Abstract text

An active device attaches a sequential number representing a notification number to update record information for recording an event that data is stored, then notifies a preliminary device of the update record information, and, accumulates the update record information. The active device again makes the notification. The preliminary device monitors the update record information of which the active device notifies and detects, based on regularity of the sequential numbers attached to the respective pieces of update record information, missing reception of the update record information. When detecting the missing reception of the update record information, the preliminary device makes a request for retransmitting the update record information and notifies of information about matching data stored in a first storage unit of the active device with data stored in a second storage unit of the preliminary device.