Re: Status of dumb http transport?

From: Jakub Narebski <jnareb@gmail.com>
Date: Jan 30, 2011, 16:01 UTC
Message-ID: <m3pqre9y9y.fsf@localhost.localdomain>
In-Reply-To: <AANLkTik99NNjNwLbj3-xEvVqiYUENiaREYAeJUC0bwVY@mail.gmail.com>

Martin Langhoff <martin.langhoff@gmail.com> writes:

> What is the state of dumb http transport today, for fetching updates?
> Is the client code smart enough to fetch indexes and use range
> requests? If so, how does that fare for latency?

A good description of "dumb" http transport can be found in Pro Git
book, in chapter 9.6 "Transfer Protocols", section "The Dumb Protocol"
http://progit.org/book/ch9-6.html
 
In short "dumb" http transport is realy dumb, it consults
.git/info/refs to know where to start from, and
.git/objects/info/packs to know what packfiles are available (both are
generated by git-update-server-info, usually run from update hook).

Starting from appropriate object, it tries to download loose object, if posible, and if not then it downloads index files to know which packfile to download. It walks the chain down checking which objects it has, and if there is one missing it downloads it. It always downloads _whole_ pack files. It doesn't use range request... what's more, as far as I know it doesn't know how to resume partially downloaded packfile (hough it wouldn't re-download packfile it already got).

Show 13 quoted lines

> Background: I am looking at whether yum repositories' data (currently
> in sqlite & xml) could benefit from being a git (or very gittish)
> database -- with a bit of re-organizing to make it git-efficient of
> course.
> 
> Not the data that would benefit, but rather, users pulling updates
> from fast-moving repos (updates, updates-testing, rawhide...).
> 
> One of the constraints is that this has to be http, and work well
> across a universe of mirrors (that won't install or configure
> software) and the good bad and ugly world of http proxies. Yum can be
> taught to use the git proto, but that won't gain widespread use
> quickly -- http is and will be the mainstay for a long time.

Note that from some time we have "smart" http protocol, which is almost git protocol proxied over http... with addition that http protocol is stateless. Default git-http-backend CGI script falls back to "dumb" protocol if client doesn't support smart http transport.

-- 
Jakub Narebski
Poland
ShadeHawk on #git