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

> 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.