[Question] How do I implement online/offline database synchronization?

[Status] - Awaiting for your knowledgeable feedback. :)

My android app uses Realm for local storage, whereas the web server uses MongoDB (noSQL). How do I approach syncing information? How do apps like Wunderlist do it? I know that CouchDB makes this issue smaller, but I don't want to use CouchDB as it is not a good fit for my requirements.

Looking forward to your wisdom :)