When I had to do that for some apps, the main issues were * while being offline some notifications get lost so one needs to get a sense of the general state of the central server * I wanted to synchronize only part of the database to each client (data it has the right to see) * pushes to the server could not be generic due to business logic (again, rights of the user)
Thus I started recording changes on the client, pushing them to the central server through functional endpoints and then pulling all changes from the server. To pull data from the server I found a paper[1] with an interesting way of encoding data so that the client can get a sense of the difference by getting a data structure of size proportional to the diff (no matter the whole dataset size). I developed a library for that, pretty limited for now [2].
[#2] http://conferences.sigcomm.org/sigcomm/2011/papers/sigcomm/p... [#1] https://github.com/3musket33rs/mathsync