 git cat-file blob "$hash1" >/dev/null)
+'
+
+test_expect_success 'invalid blobs generate errors' '
+	cd guest-repo &&
+	test_must_fail git cat-file blob "invalid"
+'
+
+test_done
diff --git a/t/t0550/read-object b/t/t0550/read-object
new file mode 100755
index 0000000000..004e9368c9
--- /dev/null
+++ b/t/t0550/read-object
@@ -0,0 +1,68 @@
+#!/usr/bin/perl
+#
+# Example implementation for the Git read-object protocol version 1
+# See Documentation/technical/read-object-protocol.txt
+#
+# Allows you to test the ability for blobs to be pulled from a host git repo
+# "on demand."  Called when git needs a blob it couldn't find locally due to
+# a lazy clone that only cloned the commits and trees.
+#
+# A lazy clone can be simulated via the following commands from the host repo
+# you wish to create a lazy clone of:
+#
+# cd /host_repo
+# git rev-parse HEAD
+# git init /guest_repo
+# git cat-file --batch-check --batch-all-objects | grep -v 'blob' |
+#	cut -d' ' -f1 | git pack-objects /e/guest_repo/.git/objects/pack/noblobs
+# cd /guest_repo
+# git config core.virtualizeobjects true
+# git reset --hard <sha from rev-parse call above>
+#
+# Please note, this sample is a minimal skeleton. No proper error handling 
+# was implemented.
+#
+
+use 5.008;
+use lib (split(/:/, $ENV{GITPERLLIB}));
+use strict;
+use warnings;
+use Git::Packet;
+
+#
+# Point $DIR to the folder where your host git repo is located so we can pull
+# missing objects from it
+#
+my $DIR = "../.git/";
+
+packet_initialize("git-read-object", 1);
+
+my %remote_caps = packet_read_and_check_capabilities("get_direct");
+packet_check_and_write_capabilities(\%remote_caps, "get_direct");
+
+while (1) {
+	my ($res, $command) = packet_txt_read();
+
+	if ( $res == -1 ) {
+		exit 0;
+	}
+
+	$command =~ s/^command=//;
+
+	if ( $command eq "init" ) {
+		packet_bin_read();
+
+		packet_txt_write("status=success");
+		packet_flush();
+	} elsif ( $command eq "get_direct" ) {
+		my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
+		packet_bin_read();
+
+		system ('git --git-dir="' . $DIR . '" cat-file blob ' . $sha1 . ' | GIT_NO_EXTERNAL_ODB=1 git hash-object -w --stdin >/dev/null 2>&1');
+
+		packet_txt_write(($?) ? "status=error" : "status=success");
+		packet_flush();
+	} else {
+		die "bad command '$command'";
+	}
+}

external-odb.c, odb-helper.c, sha1_file.c

diff --git a/external-odb.c b/external-odb.c
index de2ae70db1..5e5871c933 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -106,29 +106,11 @@ const char *external_odb_root(void)
 	return root;
 }
 
-int external_odb_has_object(const unsigned char *sha1)
-{
-	struct odb_helper *o;
-
-	external_odb_init();
-
-	for (o = helpers; o; o = o->next) {
-		if (!(o->supported_capabilities & ODB_HELPER_CAP_HAVE))
-			return 1;
-		if (odb_helper_has_object(o, sha1))
-			return 1;
-	}
-	return 0;
-}
-
-int external_odb_get_object(const unsigned char *sha1)
+static int external_odb_do_get_object(const unsigned char *sha1)
 {
 	struct odb_helper *o;
 	struct strbuf pathbuf = STRBUF_INIT;
 
-	if (!external_odb_has_object(sha1))
-		return -1;
-
 	sha1_file_name_alt(&pathbuf, external_odb_root(), sha1);
 	safe_create_leading_directories_const(pathbuf.buf);
 	prepare_external_alt_odb();
@@ -203,6 +185,32 @@ int external_odb_get_many_direct(const struct oid_array *to_get)
 	return -1;
 }
 
+int external_odb_has_object(const unsigned char *sha1)
+{
+	struct odb_helper *o;
+
+	external_odb_init();
+
+	for (o = helpers; o; o = o->next) {
+		if (!(o->supported_capabilities & ODB_HELPER_CAP_HAVE)) {
+			if (o->supported_capabilities & ODB_HELPER_CAP_GET_DIRECT)
+				return 1;
+			return !external_odb_do_get_object(sha1);
+		}
+		if (odb_helper_has_object(o, sha1))
+			return 1;
+	}
+	return 0;
+}
+
+int external_odb_get_object(const unsigned char *sha1)
+{
+	if (!external_odb_has_object(sha1))
+		return -1;
+
+	return external_odb_do_get_object(sha1);
+}
+
 int external_odb_put_object(const void *buf, size_t len,
 			    const char *type, unsigned char *sha1)
 {
diff --git a/odb-helper.c b/odb-helper.c
index fdd062ec8d..cad3d1005c 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -5,6 +5,22 @@
 #include "run-command.h"
 #include "sha1-lookup.h"
 #include "fetch-object.h"
+#include "sub-process.h"
+#include "pkt-line.h"
+#include "sigchain.h"
+
+struct object_process {
+	struct subprocess_entry subprocess;
+	unsigned int supported_capabilities;
+};
+
+static struct hashmap subprocess_map;
+
+static int check_object_process_status(int fd, struct strbuf *status)
+{
+	subprocess_read_status(fd, status);
+	return strcmp(status->buf, "success");
+}
 
 static void parse_capabilities(char *cap_buf,
 			       unsigned int *supported_capabilities,
@@ -40,6 +56,392 @@ static void parse_capabilities(char *cap_buf,
 	string_list_clear(&cap_list, 0);
 }
 
+static int start_object_process_fn(struct subprocess_entry *subprocess)
+{
+	static int versions[] = {1, 0};
+	static struct subprocess_capability capabilities[] = {
+		{ "get_git_obj", ODB_HELPER_CAP_GET_GIT_OBJ },
+		{ "get_raw_obj", ODB_HELPER_CAP_GET_RAW_OBJ },
+		{ "get_direct",  ODB_HELPER_CAP_GET_DIRECT  },
+		{ "put_git_obj", ODB_HELPER_CAP_PUT_GIT_OBJ },
+		{ "put_raw_obj", ODB_HELPER_CAP_PUT_RAW_OBJ },
+		{ "put_direct",  ODB_HELPER_CAP_PUT_DIRECT  },
+		{ "have",        ODB_HELPER_CAP_HAVE },
+		{ NULL, 0 }
+	};
+	struct object_process *entry = (struct object_process *)subprocess;
+	return subprocess_handshake(subprocess, "git-read-object", versions, NULL,
+				    capabilities,
+				    &entry->supported_capabilities);
+}
+
+static struct object_process *launch_object_process(struct odb_helper *o,
+						    unsigned int capability)
+{
+	struct object_process *entry = NULL;
+
+	if (!subprocess_map.tablesize)
+		hashmap_init(&subprocess_map, (hashmap_cmp_fn) cmd2process_cmp, NULL, 0);
+	else
+		entry = (struct object_process *)subprocess_find_entry(&subprocess_map, o->dealer);
+
+	fflush(NULL);
+
+	if (!entry) {
+		entry = xmalloc(sizeof(*entry));
+		entry->supported_capabilities = 0;
+
+		if (subprocess_start(&subprocess_map, &entry->subprocess, o->dealer, start_object_process_fn)) {
+			error("Could not launch process for cmd '%s'", o->dealer);
+			free(entry);
+			return NULL;
+		}
+	}
+
+	o->supported_capabilities = entry->supported_capabilities;
+
+	if (capability && !(capability & entry->supported_capabilities)) {
+		error("The cmd '%s' does not support capability '%d'", o->dealer, capability);
+		return NULL;
+	}
+
+	sigchain_push(SIGPIPE, SIG_IGN);
+
+	return entry;
+}
+
+static int check_object_process_error(int err,
+				      const char *status,
+				      struct object_process *entry,
+				      const char *cmd,
+				      unsigned int capability)
+{
+	sigchain_pop(SIGPIPE);
+
+	if (!err)
+		return 0;
+
+	if (!strcmp(status, "error")) {
+		/* The process signaled a problem with the file. */
+	} else if (!strcmp(status, "notfound")) {
+		/* Object was not found */
+		err = -1;
+	} else if (!strcmp(status, "abort")) {
+		/*
+		 * The process signaled a permanent problem. Don't try to read
+		 * objects with the same command for the lifetime of the current
+		 * Git process.
+		 */
+		if (capability)
+			entry->supported_capabilities &= ~capability;
+	} else {
+		/*
+		 * Something went wrong with the read-object process.
+		 * Force shutdown and restart if needed.
+		 */
+		error("external object process '%s' failed", cmd);
+		subprocess_stop(&subprocess_map, &entry->subprocess);
+		free(entry);
+	}
+
+	return err;
+}
+
+static int send_init_packets(struct object_process *entry,
+			     struct strbuf *status)
+{
+	struct child_process *process = &entry->subprocess.process;
+
+	return packet_write_fmt_gently(process->in, "command=init\n") ||
+		packet_flush_gently(process->in) ||
+		check_object_process_status(process->out, status);
+}
+
+static int init_object_process(struct odb_helper *o)
+{
+	int err;
+	struct strbuf status = STRBUF_INIT;
+	struct object_process *entry = launch_object_process(o, 0);
+	if (!entry)
+		return -1;
+
+	err = send_init_packets(entry, &status);
+
+	return check_object_process_error(err, status.buf, entry,
+					  o->dealer, 0);
+}
+
+static ssize_t read_packetized_raw_object_to_fd(struct odb_helper *o,
+						const unsigned char *sha1,
+						int fd_in, int fd_out)
+{
+	ssize_t total_read = 0;
+	unsigned long total_got = 0;
+	int packet_len;
+
+	char hdr[32];
+	int hdrlen;
+
+	int ret = Z_STREAM_END;
+	unsigned char compressed[4096];
+	git_zstream stream;
+	git_SHA_CTX hash;
+	unsigned char real_sha1[20];
+
+	off_t size;
+	enum object_type type;
+	const char *s;
+	int pkt_size;
+	char *size_buf;
+
+	size_buf = packet_read_line(fd_in, &pkt_size);
+	if (!skip_prefix(size_buf, "size=", &s))
+		return error("odb helper '%s' did not send size of plain object", o->name);
+	size = strtoumax(s, NULL, 10);
+	if (!skip_prefix(packet_read_line(fd_in, NULL), "kind=", &s))
+		return error("odb helper '%s' did not send kind of plain object", o->name);
+	/* Check if the object is not available */
+	if (!strcmp(s, "none"))
+		return -1;
+	type = type_from_string_gently(s, strlen(s), 1);
+	if (type < 0)
+		return error("odb helper '%s' sent bad type '%s'", o->name, s);
+
+	/* Set it up */
+	git_deflate_init(&stream, zlib_compression_level);
+	stream.next_out = compressed;
+	stream.avail_out = sizeof(compressed);
+	git_SHA1_Init(&hash);
+
+	/* First header.. */
+	hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %lu", type_name(type), size) + 1;
+	stream.next_in = (unsigned char *)hdr;
+	stream.avail_in = hdrlen;
+	while (git_deflate(&stream, 0) == Z_OK)
+		; /* nothing */
+	git_SHA1_Update(&hash, hdr, hdrlen);
+
+	for (;;) {
+		/* packet_read() writes a '\0' extra byte at the end */
+		char buf[LARGE_PACKET_DATA_MAX + 1];
+
+		packet_len = packet_read(fd_in, NULL, NULL,
+			buf, LARGE_PACKET_DATA_MAX + 1,
+			PACKET_READ_GENTLE_ON_EOF);
+
+		if (packet_len <= 0)
+			break;
+
+		total_got += packet_len;
+
+		/* Then the data itself.. */
+		stream.next_in = (void *)buf;
+		stream.avail_in = packet_len;
+		do {
+			unsigned char *in0 = stream.next_in;
+			ret = git_deflate(&stream, Z_FINISH);
+			git_SHA1_Update(&hash, in0, stream.next_in - in0);
+			write_or_die(fd_out, compressed, stream.next_out - compressed);
+			stream.next_out = compressed;
+			stream.avail_out = sizeof(compressed);
+		} while (ret == Z_OK);
+
+		total_read += packet_len;
+	}
+
+	if (packet_len < 0) {
+		error("unable to read from odb helper '%s': %s",
+		      o->name, strerror(errno));
+		git_deflate_end(&stream);
+		return packet_len;
+	}
+
+	if (ret != Z_STREAM_END) {
+		warning("bad zlib data from odb helper '%s' for %s",
+			o->name, sha1_to_hex(sha1));
+		return -1;
+	}
+
+	ret = git_deflate_end_gently(&stream);
+	if (ret != Z_OK) {
+		warning("deflateEnd on object %s from odb helper '%s' failed (%d)",
+			sha1_to_hex(sha1), o->name, ret);
+		return -1;
+	}
+	git_SHA1_Final(real_sha1, &hash);
+	if (hashcmp(sha1, real_sha1)) {
+		warning("sha1 mismatch from odb helper '%s' for %s (got %s)",
+			o->name, sha1_to_hex(sha1), sha1_to_hex(real_sha1));
+		return -1;
+	}
+	if (total_got != size) {
+		warning("size mismatch from odb helper '%s' for %s (%lu != %lu)",
+			o->name, sha1_to_hex(sha1), total_got, size);
+		return -1;
+	}
+
+	return total_read;
+}
+
+static ssize_t read_packetized_git_object_to_fd(struct odb_helper *o,
+						const unsigned char *sha1,
+						int fd_in, int fd_out)
+{
+	ssize_t total_read = 0;
+	unsigned long total_got = 0;
+	int packet_len;
+	git_zstream stream;
+	int zret = Z_STREAM_END;
+	git_SHA_CTX hash;
+	unsigned char real_sha1[20];
+	struct strbuf header = STRBUF_INIT;
+	unsigned long hdr_size;
+
+	memset(&stream, 0, sizeof(stream));
+	git_inflate_init(&stream);
+	git_SHA1_Init(&hash);
+
+	for (;;) {
+		/* packet_read() writes a '\0' extra byte at the end */
+		char buf[LARGE_PACKET_DATA_MAX + 1];
+
+		packet_len = packet_read(fd_in, NULL, NULL,
+			buf, LARGE_PACKET_DATA_MAX + 1,
+			PACKET_READ_GENTLE_ON_EOF);
+
+		if (packet_len <= 0)
+			break;
+
+		write_or_die(fd_out, buf, packet_len);
+
+		stream.next_in = (unsigned char *)buf;
+		stream.avail_in = packet_len;
+		do {
+			unsigned char inflated[4096];
+			unsigned long got;
+
+			stream.next_out = inflated;
+			stream.avail_out = sizeof(inflated);
+			zret = git_inflate(&stream, Z_SYNC_FLUSH);
+			got = sizeof(inflated) - stream.avail_out;
+
+			git_SHA1_Update(&hash, inflated, got);
+			/* skip header when counting size */
+			if (!total_got) {
+				const unsigned char *p = memchr(inflated, '\0', got);
+				if (p) {
+					unsigned long hdr_last = p - inflated + 1;
+					strbuf_add(&header, inflated, hdr_last);
+					got -= hdr_last;
+				} else {
+					strbuf_add(&header, inflated, got);
+					got = 0;
+				}
+			}
+			total_got += got;
+		} while (stream.avail_in && zret == Z_OK);
+
+		total_read += packet_len;
+	}
+
+	git_inflate_end(&stream);
+
+	if (packet_len < 0) {
+		strbuf_release(&header);
+		return packet_len;
+	}
+
+	git_SHA1_Final(real_sha1, &hash);
+
+	if (zret != Z_STREAM_END) {
+		warning("bad zlib data from odb helper '%s' for %s",
+			o->name, sha1_to_hex(sha1));
+		strbuf_release(&header);
+		return -1;
+	}
+	if (hashcmp(real_sha1, sha1)) {
+		warning("sha1 mismatch from odb helper '%s' for %s (got %s)",
+			o->name, sha1_to_hex(sha1), sha1_to_hex(real_sha1));
+		strbuf_release(&header);
+		return -1;
+	}
+	if (parse_sha1_header(header.buf, &hdr_size) < 0) {
+		warning("could not parse header from odb helper '%s' for %s",
+			o->name, sha1_to_hex(sha1));
+		strbuf_release(&header);
+		return -1;
+	}
+	if (total_got != hdr_size) {
+		warning("size mismatch from odb helper '%s' for %s (%lu != %lu)",
+			o->name, sha1_to_hex(sha1), total_got, hdr_size);
+		strbuf_release(&header);
+		return -1;
+	}
+
+	strbuf_release(&header);
+
+	return total_read;
+}
+
+static int send_get_packets(struct odb_helper *o,
+			    struct object_process *entry,
+			    const unsigned char *sha1,
+			    int fd,
+			    unsigned int *cur_cap,
+			    struct strbuf *status)
+{
+	const char *instruction;
+	int err;
+	struct child_process *process = &entry->subprocess.process;
+
+	if (entry->supported_capabilities & ODB_HELPER_CAP_GET_GIT_OBJ) {
+		*cur_cap = ODB_HELPER_CAP_GET_GIT_OBJ;
+		instruction = "get_git_obj";
+	} else if (entry->supported_capabilities & ODB_HELPER_CAP_GET_RAW_OBJ) {
+		*cur_cap = ODB_HELPER_CAP_GET_RAW_OBJ;
+		instruction = "get_raw_obj";
+	} else if (entry->supported_capabilities & ODB_HELPER_CAP_GET_DIRECT) {
+		*cur_cap = ODB_HELPER_CAP_GET_DIRECT;
+		instruction = "get_direct";
+	} else {
+		BUG("No known ODB_HELPER_CAP_GET_XXX capability!");
+	}
+
+	err = packet_write_fmt_gently(process->in, "command=%s\n", instruction);
+	if (err)
+		return err;
+
+	err = packet_write_fmt_gently(process->in, "sha1=%s\n", sha1_to_hex(sha1));
+	if (err)
+		return err;
+
+	err = packet_flush_gently(process->in);
+	if (err)
+		return err;
+
+	if (entry->supported_capabilities & ODB_HELPER_CAP_GET_RAW_OBJ)
+		err = read_packetized_raw_object_to_fd(o, sha1, process->out, fd) < 0;
+	else if (entry->supported_capabilities & ODB_HELPER_CAP_GET_GIT_OBJ)
+		err = read_packetized_git_object_to_fd(o, sha1, process->out, fd) < 0;
+
+	return check_object_process_status(process->out, status);
+}
+
+static int get_object_process(struct odb_helper *o, const unsigned char *sha1, int fd)
+{
+	int err;
+	struct strbuf status = STRBUF_INIT;
+	unsigned int cur_cap = 0;
+	struct object_process *entry = launch_object_process(o, 0);
+	if (!entry)
+		return -1;
+
+	err = send_get_packets(o, entry, sha1, fd, &cur_cap, &status);
+
+	return check_object_process_error(err, status.buf, entry,
+					  o->dealer, cur_cap);
+}
+
 struct odb_helper *odb_helper_new(const char *name, int namelen)
 {
 	struct odb_helper *o;
@@ -118,19 +520,12 @@ static int odb_helper_finish(struct odb_helper *o,
 	return 0;
 }
 
-int odb_helper_init(struct odb_helper *o)
+static int init_object_script(struct odb_helper *o)
 {
 	struct odb_helper_cmd cmd;
 	FILE *fh;
 	struct strbuf line = STRBUF_INIT;
 
-	if (o->initialized)
-		return 0;
-	o->initialized = 1;
-
-	if (o->type != ODB_HELPER_SCRIPT_CMD)
-		return 0;
-
 	if (odb_helper_start(o, &cmd, 0, "init") < 0)
 		return -1;
 
@@ -145,6 +540,27 @@ int odb_helper_init(struct odb_helper *o)
 	return 0;
 }
 
+int odb_helper_init(struct odb_helper *o)
+{
+	int res = 0;
+	uint64_t start;
+
+	if (o->initialized)
+		return 0;
+	o->initialized = 1;
+
+	start = getnanotime();
+
+	if (o->type == ODB_HELPER_SCRIPT_CMD)
+		res = init_object_script(o);
+	else if (o->type == ODB_HELPER_SUBPROCESS_CMD)
+		res = init_object_process(o);
+
+	trace_performance_since(start, "odb_helper_init");
+
+	return res;
+}
+
 static int parse_object_line(struct odb_helper_object *o, const char *line)
 {
 	char *end;
@@ -439,31 +855,42 @@ static int odb_helper_get_git_object(struct odb_helper *o,
 	return 0;
 }
 
+static int get_direct_script(struct odb_helper *o, const unsigned char *sha1)
+{
+	struct odb_helper_cmd cmd;
+
+	if (odb_helper_start(o, &cmd, 0, "get_direct %s", sha1_to_hex(sha1)) < 0)
+		return -1;
+	if (odb_helper_finish(o, &cmd))
+		return -1;
+	return 0;
+}
+
 int odb_helper_get_direct(struct odb_helper *o,
 			  const unsigned char *sha1)
 {
-	uint64_t start = getnanotime();
-
-	if (o->type == ODB_HELPER_GIT_REMOTE) {
-		fetch_object(o->dealer, sha1);
-	} else {
-		struct odb_helper_object *obj;
-		struct odb_helper_cmd cmd;
+	int res = 0;
+	uint64_t start;
 
-		obj = odb_helper_lookup(o, sha1);
+	if (o->supported_capabilities & ODB_HELPER_CAP_HAVE &&
+	    o->type != ODB_HELPER_GIT_REMOTE) {
+		struct odb_helper_object *obj = odb_helper_lookup(o, sha1);
 		if (!obj)
 			return -1;
+	}
 
-		if (odb_helper_start(o, &cmd, 0, "get_direct %s", sha1_to_hex(sha1)) < 0)
-			return -1;
+	start = getnanotime();
 
-		if (odb_helper_finish(o, &cmd))
-			return -1;
-	}
+	if (o->type == ODB_HELPER_GIT_REMOTE)
+		fetch_object(o->dealer, sha1);
+	else if (o->type == ODB_HELPER_SCRIPT_CMD)
+		res = get_direct_script(o, sha1);
+	else
+		res = get_object_process(o, sha1, -1);
 
 	trace_performance_since(start, "odb_helper_get_direct");
 
-	return 0;
+	return res;
 }
 
 int odb_helper_get_many_direct(struct odb_helper *o,
@@ -482,14 +909,33 @@ int odb_helper_get_many_direct(struct odb_helper *o,
 	return res;
 }
 
+static int get_object_script(struct odb_helper *o, const unsigned char *sha1, int fd)
+{
+	if (o->supported_capabilities & ODB_HELPER_CAP_GET_GIT_OBJ)
+		return odb_helper_get_git_object(o, sha1, fd);
+	if (o->supported_capabilities & ODB_HELPER_CAP_GET_RAW_OBJ)
+		return odb_helper_get_raw_object(o, sha1, fd);
+	if (o->supported_capabilities & ODB_HELPER_CAP_GET_DIRECT)
+		return 0;
+
+	BUG("invalid get capability (capabilities: '%d')", o->supported_capabilities);
+}
+
 int odb_helper_get_object(struct odb_helper *o,
 			  const unsigned char *sha1,
 			  int fd)
 {
-	if (o->supported_capabilities & ODB_HELPER_CAP_GET_RAW_OBJ)
-		return odb_helper_get_raw_object(o, sha1, fd);
-	else
-		return odb_helper_get_git_object(o, sha1, fd);
+	int res = 0;
+	uint64_t start = getnanotime();
+
+	if (o->type == ODB_HELPER_SCRIPT_CMD)
+		res = get_object_script(o, sha1, fd);
+	else if (o->type == ODB_HELPER_SUBPROCESS_CMD)
+		res = get_object_process(o, sha1, fd);
+
+	trace_performance_since(start, "odb_helper_get_object");
+
+	return res;
 }
 
 int odb_helper_put_object(struct odb_helper *o,
diff --git a/sha1_file.c b/sha1_file.c
index 64679fae36..9ccdae35bc 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -730,11 +730,20 @@ int check_and_freshen_file(const char *fn, int freshen)
 static int check_and_freshen_local(const unsigned char *sha1, int freshen)
 {
 	static struct strbuf buf = STRBUF_INIT;
+	int ret;
+	int tried_hook = 0;
 
 	strbuf_reset(&buf);
 	sha1_file_name(&buf, sha1);
 
-	return check_and_freshen_file(buf.buf, freshen);
+retry:
+	ret = check_and_freshen_file(buf.buf, freshen);
+	if (!ret && !tried_hook) {
+		tried_hook = 1;
+		if (!external_odb_get_direct(sha1))
+			goto retry;
+	}
+	return ret;
 }
 
 static int check_and_freshen_nonlocal(const unsigned char *sha1, int freshen)
@@ -746,7 +755,7 @@ static int check_and_freshen_nonlocal(const unsigned char *sha1, int freshen)
 		if (check_and_freshen_file(path, freshen))
 			return 1;
 	}
-	return 0;
+	return external_odb_has_object(sha1);
 }
 
 static int check_and_freshen(const unsigned char *sha1, int freshen)

diff --git a/odb-helper.c b/odb-helper.c
index 2734af793b..fdd062ec8d 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -235,8 +235,107 @@ int odb_helper_has_object(struct odb_helper *o, const unsigned char *sha1)
 	return !!odb_helper_lookup(o, sha1);
 }
 
-int odb_helper_get_object(struct odb_helper *o, const unsigned char *sha1,
-			    int fd)
+static int odb_helper_get_raw_object(struct odb_helper *o,
+				     const unsigned char *sha1,
+				     int fd)
+{
+	struct odb_helper_object *obj;
+	struct odb_helper_cmd cmd;
+	unsigned long total_got = 0;
+
+	char hdr[32];
+	int hdrlen;
+
+	int ret = Z_STREAM_END;
+	unsigned char compressed[4096];
+	git_zstream stream;
+	git_SHA_CTX hash;
+	unsigned char real_sha1[20];
+
+	obj = odb_helper_lookup(o, sha1);
+	if (!obj)
+		return -1;
+
+	if (odb_helper_start(o, &cmd, 0, "get_raw_obj %s", sha1_to_hex(sha1)) < 0)
+		return -1;
+
+	/* Set it up */
+	git_deflate_init(&stream, zlib_compression_level);
+	stream.next_out = compressed;
+	stream.avail_out = sizeof(compressed);
+	git_SHA1_Init(&hash);
+
+	/* First header.. */
+	hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %lu", type_name(obj->type), obj->size) + 1;
+	stream.next_in = (unsigned char *)hdr;
+	stream.avail_in = hdrlen;
+	while (git_deflate(&stream, 0) == Z_OK)
+		; /* nothing */
+	git_SHA1_Update(&hash, hdr, hdrlen);
+
+	for (;;) {
+		unsigned char buf[4096];
+		int r;
+
+		r = xread(cmd.child.out, buf, sizeof(buf));
+		if (r < 0) {
+			error("unable to read from odb helper '%s': %s",
+			      o->name, strerror(errno));
+			close(cmd.child.out);
+			odb_helper_finish(o, &cmd);
+			git_deflate_end(&stream);
+			return -1;
+		}
+		if (r == 0)
+			break;
+
+		total_got += r;
+
+		/* Then the data itself.. */
+		stream.next_in = (void *)buf;
+		stream.avail_in = r;
+		do {
+			unsigned char *in0 = stream.next_in;
+			ret = git_deflate(&stream, Z_FINISH);
+			git_SHA1_Update(&hash, in0, stream.next_in - in0);
+			write_or_die(fd, compressed, stream.next_out - compressed);
+			stream.next_out = compressed;
+			stream.avail_out = sizeof(compressed);
+		} while (ret == Z_OK);
+	}
+
+	close(cmd.child.out);
+	if (ret != Z_STREAM_END) {
+		warning("bad zlib data from odb helper '%s' for %s",
+			o->name, sha1_to_hex(sha1));
+		return -1;
+	}
+	ret = git_deflate_end_gently(&stream);
+	if (ret != Z_OK) {
+		warning("deflateEnd on object %s from odb helper '%s' failed (%d)",
+			sha1_to_hex(sha1), o->name, ret);
+		return -1;
+	}
+	git_SHA1_Final(real_sha1, &hash);
+	if (hashcmp(sha1, real_sha1)) {
+		warning("sha1 mismatch from odb helper '%s' for %s (got %s)",
+			o->name, sha1_to_hex(sha1), sha1_to_hex(real_sha1));
+		return -1;
+	}
+	if (odb_helper_finish(o, &cmd))
+		return -1;
+	if (total_got != obj->size) {
+		warning("size mismatch from odb helper '%s' for %s (%lu != %lu)",
+			o->name, sha1_to_hex(sha1), total_got, obj->size);
+		return -1;
+	}
+
+	return 0;
+}
+
+static int odb_helper_get_git_object(struct odb_helper *o,
+				     const unsigned char *sha1,
+				     int fd)
 {
 	struct odb_helper_object *obj;
 	struct odb_helper_cmd cmd;
@@ -383,6 +482,16 @@ int odb_helper_get_many_direct(struct odb_helper *o,
 	return res;
 }
 
+int odb_helper_get_object(struct odb_helper *o,
+			  const unsigned char *sha1,
+			  int fd)
+{
+	if (o->supported_capabilities & ODB_HELPER_CAP_GET_RAW_OBJ)
+		return odb_helper_get_raw_object(o, sha1, fd);
+	else
+		return odb_helper_get_git_object(o, sha1, fd);
+}
+
 int odb_helper_put_object(struct odb_helper *o,
 			  const void *buf, size_t len,
 			  const char *type, unsigned char *sha1)

diff --git a/t/t0510-transfer-e-odb.sh b/t/t0510-transfer-e-odb.sh
new file mode 100755
index 0000000000..065ec7d759
--- /dev/null
+++ b/t/t0510-transfer-e-odb.sh
@@ -0,0 +1,144 @@
+#!/bin/sh
+
+test_description='basic tests for transfering external ODBs'
+
+. ./test-lib.sh
+
+ORIG_SOURCE="$PWD/.git"
+export ORIG_SOURCE
+
+ALT_SOURCE1="$PWD/alt-repo1/.git"
+export ALT_SOURCE1
+write_script odb-helper1 <<\EOF
+die() {
+	printf >&2 "%s\n" "$@"
+	exit 1
+}
+GIT_DIR=$ALT_SOURCE1; export GIT_DIR
+case "$1" in
+init)
+	echo "capability=get_git_obj"
+	echo "capability=have"
+	;;
+have)
+	git cat-file --batch-check --batch-all-objects |
+	awk '{print $1 " " $3 " " $2}'
+	;;
+get_git_obj)
+	cat "$GIT_DIR"/objects/$(echo $2 | sed 's#..#&/#')
+	;;
+put_raw_obj)
+	sha1="$2"
+	size="$3"
+	kind="$4"
+	writen=$(git hash-object -w -t "$kind" --stdin)
+	test "$writen" = "$sha1" || die "bad sha1 passed '$sha1' vs writen '$writen'"
+	ref_hash=$(echo "$sha1 $size $kind" | GIT_DIR=$ORIG_SOURCE GIT_NO_EXTERNAL_ODB=1 git hash-object -w -t blob --stdin) || exit
+	GIT_DIR=$ORIG_SOURCE git update-ref refs/odbs/magic/"$sha1" "$ref_hash"
+	;;
+*)
+	die "unknown command '$1'"
+	;;
+esac
+EOF
+HELPER1="\"$PWD\"/odb-helper1"
+
+OTHER_SOURCE="$PWD/.git"
+export OTHER_SOURCE
+
+ALT_SOURCE2="$PWD/alt-repo2/.git"
+export ALT_SOURCE2
+write_script odb-helper2 <<\EOF
+die() {
+	printf >&2 "%s\n" "$@"
+	exit 1
+}
+GIT_DIR=$ALT_SOURCE2; export GIT_DIR
+case "$1" in
+init)
+	echo "capability=get_git_obj"
+	echo "capability=have"
+	;;
+have)
+	GIT_DIR=$OTHER_SOURCE git for-each-ref --format='%(objectname)' refs/odbs/magic/ | GIT_DIR=$OTHER_SOURCE xargs git show
+	;;
+get_git_obj)
+	OBJ_FILE="$GIT_DIR"/objects/$(echo $2 | sed 's#..#&/#')
+	if ! test -f "$OBJ_FILE"
+	then
+		# "Download" the missing object by copying it from alt-repo1
+		OBJ_DIR=$(echo $2 | sed 's/\(..\).*/\1/')
+		OBJ_BASE=$(basename "$OBJ_FILE")
+		ALT_OBJ_DIR1="$ALT_SOURCE1/objects/$OBJ_DIR"
+		ALT_OBJ_DIR2="$ALT_SOURCE2/objects/$OBJ_DIR"
+		mkdir -p "$ALT_OBJ_DIR2" || die "Could not mkdir '$ALT_OBJ_DIR2'"
+		OBJ_SRC="$ALT_OBJ_DIR1/$OBJ_BASE"
+		cp "$OBJ_SRC" "$ALT_OBJ_DIR2" ||
+		die "Could not cp '$OBJ_SRC' into '$ALT_OBJ_DIR2'"
+	fi
+	cat "$OBJ_FILE" || die "Could not cat '$OBJ_FILE'"
+	;;
+put_raw_obj)
+	sha1="$2"
+	size="$3"
+	kind="$4"
+	writen=$(git hash-object -w -t "$kind" --stdin)
+	test "$writen" = "$sha1" || die "bad sha1 passed '$sha1' vs writen '$writen'"
+	ref_hash=$(echo "$sha1 $size $kind" | GIT_DIR=$OTHER_SOURCE GIT_NO_EXTERNAL_ODB=1 git hash-object -w -t blob --stdin) || exit
+	GIT_DIR=$OTHER_SOURCE git update-ref refs/odbs/magic/"$sha1" "$ref_hash"
+	;;
+*)
+	die "unknown command '$1'"
+	;;
+esac
+EOF
+HELPER2="\"$PWD\"/odb-helper2"
+
+test_expect_success 'setup first alternate repo' '
+	git init alt-repo1 &&
+	test_commit zero &&
+	git config odb.magic.scriptCommand "$HELPER1"
+'
+
+test_expect_success 'setup other repo and its alternate repo' '
+	git init other-repo &&
+	git init alt-repo2 &&
+	(cd other-repo &&
+	 git remote add origin .. &&
+	 git pull origin master &&
+	 git checkout master &&
+	 git log)
+'
+
+test_expect_success 'new blobs are put in first object store' '
+	test_commit one &&
+	hash1=$(git ls-tree HEAD | grep one.t | cut -f1 | cut -d\  -f3) &&
+	content=$(cd alt-repo1 && git show "$hash1") &&
+	test "$content" = "one" &&
+	test_commit two &&
+	hash2=$(git ls-tree HEAD | grep two.t | cut -f1 | cut -d\  -f3) &&
+	content=$(cd alt-repo1 && git show "$hash2") &&
+	test "$content" = "two"
+'
+
+test_expect_success 'other repo gets the blobs from object store' '
+	(cd other-repo &&
+	 git fetch origin "refs/odbs/magic/*:refs/odbs/magic/*" &&
+	 test_must_fail git cat-file blob "$hash1" &&
+	 test_must_fail git cat-file blob "$hash2" &&
+	 git config odb.magic.scriptCommand "$HELPER2" &&
+	 git cat-file blob "$hash1" &&
+	 git cat-file blob "$hash2"
+	)
+'
+
+test_expect_success 'other repo gets everything else' '
+	(cd other-repo &&
+	 git fetch origin &&
+	 content=$(git show "$hash1") &&
+	 test "$content" = "one" &&
+	 content=$(git show "$hash2") &&
+	 test "$content" = "two")
+'
+
+test_done

diff --git a/t/t0500-external-odb.sh b/t/t0500-external-odb.sh
index 4ccca1e965..f924de870f 100755
--- a/t/t0500-external-odb.sh
+++ b/t/t0500-external-odb.sh
@@ -69,4 +69,12 @@ test_expect_success 'helper can add objects to alt repo' '
 	test "$size" -eq "$alt_size"
 '
 
+test_expect_success 'commit adds objects to alt repo' '
+	test_config odb.magic.scriptCommand "$HELPER" &&
+	test_commit three &&
+	hash3=$(git ls-tree HEAD | grep three.t | cut -f1 | cut -d\  -f3) &&
+	content=$(git -C alt-repo show "$hash3") &&
+	test "$content" = "three"
+'
+
 test_done

diff --git a/t/lib-httpd/apache-e-odb.conf b/t/lib-httpd/apache-e-odb.conf
new file mode 100644
index 0000000000..19a1540c82
--- /dev/null
+++ b/t/lib-httpd/apache-e-odb.conf
@@ -0,0 +1,214 @@
+ServerName dummy
+PidFile httpd.pid
+DocumentRoot www
+LogFormat "%h %l %u %t \"%r\" %>s %b" common
+CustomLog access.log common
+ErrorLog error.log
+<IfModule !mod_log_config.c>
+	LoadModule log_config_module modules/mod_log_config.so
+</IfModule>
+<IfModule !mod_alias.c>
+	LoadModule alias_module modules/mod_alias.so
+</IfModule>
+<IfModule !mod_cgi.c>
+	LoadModule cgi_module modules/mod_cgi.so
+</IfModule>
+<IfModule !mod_env.c>
+	LoadModule env_module modules/mod_env.so
+</IfModule>
+<IfModule !mod_rewrite.c>
+	LoadModule rewrite_module modules/mod_rewrite.so
+</IFModule>
+<IfModule !mod_version.c>
+	LoadModule version_module modules/mod_version.so
+</IfModule>
+<IfModule !mod_headers.c>
+	LoadModule headers_module modules/mod_headers.so
+</IfModule>
+
+<IfVersion < 2.4>
+LockFile accept.lock
+</IfVersion>
+
+<IfVersion < 2.1>
+<IfModule !mod_auth.c>
+	LoadModule auth_module modules/mod_auth.so
+</IfModule>
+</IfVersion>
+
+<IfVersion >= 2.1>
+<IfModule !mod_auth_basic.c>
+	LoadModule auth_basic_module modules/mod_auth_basic.so
+</IfModule>
+<IfModule !mod_authn_file.c>
+	LoadModule authn_file_module modules/mod_authn_file.so
+</IfModule>
+<IfModule !mod_authz_user.c>
+	LoadModule authz_user_module modules/mod_authz_user.so
+</IfModule>
+<IfModule !mod_authz_host.c>
+	LoadModule authz_host_module modules/mod_authz_host.so
+</IfModule>
+</IfVersion>
+
+<IfVersion >= 2.4>
+<IfModule !mod_authn_core.c>
+	LoadModule authn_core_module modules/mod_authn_core.so
+</IfModule>
+<IfModule !mod_authz_core.c>
+	LoadModule authz_core_module modules/mod_authz_core.so
+</IfModule>
+<IfModule !mod_access_compat.c>
+	LoadModule access_compat_module modules/mod_access_compat.so
+</IfModule>
+<IfModule !mod_mpm_prefork.c>
+	LoadModule mpm_prefork_module modules/mod_mpm_prefork.so
+</IfModule>
+<IfModule !mod_unixd.c>
+	LoadModule unixd_module modules/mod_unixd.so
+</IfModule>
+</IfVersion>
+
+PassEnv GIT_VALGRIND
+PassEnv GIT_VALGRIND_OPTIONS
+PassEnv GNUPGHOME
+PassEnv ASAN_OPTIONS
+PassEnv GIT_TRACE
+PassEnv GIT_CONFIG_NOSYSTEM
+
+Alias /dumb/ www/
+Alias /auth/dumb/ www/auth/dumb/
+
+<LocationMatch /smart/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+	SetEnv GIT_HTTP_EXPORT_ALL
+</LocationMatch>
+<LocationMatch /smart_noexport/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+</LocationMatch>
+<LocationMatch /smart_custom_env/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+	SetEnv GIT_HTTP_EXPORT_ALL
+	SetEnv GIT_COMMITTER_NAME "Custom User"
+	SetEnv GIT_COMMITTER_EMAIL custom@example.com
+</LocationMatch>
+<LocationMatch /smart_namespace/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+	SetEnv GIT_HTTP_EXPORT_ALL
+	SetEnv GIT_NAMESPACE ns
+</LocationMatch>
+<LocationMatch /smart_cookies/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+	SetEnv GIT_HTTP_EXPORT_ALL
+	Header set Set-Cookie name=value
+</LocationMatch>
+<LocationMatch /smart_headers/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+	SetEnv GIT_HTTP_EXPORT_ALL
+</LocationMatch>
+ScriptAlias /upload/ upload.sh/
+ScriptAlias /list/ list.sh/
+<Directory ${GIT_EXEC_PATH}>
+	Options FollowSymlinks
+</Directory>
+<Files upload.sh>
+  Options ExecCGI
+</Files>
+<Files list.sh>
+  Options ExecCGI
+</Files>
+<Files ${GIT_EXEC_PATH}/git-http-backend>
+	Options ExecCGI
+</Files>
+
+RewriteEngine on
+RewriteRule ^/smart-redir-perm/(.*)$ /smart/$1 [R=301]
+RewriteRule ^/smart-redir-temp/(.*)$ /smart/$1 [R=302]
+RewriteRule ^/smart-redir-auth/(.*)$ /auth/smart/$1 [R=301]
+RewriteRule ^/smart-redir-limited/(.*)/info/refs$ /smart/$1/info/refs [R=301]
+RewriteRule ^/ftp-redir/(.*)$ ftp://localhost:1000/$1 [R=302]
+
+RewriteRule ^/loop-redir/x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-(.*) /$1 [R=302]
+RewriteRule ^/loop-redir/(.*)$ /loop-redir/x-$1 [R=302]
+
+# Apache 2.2 does not understand <RequireAll>, so we use RewriteCond.
+# And as RewriteCond does not allow testing for non-matches, we match
+# the desired case first (one has abra, two has cadabra), and let it
+# pass by marking the RewriteRule as [L], "last rule, do not process
+# any other matching RewriteRules after this"), and then have another
+# RewriteRule that matches all other cases and lets them fail via '[F]',
+# "fail the request".
+RewriteCond %{HTTP:x-magic-one} =abra
+RewriteCond %{HTTP:x-magic-two} =cadabra
+RewriteRule ^/smart_headers/.* - [L]
+RewriteRule ^/smart_headers/.* - [F]
+
+<IfDefine SSL>
+LoadModule ssl_module modules/mod_ssl.so
+
+SSLCertificateFile httpd.pem
+SSLCertificateKeyFile httpd.pem
+SSLRandomSeed startup file:/dev/urandom 512
+SSLRandomSeed connect file:/dev/urandom 512
+SSLSessionCache none
+SSLMutex file:ssl_mutex
+SSLEngine On
+</IfDefine>
+
+<Location /auth/>
+	AuthType Basic
+	AuthName "git-auth"
+	AuthUserFile passwd
+	Require valid-user
+</Location>
+
+<LocationMatch "^/auth-push/.*/git-receive-pack$">
+	AuthType Basic
+	AuthName "git-auth"
+	AuthUserFile passwd
+	Require valid-user
+</LocationMatch>
+
+<LocationMatch "^/auth-fetch/.*/git-upload-pack$">
+	AuthType Basic
+	AuthName "git-auth"
+	AuthUserFile passwd
+	Require valid-user
+</LocationMatch>
+
+RewriteCond %{QUERY_STRING} service=git-receive-pack [OR]
+RewriteCond %{REQUEST_URI} /git-receive-pack$
+RewriteRule ^/half-auth-complete/ - [E=AUTHREQUIRED:yes]
+
+<Location /half-auth-complete/>
+  Order Deny,Allow
+  Deny from env=AUTHREQUIRED
+
+  AuthType Basic
+  AuthName "Git Access"
+  AuthUserFile passwd
+  Require valid-user
+  Satisfy Any
+</Location>
+
+<IfDefine DAV>
+	LoadModule dav_module modules/mod_dav.so
+	LoadModule dav_fs_module modules/mod_dav_fs.so
+
+	DAVLockDB DAVLock
+	<Location /dumb/>
+		Dav on
+	</Location>
+	<Location /auth/dumb>
+		Dav on
+	</Location>
+</IfDefine>
+
+<IfDefine SVN>
+	LoadModule dav_svn_module modules/mod_dav_svn.so
+
+	<Location /${LIB_HTTPD_SVN}>
+		DAV svn
+		SVNPath "${LIB_HTTPD_SVNPATH}"
+	</Location>
+</IfDefine>

t/lib-httpd.sh, t/lib-httpd/list.sh

diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index d80b004549..f31ea261f5 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -133,6 +133,7 @@ prepare_httpd() {
 	install_script broken-smart-http.sh
 	install_script error.sh
 	install_script upload.sh
+	install_script list.sh
 
 	ln -s "$LIB_HTTPD_MODULE_PATH" "$HTTPD_ROOT_PATH/modules"
 
diff --git a/t/lib-httpd/list.sh b/t/lib-httpd/list.sh
new file mode 100644
index 0000000000..b6d6c29a2f
--- /dev/null
+++ b/t/lib-httpd/list.sh
@@ -0,0 +1,41 @@
+#!/bin/sh
+
+FILES_DIR="www/files"
+
+OLDIFS="$IFS"
+IFS='&'
+set -- $QUERY_STRING
+IFS="$OLDIFS"
+
+while test $# -gt 0
+do
+	key=${1%%=*}
+	val=${1#*=}
+
+	case "$key" in
+	"sha1") sha1="$val" ;;
+	*) echo >&2 "unknown key '$key'" ;;
+	esac
+
+	shift
+done
+
+if test -d "$FILES_DIR"
+then
+	if test -z "$sha1"
+	then
+		echo 'Status: 200 OK'
+		echo
+		ls "$FILES_DIR" | tr '-' ' '
+	else
+		if test -f "$FILES_DIR/$sha1"-*
+		then
+			echo 'Status: 200 OK'
+			echo
+			cat "$FILES_DIR/$sha1"-*
+		else
+			echo 'Status: 404 Not Found'
+			echo
+		fi
+	fi
+fi

external-odb.c, odb-helper.c

diff --git a/external-odb.c b/external-odb.c
index ec1ec82dd9..9f797d66f4 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -41,6 +41,10 @@ static int external_odb_config(const char *var, const char *value, void *data)
 		o->type = ODB_HELPER_SCRIPT_CMD;
 		return git_config_string(&o->dealer, var, value);
 	}
+	if (!strcmp(subkey, "subprocesscommand")) {
+		o->type = ODB_HELPER_SUBPROCESS_CMD;
+		return git_config_string(&o->dealer, var, value);
+	}
 
 	return 0;
 }
diff --git a/odb-helper.c b/odb-helper.c
index 677ea850a5..bf3c1ad8ac 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -116,16 +116,12 @@ static int odb_helper_object_cmp(const void *va, const void *vb)
 	return oidcmp(&a->oid, &b->oid);
 }
 
-static void odb_helper_load_have(struct odb_helper *o)
+static void have_object_script(struct odb_helper *o)
 {
 	struct odb_helper_cmd cmd;
 	FILE *fh;
 	struct strbuf line = STRBUF_INIT;
 
-	if (o->have_valid)
-		return;
-	o->have_valid = 1;
-
 	if (odb_helper_start(o, &cmd, "have") < 0)
 		return;
 
@@ -137,6 +133,16 @@ static void odb_helper_load_have(struct odb_helper *o)
 	strbuf_release(&line);
 	fclose(fh);
 	odb_helper_finish(o, &cmd);
+}
+
+static void odb_helper_load_have(struct odb_helper *o)
+{
+	if (o->have_valid)
+		return;
+	o->have_valid = 1;
+
+	if (o->type == ODB_HELPER_SCRIPT_CMD)
+		have_object_script(o);
 
 	qsort(o->have, o->have_nr, sizeof(*o->have), odb_helper_object_cmp);
 }

external-odb.c, external-odb.h, odb-helper.c, odb-helper.h, sha1_file.c, t/t0500-external-odb.sh

diff --git a/external-odb.c b/external-odb.c
index 82ac80aa04..ec1ec82dd9 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -33,8 +33,14 @@ static int external_odb_config(const char *var, const char *value, void *data)
 
 	o = find_or_create_helper(name, namelen);
 
-	if (!strcmp(subkey, "promisorremote"))
+	if (!strcmp(subkey, "promisorremote")) {
+		o->type = ODB_HELPER_GIT_REMOTE;
 		return git_config_string(&o->dealer, var, value);
+	}
+	if (!strcmp(subkey, "scriptcommand")) {
+		o->type = ODB_HELPER_SCRIPT_CMD;
+		return git_config_string(&o->dealer, var, value);
+	}
 
 	return 0;
 }
@@ -100,6 +106,53 @@ int external_odb_has_object(const unsigned char *sha1)
 	return 0;
 }
 
+int external_odb_get_object(const unsigned char *sha1)
+{
+	struct odb_helper *o;
+	struct strbuf pathbuf = STRBUF_INIT;
+
+	if (!external_odb_has_object(sha1))
+		return -1;
+
+	sha1_file_name_alt(&pathbuf, external_odb_root(), sha1);
+	safe_create_leading_directories_const(pathbuf.buf);
+	prepare_external_alt_odb();
+
+	for (o = helpers; o; o = o->next) {
+		struct strbuf tmpfile = STRBUF_INIT;
+		int ret;
+		int fd;
+
+		if (!odb_helper_has_object(o, sha1))
+			continue;
+
+		fd = create_object_tmpfile(&tmpfile, pathbuf.buf);
+		if (fd < 0) {
+			strbuf_release(&tmpfile);
+			strbuf_release(&pathbuf);
+			return -1;
+		}
+
+		if (odb_helper_get_object(o, sha1, fd) < 0) {
+			close(fd);
+			unlink(tmpfile.buf);
+			strbuf_release(&tmpfile);
+			continue;
+		}
+
+		close_sha1_file(fd);
+		ret = finalize_object_file(tmpfile.buf, pathbuf.buf);
+		strbuf_release(&tmpfile);
+		strbuf_release(&pathbuf);
+		if (!ret)
+			return 0;
+	}
+
+	strbuf_release(&pathbuf);
+
+	return -1;
+}
+
 int external_odb_get_direct(const unsigned char *sha1)
 {
 	struct odb_helper *o;
diff --git a/external-odb.h b/external-odb.h
index 254070994a..c5ac071995 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -15,6 +15,7 @@ extern struct odb_helper *find_odb_helper(const char *dealer,
 					  enum odb_helper_type type);
 extern const char *external_odb_root(void);
 extern int external_odb_has_object(const unsigned char *sha1);
+extern int external_odb_get_object(const unsigned char *sha1);
 extern int external_odb_get_direct(const unsigned char *sha1);
 
 #endif /* EXTERNAL_ODB_H */
diff --git a/odb-helper.c b/odb-helper.c
index c6127ce81f..677ea850a5 100644
--- a/odb-helper.c
+++ b/odb-helper.c
@@ -21,13 +21,124 @@ struct odb_helper_cmd {
 	struct child_process child;
 };
 
+/*
+ * Callers are responsible to ensure that the result of vaddf(fmt, ap)
+ * is properly shell-quoted.
+ */
+static void prepare_helper_command(struct argv_array *argv, const char *cmd,
+				   const char *fmt, va_list ap)
+{
+	struct strbuf buf = STRBUF_INIT;
+
+	strbuf_addstr(&buf, cmd);
+	strbuf_addch(&buf, ' ');
+	strbuf_vaddf(&buf, fmt, ap);
+
+	argv_array_push(argv, buf.buf);
+	strbuf_release(&buf);
+}
+
+__attribute__((format (printf,3,4)))
+static int odb_helper_start(struct odb_helper *o,
+			    struct odb_helper_cmd *cmd,
+			    const char *fmt, ...)
+{
+	va_list ap;
+
+	memset(cmd, 0, sizeof(*cmd));
+	argv_array_init(&cmd->argv);
+
+	if (!o->dealer)
+		return -1;
+
+	va_start(ap, fmt);
+	prepare_helper_command(&cmd->argv, o->dealer, fmt, ap);
+	va_end(ap);
+
+	cmd->child.argv = cmd->argv.argv;
+	cmd->child.use_shell = 1;
+	cmd->child.no_stdin = 1;
+	cmd->child.out = -1;
+
+	if (start_command(&cmd->child) < 0) {
+		argv_array_clear(&cmd->argv);
+		return -1;
+	}
+
+	return 0;
+}
+
+static int odb_helper_finish(struct odb_helper *o,
+			     struct odb_helper_cmd *cmd)
+{
+	int ret = finish_command(&cmd->child);
+	argv_array_clear(&cmd->argv);
+	if (ret) {
+		warning("odb helper '%s' reported failure", o->name);
+		return -1;
+	}
+	return 0;
+}
+
+static int parse_object_line(struct odb_helper_object *o, const char *line)
+{
+	char *end;
+	if (get_oid_hex(line, &o->oid) < 0)
+		return -1;
+
+	line += 40;
+	if (*line++ != ' ')
+		return -1;
+
+	o->size = strtoul(line, &end, 10);
+	if (line == end || *end++ != ' ')
+		return -1;
+
+	o->type = type_from_string(end);
+	return 0;
+}
+
+static int add_have_entry(struct odb_helper *o, const char *line)
+{
+	ALLOC_GROW(o->have, o->have_nr+1, o->have_alloc);
+	if (parse_object_line(&o->have[o->have_nr], line) < 0) {
+		warning("bad 'have' input from odb helper '%s': %s",
+			o->name, line);
+		return 1;
+	}
+	o->have_nr++;
+	return 0;
+}
+
+static int odb_helper_object_cmp(const void *va, const void *vb)
+{
+	const struct odb_helper_object *a = va, *b = vb;
+	return oidcmp(&a->oid, &b->oid);
+}
+
 static void odb_helper_load_have(struct odb_helper *o)
 {
+	struct odb_helper_cmd cmd;
+	FILE *fh;
+	struct strbuf line = STRBUF_INIT;
+
 	if (o->have_valid)
 		return;
 	o->have_valid = 1;
 
-	/* TODO */
+	if (odb_helper_start(o, &cmd, "have") < 0)
+		return;
+
+	fh = xfdopen(cmd.child.out, "r");
+	while (strbuf_getline(&line, fh) != EOF)
+		if (add_have_entry(o, line.buf))
+			break;
+
+	strbuf_release(&line);
+	fclose(fh);
+	odb_helper_finish(o, &cmd);
+
+	qsort(o->have, o->have_nr, sizeof(*o->have), odb_helper_object_cmp);
 }
 
 static const unsigned char *have_sha1_access(size_t index, void *table)
@@ -53,6 +164,111 @@ int odb_helper_has_object(struct odb_helper *o, const unsigned char *sha1)
 	return !!odb_helper_lookup(o, sha1);
 }
 
+int odb_helper_get_object(struct odb_helper *o, const unsigned char *sha1,
+			    int fd)
+{
+	struct odb_helper_object *obj;
+	struct odb_helper_cmd cmd;
+	unsigned long total_got;
+	git_zstream stream;
+	int zret = Z_STREAM_END;
+	git_SHA_CTX hash;
+	unsigned char real_sha1[20];
+	struct strbuf header = STRBUF_INIT;
+	unsigned long hdr_size;
+
+	obj = odb_helper_lookup(o, sha1);
+	if (!obj)
+		return -1;
+
+	if (odb_helper_start(o, &cmd, "get_git_obj %s", sha1_to_hex(sha1)) < 0)
+		return -1;
+
+	memset(&stream, 0, sizeof(stream));
+	git_inflate_init(&stream);
+	git_SHA1_Init(&hash);
+	total_got = 0;
+
+	for (;;) {
+		unsigned char buf[4096];
+		int r;
+
+		r = xread(cmd.child.out, buf, sizeof(buf));
+		if (r < 0) {
+			error("unable to read from odb helper '%s': %s",
+			      o->name, strerror(errno));
+			close(cmd.child.out);
+			odb_helper_finish(o, &cmd);
+			git_inflate_end(&stream);
+			return -1;
+		}
+		if (r == 0)
+			break;
+
+		write_or_die(fd, buf, r);
+
+		stream.next_in = buf;
+		stream.avail_in = r;
+		do {
+			unsigned char inflated[4096];
+			unsigned long got;
+
+			stream.next_out = inflated;
+			stream.avail_out = sizeof(inflated);
+			zret = git_inflate(&stream, Z_SYNC_FLUSH);
+			got = sizeof(inflated) - stream.avail_out;
+
+			git_SHA1_Update(&hash, inflated, got);
+			/* skip header when counting size */
+			if (!total_got) {
+				const unsigned char *p = memchr(inflated, '\0', got);
+				if (p) {
+					unsigned long hdr_last = p - inflated + 1;
+					strbuf_add(&header, inflated, hdr_last);
+					got -= hdr_last;
+				} else {
+					strbuf_add(&header, inflated, got);
+					got = 0;
+				}
+			}
+			total_got += got;
+		} while (stream.avail_in && zret == Z_OK);
+	}
+
+	close(cmd.child.out);
+	git_inflate_end(&stream);
+	git_SHA1_Final(real_sha1, &hash);
+	if (odb_helper_finish(o, &cmd))
+		return -1;
+	if (zret != Z_STREAM_END) {
+		warning("bad zlib data from odb helper '%s' for %s",
+			o->name, sha1_to_hex(sha1));
+		return -1;
+	}
+	if (total_got != obj->size) {
+		warning("size mismatch from odb helper '%s' for %s (%lu != %lu)",
+			o->name, sha1_to_hex(sha1), total_got, obj->size);
+		return -1;
+	}
+	if (hashcmp(real_sha1, sha1)) {
+		warning("sha1 mismatch from odb helper '%s' for %s (got %s)",
+			o->name, sha1_to_hex(sha1), sha1_to_hex(real_sha1));
+		return -1;
+	}
+	if (parse_sha1_header(header.buf, &hdr_size) < 0) {
+		warning("could not parse header from odb helper '%s' for %s",
+			o->name, sha1_to_hex(sha1));
+		return -1;
+	}
+	if (total_got != hdr_size) {
+		warning("size mismatch from odb helper '%s' for %s (%lu != %lu)",
+			o->name, sha1_to_hex(sha1), total_got, hdr_size);
+		return -1;
+	}
+
+	return 0;
+}
+
 int odb_helper_get_direct(struct odb_helper *o,
 			  const unsigned char *sha1)
 {
diff --git a/odb-helper.h b/odb-helper.h
index d4b968693f..819182f76a 100644
--- a/odb-helper.h
+++ b/odb-helper.h
@@ -23,6 +23,10 @@ struct odb_helper {
 extern struct odb_helper *odb_helper_new(const char *name, int namelen);
 extern int odb_helper_has_object(struct odb_helper *o,
 				 const unsigned char *sha1);
+extern int odb_helper_get_object(struct odb_helper *o,
+				 const unsigned char *sha1,
+				 int fd);
 extern int odb_helper_get_direct(struct odb_helper *o,
 				 const unsigned char *sha1);
+
 #endif /* ODB_HELPER_H */
diff --git a/sha1_file.c b/sha1_file.c
index c460eb2658..847883f261 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -907,6 +907,9 @@ static int stat_sha1_file(const unsigned char *sha1, struct stat *st,
 			return 0;
 	}
 
+	if (!external_odb_get_object(sha1) && !lstat(*path, st))
+		return 0;
+
 	return -1;
 }
 
@@ -946,7 +949,14 @@ static int open_sha1_file(const unsigned char *sha1, const char **path)
 	if (fd >= 0)
 		return fd;
 
-	return open_sha1_file_alt(sha1, path);
+	fd = open_sha1_file_alt(sha1, path);
+	if (fd >= 0)
+		return fd;
+
+	if (!external_odb_get_object(sha1))
+		fd = open_sha1_file_alt(sha1, path);
+
+	return fd;
 }
 
 /*
diff --git a/t/t0500-external-odb.sh b/t/t0500-external-odb.sh
new file mode 100755
index 0000000000..eaa3688a31
--- /dev/null
+++ b/t/t0500-external-odb.sh
@@ -0,0 +1,44 @@
+#!/bin/sh
+
+test_description='basic tests for external object databases'
+
+. ./test-lib.sh
+
+ALT_SOURCE="$PWD/alt-repo/.git"
+export ALT_SOURCE
+write_script odb-helper <<\EOF
+GIT_DIR=$ALT_SOURCE; export GIT_DIR
+case "$1" in
+have)
+	git cat-file --batch-check --batch-all-objects |
+	awk '{print $1 " " $3 " " $2}'
+	;;
+get_git_obj)
+	cat "$GIT_DIR"/objects/$(echo $2 | sed 's#..#&/#')
+	;;
+esac
+EOF
+HELPER="\"$PWD\"/odb-helper"
+
+test_expect_success 'setup alternate repo' '
+	git init alt-repo &&
+	test_commit -C alt-repo one &&
+	test_commit -C alt-repo two &&
+	alt_head=$(git -C alt-repo rev-parse HEAD)
+'
+
+test_expect_success 'alt objects are missing' '
+	test_must_fail git log --format=%s $alt_head
+'
+
+test_expect_success 'helper can retrieve alt objects' '
+	test_config odb.magic.scriptCommand "$HELPER" &&
+	cat >expect <<-\EOF &&
+	two
+	one
+	EOF
+	git log --format=%s $alt_head >actual &&
+	test_cmp expect actual
+'
+
+test_done

external-odb.c, external-odb.h

diff --git a/external-odb.c b/external-odb.c
index 9c77180d6c..82ac80aa04 100644
--- a/external-odb.c
+++ b/external-odb.c
@@ -39,17 +39,27 @@ static int external_odb_config(const char *var, const char *value, void *data)
 	return 0;
 }
 
-static void external_odb_init(void)
+static void external_odb_do_init(int force)
 {
 	static int initialized;
 
-	if (initialized || !use_external_odb)
+	if ((!force && initialized) || !use_external_odb)
 		return;
 	initialized = 1;
 
 	git_config(external_odb_config, NULL);
 }
 
+static inline void external_odb_init(void)
+{
+	external_odb_do_init(0);
+}
+
+inline void external_odb_reinit(void)
+{
+	external_odb_do_init(1);
+}
+
 int has_external_odb(void)
 {
 	external_odb_init();
diff --git a/external-odb.h b/external-odb.h
index 27c3d39c1b..254070994a 100644
--- a/external-odb.h
+++ b/external-odb.h
@@ -9,6 +9,7 @@ enum odb_helper_type {
 	OBJ_HELPER_MAX
 };
 
+extern void external_odb_reinit(void);
 extern int has_external_odb(void);
 extern struct odb_helper *find_odb_helper(const char *dealer,
 					  enum odb_helper_type type);