A Comparison of Offline Sync Protocols and Implementations

Sync Protocol or Implementation	Conflict Resolution Strategies	Client Implementations
CouchDB Replication Protocol	deterministically-chosen winner[1] developer-supplied strategy (selecting from a revision tree of conflicts)[2]	PouchDB Cloudant Mobile Sync (iOS, Android) Couchbase Mobile (iOS, Android, Java, .NET, Xamarin, PhoneGap)[3]
Realm Sync Engine	last write wins (when updating or deleting objects)[4] appending items (when inserting into a list)[5]	Realm JavaScript (React Native, Node.js) Realm Objective‑C (iOS) Realm Swift (iOS) Realm Java (Android) Realm Xamarin (.NET)
Firebase Realtime Database	last write wins (when using set() or update())[6] appending items (when using push())[7] developer-supplied strategy (when using transaction())[8]	Firebase SDK: JavaScript Firebase SDK: iOS Firebase SDK: Android Firebase SDK: C++ Firebase SDK: Server (Node.js, Java, Python) Firebase SDK: Unity

1. Apache CouchDB implementations use a deterministic algorithm to choose a "winning" document. See: Working with conflicting documents. ↩

2. Apache CouchDB implementations will maintain a revision tree of conflicted documents, allowing for the selection of a "winning" document or the merging of document data using application logic. See: Working with conflicting documents. ↩

3. Couchbase Mobile 2.0 will not be compatible with the CouchDB Replication Protocol. ↩

4. See the Realm Objective Server documentation on conflict resolution. ↩

5. See the Realm Objective Server documentation on conflict resolution. ↩

6. See the Firebase documentation on saving data. ↩

7. See the Firebase documentation on saving data. ↩

8. Transactions in Firebase can be slow and complex as they require one or more round trips to the server. See the Firebase documentation on saving data. ↩