Re: [PATCH v2 4/4] bundle v3: the beginning

[Christian Couder <christian.couder@gmail.com>]

On Wed, Jun 1, 2016 at 3:37 PM, Duy Nguyen <pclouds@gmail.com> wrote:
> On Tue, May 31, 2016 at 8:18 PM, Christian Couder
> <christian.couder@gmail.com> wrote:
>>>> I wonder if this mechanism could also be used or extended to clone and
>>>> fetch an alternate object database.
>>>>
>>>> In [1], [2] and [3], and this was also discussed during the
>>>> Contributor Summit last month, Peff says that he started working on
>>>> alternate object database support a long time ago, and that the hard
>>>> part is a protocol extension to tell remotes that you can access some
>>>> objects in a different way.
>>>>
>>>> If a Git client would download a "$name.bndl" v3 bundle file that
>>>> would have a "data: $URL/alt-odb-$name.odb" extended header, the Git
>>>> client would just need to download "$URL/alt-odb-$name.odb" and use
>>>> the alternate object database support on this file.
>>>
>>> What does this file contain exactly? A list of SHA-1 that can be
>>> retrieved from this remote/alternate odb?
>>
>> It would depend on the external odb. Git could support different
>> external odb that have different trade-offs.
>>
>>> I wonder if we could just
>>> git-replace for this marking. The replaced content could contain the
>>> uri pointing to the alt odb.
>>
>> Yeah, interesting!
>> That's indeed another possibility that might not need the transfer of
>> any external odb.
>>
>> But in this case it might be cleaner to just have a separate ref hierarchy like:
>>
>> refs/external-odbs/my-ext-odb/<sha1>
>>
>> instead of using the replace one.
>>
>> Or maybe:
>>
>> refs/replace/external-odbs/my-ext-odb/<sha1>
>>
>> if we really want to use the replace hierarchy.
>
> Yep. replace hierarchy crossed my mind. But then I thought about
> performance degradation when there are more than one pack (we have to
> search through them all for every SHA-1) and discarded it because we
> would need to do the same linear search here. I guess we will most
> likely have one or two name spaces so it probably won't matter.

Yeah.

>>> We could optionally contact alt odb to
>>> retrieve real content, or just show the replaced/fake data when alt
>>> odb is out of reach.
>>
>> Yeah, I wonder if that really needs the replace mechanism.
>
> Replace mechanism provides good hook point. But it really depends how
> invasive this remote odb is. If a fake content is enough to avoid
> breakages up high, git-replace is enough. If you really need to pass
> remote odb info up so higher levels can do something more fancy, then
> it's insufficient.
>
>> By the way this makes me wonder if we could implement resumable clone
>> using some kind of replace ref.
>>
>> The client while cloning nearly as usual would download one or more
>> special replace refs that would points to objects with links to
>> download bundles using standard protocols.
>> Just after the clone, the client would read these objects and download
>> the bundles from these objects.
>> And then it would clone from these bundles.
>
> I thought we have settled on resumable clone, just waiting for an
> implementation :) Doing it your way, you would need to download these
> special objects too (in a pack?) and come back download some more
> bundles. It would be more efficient to show the bundle uri early and
> go download the bundle on the side while you go on to get the
> addition/smaller pack that contains the rest.

Yeah, something like the bundle v3 mechanism is probably more efficient.

Thanks,
Christian.