1. A method, comprising: receiving a request to update data of a user of a multi-user on-demand database service, wherein the data of the user is stored by the on-demand database service on a multi-tenant on-demand database system; identifying data stored by a first server separate from the on-demand database service, the data being relevant to the user; utilizing a data structure stored on a non-transitory computer readable storage medium to determine, by a processor, a difference between the data stored on the first server and the data stored by the on-demand database service, wherein the data structure includes: a plurality of user identifiers, wherein each user identifier identifies one of a plurality of users of the on-demand database service, and a plurality of data identifiers, wherein each data identifier correlates with at least one of the user identifiers and identifies data accessible to the user associated with the correlated user identifier; and periodically and/or on-demand, synchronizing the first server and the on-demand database service, based on the determined difference, wherein the synchronizing includes updating the data stored by the on-demand database service with at least a portion of the data stored by the first server that is determined to be different from the data stored by the on-demand database service.