A data synchronization method for a server in a system comprising three or more networked devices including at least two clients and the server, the method comprising: determining, by the server, whether a data item indicated by synchronization information comprised in a synchronization request message exists in a change log table of the server, when the server receives the synchronization request message transmitted by a first client; if a data item indicated by the synchronization information does not exist: updating, by the server the change log table by adding the data item indicated by the synchronization information and transmitting a synchronization request message comprising a data identifier and change log of the data item to a second client, creating, by the server a Globally Unique IDentifier, GUID, for the data item, wherein the GUID comprises a data ID assigned by the server, mapping the GUID to a client Locally Unique IDentifier, LUID, for the data item, wherein the LUID is included in the synchronization request message and comprises a data ID assigned by the first client, and adding the ID mapping, including the GUID and client LUID, to the change log table of the server; and if a data item indicated by the synchronization information exists: ending, by the server, data synchronization by transmitting a synchronization response message comprising status information indicating that the data is already synchronized to the first client, wherein the change log table of the server comprises a GUID field for storing GUIDs of data items, and a number of client LUID fields, corresponding to respective devices of the three or more networked devices other than the server, for storing client LUIDs of data items, and wherein the ending of the data synchronization comprises: deleting a change log of the data item; and creating the synchronization response message comprising the status information which indicates the data item is already synchronized. determining, by the server, whether a data item indicated by synchronization information comprised in a synchronization request message exists in a change log table of the server, when the server receives the synchronization request message transmitted by a first client; if a data item indicated by the synchronization information does not exist: updating, by the server the change log table by adding the data item indicated by the synchronization information and transmitting a synchronization request message comprising a data identifier and change log of the data item to a second client, creating, by the server a Globally Unique IDentifier, GUID, for the data item, wherein the GUID comprises a data ID assigned by the server, mapping the GUID to a client Locally Unique IDentifier, LUID, for the data item, wherein the LUID is included in the synchronization request message and comprises a data ID assigned by the first client, and adding the ID mapping, including the GUID and client LUID, to the change log table of the server; and updating, by the server the change log table by adding the data item indicated by the synchronization information and transmitting a synchronization request message comprising a data identifier and change log of the data item to a second client, creating, by the server a Globally Unique IDentifier, GUID, for the data item, wherein the GUID comprises a data ID assigned by the server, mapping the GUID to a client Locally Unique IDentifier, LUID, for the data item, wherein the LUID is included in the synchronization request message and comprises a data ID assigned by the first client, and adding the ID mapping, including the GUID and client LUID, to the change log table of the server; and if a data item indicated by the synchronization information exists: ending, by the server, data synchronization by transmitting a synchronization response message comprising status information indicating that the data is already synchronized to the first client, ending, by the server, data synchronization by transmitting a synchronization response message comprising status information indicating that the data is already synchronized to the first client, wherein the change log table of the server comprises a GUID field for storing GUIDs of data items, and a number of client LUID fields, corresponding to respective devices of the three or more networked devices other than the server, for storing client LUIDs of data items, and wherein the ending of the data synchronization comprises: deleting a change log of the data item; and creating the synchronization response message comprising the status information which indicates the data item is already synchronized. deleting a change log of the data item; and creating the synchronization response message comprising the status information which indicates the data item is already synchronized.