> On Oct 17, 2014, at 3:41 PM, Jens Alfke <[email protected]> wrote: > > >> On Oct 17, 2014, at 12:15 PM, Brian Mitchell <[email protected]> >> wrote: >> >> Simply put: if and only if the revs match we should assume some optimism >> just like we >> do with things like atts_since. There’s already a lot of trust between two >> nodes for replication >> and we should assume that matching revs were either unique (or random) or >> based on some >> deterministic property that isn’t likely to collide unless it was an >> equivalent operation. > > I'm sorry, I've read this a few times and I can't figure out exactly what > your meaning is. Could you elaborate? Particularly, what does "if the revs > match" mean, exactly? > > Also, I don't think your statement "there’s already a lot of trust between > two nodes for replication" is accurate in all cases. You seem to be thinking > of a server cluster (a la BigCouch) but CouchDB-style replication is often > used in a more distributed way. Both PouchDB and Couchbase Lite use > replication between servers and clients. A client can be trusted to be acting > on behalf of a user, but not beyond that. > > —Jens
No problem. I probably kept the message too short. The issue is that requiring revs to match is a bit assuming about the context different implementations are designed to operate in. The case of the optimization makes a lot of sense in some cases (clustering for availability being the most obvious). This implies there is a contract to how any implementation should treat revisions: 1. Any revs that match between two documents should be assumed to be the same revision of the document. This is important outside of optimization (N-way replications for example). 2. Each implementation must be trusted to generate unique revisions. 3. Optionally: revisions can be generated deterministically to allow idempotent operations. This is really important for clusters (non-optional in practice) but has very little important for PouchDB. I’d urge implementations to document what guarantees their revs have but I would stop short in exposing the implementation (like the digest used or RNG function) as that is out of scope for the _rev contract for compatible implementations. There are many reasons to settle at this level of detail, backwards compatibility being the most important. The other is that it could allow other sorts of rev encoding in the future for some implementations (cheaper tree merges being one thing worth revisiting). So PouchDB should generate revs that make sense for PouchDB’s implementation. The contract of how these revs are interpreted shouldn’t constrain it to implementing the same JSON normalization and digest that others do. Same goes for other Couch’s. Brian.
