Re: RFC: Resumable clone based on hybrid "smart" and "dumb" HTTP

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Feb 10, 2016, 21:22 UTC
Message-ID: <20160210212207.GB10155@google.com>
In-Reply-To: <xmqqh9hgz3km.fsf@gitster.mtv.corp.google.com>

Junio C Hamano wrote:

> I somehow doubt it.  Both index-pack and lost-found need to trace
> "object A depends on object B", but the similarity ends there.

You already responded about this in another side-thread, so the following is mostly redundant. :) I think this message can still be useful to future readers since it more explicitly goes through the trade-offs.

The relevant similarity is that both index-pack and lost-found need to inflate all commit, tag, and tree objects.

This is the same idea that led to the introduction of "git index-pack --strict".

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> I am not quite sure if that is an advantage, though.  The second
> message proposes that the lost-found computation to be done by the
> client using *.pack, but any client, given the same *.pack, will
> compute the same result, so if the result is computed on the server
> side just once when the *.pack is prepared and downloaded to the
> client, it would give us a better overall resource utilization.  And
> in essence, that was what the *.info file in the first message was.

Advantages of not providing the list of roots:
 1. only need one round-trip to serve the packfile as-is
 2. less data sent over the wire (not important unless the list of roots
    is long)
 3. can be enabled on the server for existing repositories without an
    extra step of generating .info files

Advantage of providing the list of roots:
- speedup because the client does not have to compute the list of roots

For a client that is already iterating over all objects and inspecting FLAG_LINK, the advantage (3) seems compelling enough to prefer the protocol that doesn't sent a list of roots.

Except when people pass --depth, "git clone" sets 'check_self_contained_and_connected = 1'. That means clients that already iterate over all objects and inspect FLAG_LINK are the usual case.

Thanks for your thoughtfulness, Jonathan