I'm always sad to see that CouchDB and its PouchDB counterpart in the browser (or even on the server) are always forgotten, even though they've been solving that problem for years now:

* easy replication (protocol is documented (http://docs.couchdb.org/en/latest/replication/protocol.html), and it's only HTTP+JSON)

* The replication is reactive, ie you only get the changes since last time you synced

* The replication is realtime, it can use longpolling or server-sent events.

* The replication is two-way; the browser doesn't have a special status in the replication protocol. It is effectively the same database. The application speaks only with the local database, which is sync'ed in the background.

* CouchDB effectively acts as a log of all transformations. If you modify a document, there will be a trace that there is a "marker" that tells you that the document changed since last time you saw it.

* There is filtering, although it can be inefficient and is completely controlled by the client (so no security filtering from the server)

* CouchDB does lack granularity, it is only at the database level meaning that each user must have a different database. Couchbase is going another way with channels in the Sync gateway (http://developer.couchbase.com/mobile/develop/guides/sync-ga...) but it's non standard

We need to iterate on the CouchDB ecosystem, because it already provides a lot of what we need.