[NOTES 05/07] What can we do next with pluggable ODB?

[Taylor Blau <ttaylorr@openai.com>]

Topic: What can we do next with pluggable ODB?

* Emily: We saw this working in Patrick's talk. What do we want to do
  with it next?

* Patrick: Most things work, but we are not all the way there yet;
  commit-graph and MIDX are examples. The plan is to add a repository
  extension in 2.57.

* Peff: From the Mercurial talk, you can convince Git to make good
  deltas, but doing so has a cost. I do not think the ODB API is at a
  level where deltas can be created on the fly.

* Patrick: I think we can do that. They have information we do not have.
  Renames are tricky.

* Peff: The ODB does not know anything beyond the content. Could we give
  it more information?

* Patrick: We may want to evolve this based on the representation. We
  could pass a `struct commit *` into the ODB API instead of just
  content.

* Peff: What we have now is a reasonable stopgap.

* Patrick: Content-defined chunking is another possibility.

* Taylor: Would that be at the object layer, or a property of a
  particular ODB store implementation?

* Patrick: I would like to do it natively, but that seems like a large
  change.

* brian: It could be part of a new pack/index format. I am very much in
  favor of content-defined chunking.

* Peff: There is a logical object model. Would this be a new object
  type, where a chunked object and a blob have different hashes, or
  something below that?

* Patrick: We could start at the storage layer and eventually move it to
  the object layer.

* Taylor: Perhaps, but perhaps not. This may be easier than we are
  thinking.

* Patrick: Moving the ecosystem may be difficult.

* brian: We could have a compatibility fallback.

* Peff: Then we would have two equivalent representations of an object
  which do not hash to the same thing.

* brian: We could introduce a pack-only object type.

* Peff: Which OID would I refer to?

* brian: The blob's OID.

* Taylor: That is analogous to REF_DELTA and OFS_DELTA: multiple
  representations of the same thing.

* brian: We would need a format extension, because we are out of bits.

* Patrick: Would we still want to put it in a pack?

* brian: Yes.

* Patrick: Start with the storage layer, then see where it goes.

* Patrick: Would this only be for blobs?

* Taylor: Is "chunked" a refinement of a blob, or an attribute
  independent of object type?

* brian: Trees are not currently binary-searchable. It would be nice to
  fix that too.

* Patrick: Perhaps reftable could provide inspiration there.

* Emily: After Patrick's talk, my skip-level manager asked whether we
  could build a commit cloud with Git. What else could we do that is
  totally wild?