Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for data synchronization. In general, one aspect of the subject matter described in this specification can be embodied in methods that include the actions of receiving a knowledge vector from a first device of the pair of devices, the knowledge vector identifying data items and latest changes to the data items known to the peer device; comparing, at a second device of the pair of devices, the received knowledge vector with a local knowledge vector for changes unknown to the first device; and sending a list of changes to the first device.