1. A system for synchronizing data between a first-database and a second-database comprising: a memory; and a client configured for determining whether a retrieval request requires a synchronization of the second-database before processing the received retrieval request, wherein the system partially synchronizes the second-database from the first-database with only that portion of the first-database associated with the retrieval request in response to determining whether synchronization is required and for processing the retrieval request from the second-database, wherein the system, in response to issuing the retrieval request to the second-database, extracts from the first-database, differential data corresponding with the retrieval request between a first data set in the first-database and a second data set in the second-database as a retrieval target of the retrieval request, wherein the retrieval request includes a current retrieval request and a future retrieval request, wherein the future retrieval request is configured for issuance after the current retrieval request, and wherein the system, updates the second data set by applying only the differential data corresponding with the retrieval request to the second-database and by recording a time of updating the second data set, wherein the updated second data set is the retrieval target of the retrieval request, and wherein the differential data corresponding with the retrieval request represents only a portion of data out of all possible data sets that differ between the first-database and the second-database, and wherein the differential data indicates a difference between the second data set at a time of issue of a past retrieval request wherein the time of issue of the past retrieval issued before the retrieval request, and the second data set at the time of issue of the retrieval request.