Subject: Re: [PATCH 0/6] odb: track commit graphs via object source

From: Junio C Hamano <[email-masked]>
Date: Thu, 25 Sep 2025 12:17:50 -0700

Patrick Steinhardt <[email-masked]> writes:

> There is no inherent reason why a new backend would not be able to use
> the existing commit-graph infrastructure indeed. But there are reasons
> that specific backends may not want to do so. If objects are already
> stored in a database table, then it may make way more sense to store
> additional metadata that is currently stored in the commit-graph in a
> secondary database table instead of in the commit graph.
> ...
> This is roughly what I have in my head right now. And I realize that
> this information really should be sitting in a design document. I'm
> working on that, but still need to land two more patch series before I
> want to send such a patch series to the list.

So is everybody happy with this line of thought that makes it
mandatory for each backend to decide and implement the commit-graph
support if they want to?

My reading of the later part of Taylor's message[*] tells me that at
least Taylor does not agree with that position, and I am not sure
about this design choice, either.  Surely, each backend can have its
own optimization, but looking at the way data from the commit-graph
and other auxiliary data files are used to optimize real operations
(like populating the essential fields of the commit object first
from the graph, only to read other things lazily from the object
database, or switching to completely different traversal machinery
when reachability bitmap is available), we cannot say that each
backend can store whatever side data they please and leave it at
that.  The code paths that are supposed to be generic need to be
aware of these side data used for optimization to some degree, so
conceptually it is much cleaner (well, at least to my eyes, that is)
to declare that the auxiliary data files like commit-graph and
reachability bitmaps are defined on the objects in the repository,
no matter what backend is used to store them.

[Reference]

 * https://lore.kernel.org/git/[email-masked]/