Among other things, techniques and systems are disclosed for syncing data between a client device and a server. Synchronizing data includes initiating a sync session by negotiating a sync mode between a client device and a server for each of one or more dataclasses. A status code is generated based on a result of the negotiating. Based on the generated status code, the client device and the server exchange one or more data items to be updated for the one or more dataclasses using the negotiated sync mode for each dataclass. The exchanged one or more data items are updated at the client device or the server. The updated one or more data items are committed at the client or the server.