Re: [ANNOUNCE] Git Merge Contributor Summit topic planning

Jeff King [off-list ref] writes:

For instance, if the server knew that XYZ meant

  - send bytes m through n of packfile p, then...
  
  - send the object at position i of packfile p, as a delta against the
    object at position j of packfile q

  - ...and so on

Then you could store very small "instruction sheets" for each XYZ that
rely on the data in the packfiles. If those packfiles go away (e.g., due
to a repack) that invalidates all of your current XYZ tags. That's OK as
long as this is an optimization, not a correctness requirement.

Yes.  You can play optimization games.

quoted

So in the real life, I think that the exchange needs to be more
like this:

    C: I need a packfile with this want/have
    ... C/S negotiate what "have"s are common ...
    S: Sorry, but our negitiation indicates that you are way too
       behind.  I'll send you a packfile that brings you up to a
       slightly older set of "want", so pretend that you asked for
       these slightly older "want"s instead.  The opaque id of that
       packfile is XYZ.  After getting XYZ, come back to me with
       your original set of "want"s.  You would give me more recent
       "have" in that request.  
    ... connection interrupted ...
    C: It's me again.  I have up to byte N of pack XYZ
    S: OK, resuming (or: I do not have it anymore, start from scratch)
    ... after 0 or more iterations C fully receives and digests XYZ ...

and then the above will iterate until the server does not have to
say "Sorry but you are way too behind" and returns a packfile
without having to tweak the "want".

Yes, I think that is a reasonable variant. The client knows about
seeding, but the XYZ conversation continues to happen inside the git
protocol. So it loses flexibility versus a true CDN redirection, but it
would "just work" when the server/client both understand the feature,
without the server admin having to set up a separate bundle-over-http
infrastructure.

You can also do a CDN offline as a natural extension.  When the
server says "Sorry, you are way too behind.", the above example
tells "I'll update you to a slightly stale version first" to the
client.  An natural extension could say "Go update yourself to a
slightly stale version first by grabbing that bundle over there."

But I agree that doing everything in-line may be a logical and
simpler first step to get there.