1. In an information processing system, a method for synchronizing a first dataset with a second dataset, wherein the first and second datasets each includes user data, and the user data of each of the first and second datasets is capable of having been changed independently of the other of the first and second datasets, the method comprising: maintaining, for the first dataset, information that is descriptive of synchronization between the first dataset and a third dataset, the information hereinafter referred to as the third-party information; communicating the third-party information; and synchronizing the first dataset with the second dataset using the communicated third-party information.