Mongo 2: Rollback API

[subdavis]

Reviewer notes

This could have been included in #1066 but it's a new feature, so I made it separate. It provides a read API for rolling back to earlier versions.

Important

This implementation is naive and I don't want to keep it forever. It's a true rollback such that it actually deletes rows from the DB. I would prefer a soft rollback like git revert which replays changes in reverse. But that sounded complicated and I didn't feel like writing it.

[BryonLewis]

Much simpler with the re-base. I pulled and tested it and it seems to work properly removing the annotations after the revision number.

The only thing I would comment on is that it's a little unclear that when you give a DELETE with Revision = 5 - it really means DELETE all those revisions that are > 5 and not remove everything including 5. I think as long as it's clear in the documentation that should be okay though.

[waxlamp]

Much simpler with the re-base. I pulled and tested it and it seems to work properly removing the annotations after the revision number.

The only thing I would comment on is that it's a little unclear that when you give a DELETE with Revision = 5 - it really means DELETE all those revisions that are > 5 and not remove everything including 5. I think as long as it's clear in the documentation that should be okay though.

I'm uneasy with this being a DELETE operation at all, because it's not really (in the REST or CRUD sense). However, since this is a tentative implementation with a long-term aim of a replacement, non-destructive rollback mechanism, it's probably ok.

The main difficulty is that the clients will need to change out DELETE calls for a PATCH or POST call later. To the extent that performing a rollback is a state change from the user's POV, rather than a change in the model itself, a POST operation with a verb like rollback sounds like the right abstraction to me; that would also avoid the need for clients to change their calling sequences later. (There's a name for this supplementation of CRUD operations with arbitrary (English) verb-based actions, but I can't find it right now; it's related to DRF's use of the @action decorator in the same context.)