Re: "IMAP IDLE"-like long-polling "git fetch"

From: Eric Wong <e@80x24.org>
Date: May 2, 2019, 09:42 UTC
Message-ID: <20190502094255.kbpzffokvdch63qg@dcvr>
In-Reply-To: <87ftpxqkji.fsf@evledraar.gmail.com>

Ævar Arnfjörð Bjarmason <avarab@gmail.com> wrote:

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> 
> On Thu, May 02 2019, Eric Wong wrote:
> 
> > Stefan Beller <sbeller@google.com> wrote:
> >> IIRC, More than half the bandwidth of Googles git servers are used
> >> for ls-remote calls (i.e. polling a lot of repos, most of them did *not*
> >> change, by build bots which are really eager to try again after a minute).
> >
> > Thinking back at that statement; I think polling can be
> > optimized in git, at least.
> >
> > IIRC, your repos have lots of refs; right?
> > (which is why it's a bandwidth problem)
> >
> > Since info/refs is a static file (hopefully updated by a
> > post-update hook), the smart client can make an HTTP request
> > to check If-Modified-Since: to avoid the big response.
> >
> > The client would need to cache the mtime of the last requested
> > refs file; somewhere.
> >
> > IOW, do refs negotiation the "dumb" way; since it's no better
> > than the smart way, really.  Keep doing object transfers the
> > smart way.
> >
> > During the initial clone, smart servers could probably
> > have a header informing clients that their info/refs
> > is up-to-date and clients can do dumb refs negotiation.
> 
> Doing this with If-Modified-Since sounds like an easier drop-in
> replacement (just needs a client change), but I wonder if ETag isn't a
> better fit for this.

ETags overall could work.

> I.e. we'd document some convention where the ETag is a hash of the refs
> the client expects to be advertised in some format, it then sends that
> to the server.

But I was hoping to avoid the overhead of spawning git-http-backend entirely. And there's no consistent way to configure ETags on different static servers.

> That allows the same thing without anyone keeping more state than they
> keep now in their local ref store

I think caching the remote info/refs is useful anyways in case the user changes their fetch refspec, and it could speed up invocations of "git ls-remote".

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> On the fancier side I think bloom filters are something that's been
> discussed (and I believe someone (Twitter?) had such an internal patch),
> i.e. the client sends a bloom filter of refs they have, and the server
> advertises things they don't know about yet (and due to how bloom
> filters work, some things they *do* know about already but tripped up
> the bloom filter...).

I'm not smart enough to understand such fancy things :)