Re: [JGit-io-RFC-PATCH v2 2/4] Add JGit IO SPI and default implementation

From: Imran M Yousuf <imyousuf@gmail.com>
Date: Oct 13, 2009, 01:30 UTC
Message-ID: <7bfdc29a0910121830y4dc9b3efpe17860e04457988d@mail.gmail.com>
In-Reply-To: <20091012145741.GM9261@spearce.org>

On Mon, Oct 12, 2009 at 9:57 PM, Shawn O. Pearce <spearce@spearce.org> wrote:

Show 7 quoted lines

> imyousuf@gmail.com wrote:
>> The SPI mainly focus's in providing an API to JGit to be able to perform
>> similar operations to that of java.io.File. All direct I/O is based on the
>> java.io.Input/OutputStream classes.
>>
>> Different JGit IO SPI provider is designed to be URI scheme based and thus
>> the default implementation is that of "file" scheme. SPI provider will be

<snip />

Show 32 quoted lines

> I think this may be a bit in the wrong direction for what we are
> trying to accomplish.
>
> A number of people really want to map Git onto what is essentially
> Google's BigTable schema.  Aside from Google's own BigTable product
> (which I want to use Git on at work, because it would vastly simplfiy
> my system administration duties at $DAYJOB) there is Cassandra and
> Hadoop HBase which implement the same schema semantics.
>
> None of those systems implement file streams, they implement cell
> storage in a non-transactional system with a semi-dynamic schema.
>
> Some people have built transactional semantics on top of these
> storage layers, e.g. Google AppEngine provides multiple row
> transactions through some magic sauce layered on top of BigTable.
> I'm sure people will build similar tools on top of Cassandra
> and HBase.
>
> Where I'm trying to go with this is that things that are stored
> in files on the filesystem in traditional Git wouldn't normally be
> mapped into "byte streams" in a BigTable-ish system, or even the
> JDBC-ish system you were describing.
>
> For .git/config we might want to map config variable names into
> keys in the table, with values stored in cells.  This makes it
> easier to query or edit the data.
>
> Fortunately, "Config" is abstract enough that we could subclass
> it with a CassandraConfig and simply use that instance when on a
> based Cassandra storage system.  No file streams required.  Ditto
> for a JdbcConfig.
>

Firstly, I am sorry but I am not intelligent enough to perceive, how do the user decide which instance of Config to use? I personally think that there is no API to achieve what you just mentioned :(; i.e. the user will have know CassandraConfig directly. Secondly, I instead was thinking of porting JGit for that matter to any system supporting streams (not any specific sub-class of them), such HBase/BigTable or HDFS anything.... Thirdly, I think we actually have several task in hand and I would state them as -

1. First introduce the I/O API such that it completely replaces java.io.File
2. Secondly segregate persistence of for config (or config like
objects) and introduce a SPI for them for smarter storage.

I am not thinking of storing "only" the bare content of a git repository, but I intent to be able to also store the versioned contents it self as well. If we choose the above 2 steps I mentioned I am of the opinion that we will be able to achieve both our ideas. In addition I hope that if one day Git itself introduces a similar I/O API then Git can also support the I/O SPI implementations JGit will.

Waiting eagerly to learn what you think :).

Best regards,

Imran

Show 14 quoted lines

> For RefDatabase, we'd want to do the same and avoid the concept of
> packed-refs altogether.  Each Ref should go into its own row in a
> Cassandra storage system, and essentially act as a loose object.
> Ditto with JDBC.
>
> We'd probably never need to read-or-write the info/refs or
> objects/info/packs listings.
>
> And I think that's everything that a bare repository needs, aside
> from ObjectDatabase, which is already mostly abstract anyway.
>
> --
> Shawn.
>

-- 
Imran M Yousuf
Entrepreneur & Software Engineer
Smart IT Engineering
Dhaka, Bangladesh
Email: imran@smartitengineering.com
Blog: http://imyousuf-tech.blogs.smartitengineering.com/
Mobile: +880-1711402557