Subject: Re: What's cooking in git.git (May 2025, #07; Fri, 23)

From: Patrick Steinhardt <[email-masked]>
Date: Fri, 30 May 2025 11:47:53 +0200

On Tue, May 27, 2025 at 02:45:43PM -0500, Justin Tobler wrote:
> On 25/05/27 09:50AM, Junio C Hamano wrote:
> > Patrick Steinhardt <[email-masked]> writes:
> > > I think the only outstanding discussion is whether to name things
> > > `odb_alternate` or `odb_source` [1]. In case others agree that
> > > `odb_source` is a better name I'm happy to revise, but if not I'd rather
> > > keep it as-is.
> > 
> > The model in which the term "alternates" was born is "A repository
> > has its own object directory, the primary one, and in addition it
> > can borrow from zero or more alternate object directories that are
> > used by other repositories".  The presence of the primary makes the
> > word "alternate" meaningful.
> > 
> > Is the model now "A repository has one object store, which consists
> > of one or more X, all of which are equals"?  If there is no primary
> > that is more special than others, then calling X an "alternate" may
> > indeed sound funny, although (1) I do not find it terribly confusing
> > and (2) I do not find "source" much better, either.
> 
> My understanding is that the object store still has a primary X and zero
> or more alternative X. The idea is that eventually, with pluggable ODBs,
> X can be a different backend/provider instead of just being "files". If
> this is the case, calling X an "alternate" would mean we have a primary
> "alternate" and potentially a set of "alternate" alternates.
> 
> This sounds a bit odd and doesn't quite match what I would intuitively
> expect. But, I also don't find it super confusing either.

Yeah, I understand that confusion indeed. I don't think that the other
proposals we've got are a lot better, either:

  - `odb_backend` was shot down because it may cause the association
    that one object database has one backend. But backends are per
    alternate, so there's a mismatch in expectations.

  - `odb_source` is better, but we now have the problem that we use
    "alternate" interchangably in most cases where we also use
    `odb_source`. This will likely lead to somewhat awkward interfaces.

The problem with `odb_source` might eventually go away once we clearly
distinguish the "alternates" concept from the low-level mechanism to
access objects. But I'm just not certain at all whether it won't cause
more confusion when in most cases "alternates" and "sources" can be used
somewhat interchangably.

I dunno. The more I think about this the more I start to like the
`odb_source` name.

> > The names we use to call the collection and the underlying
> > implementations of the collection in the reference world
> > unfortunately does not quite help to guide us, as we do not take two
> > implementations and compose into one unified view, which is what we
> > are doing in the object store.  Hmmm...
> 
> Similar to references, I still think of a pluggable ODB as a "backend".
> The main difference being that with references there is only a single
> backend active ("file" or "reftables") at a time, while for the object
> store there could be multiple.

Yeah, that's basically the major source of confusion I want to avoid
with the "backend" terminology. I really do want to make it explicit
that there is not only one backend, but multiple ones.

Patrick