In recent years, a database system has been effectively used to process a huge amount of digital information. To improve the reliability of the database system, a database duplexing configuration is employed in some cases. For example, the publication of Japanese Unexamined Patent Application Publication No. 2002-202906 discloses a system which achieves database duplexing by matching the contents of the database in the first server and the contents of the database in the second server connected to the first server. According to the system of the publication of Japanese Unexamined Patent Application Publication No. 2002-202906, if the copying of information from the first server to the second server fails due to a communication failure between the servers, the information is stored in a difference file. Then, upon recovery, the difference file is transferred to the second server, to thereby reflect the unsuccessfully copied information in the server.
Meanwhile, to guarantee the consistency between two databases, one of the databases needs to be updated in accordance with the order of updates in the other database. However, if an error, such as a failure of a storage area or a file loss, occurs and is later eliminated, the continuity of the updated contents in the database may be interrupted, as in a case in which a change in order occurs between the update performed before the occurrence of the error and the update performed after the elimination of the error.