Synchronization architectures, methods, systems, and computer readable media are described. In one embodiment, a synchronization session is initiated between a first data processing system and a second data processing system. First data, representing changes to a first data class, is transmitted from the first data processing system to the second data processing system. Second data, representing changes to a second data class, is transmitted from the first data processing system to the second data processing system. Transmission of the second data begins before transmission of the first data is complete. The first data may comprise structured data such as contacts information, to do information, calendar information, or web browsing bookmarks.