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

> On Thu, Sep 25, 2025 at 12:17:50PM -0700, Junio C Hamano wrote:
> > Patrick Steinhardt <ps@pks.im> 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.
> 
> My intent here is mostly to allow us to swap out how exactly the data is
> being cached. During the Git Merge I heard from some JJ developer (I
> think) that they also have a pluggable cache, but they approach the
> issue differently: instead of making the cache a property of the object
> backend, they instead make the cache itself pluggable.
> 
> I think that's a worthwhile angle to explore. The cache would still sit
> on the repository level, and it wouldn't have to care at all whether we
> use loose objects/packfiles or any other backend. But in theory, we can
> still swap it out for a different representation as desired.
> 
> Which overall means that we can defer this to a later point in time, as
> we can make it pluggable independent from making the object database
> itself pluggable.
> 
> So I'd propose to merge the first six patches, as everyone seemed to

> agree that they improve the status quo, but drop the last patch that
> moves the commit-graph into the ODB sources.
> 
> Does that seem reasonable to everyone? If so, I don't really see a
> reason to reroll at this point. But please let me know in case I miss
> anything that needs addressing.
> 
> Thanks!
> 
> Patrick
>