A fault-tolerant client-server system has a primary server, a backup server; and a client. The client sends a request to the primary server, which receives and processes the request, including sending the response to the client, independent of any backup processing. The response includes the primary server state information. The primary server also performs backup processing that includes periodically sending the primary server state information to the backup server. The client receives the response from the primary server, and sends the primary server state information to the backup server. The primary server state information includes all request-reply pairs that the primary server has handled since a most recent transmission of primary server state information from the primary server to the backup server. The primary server's backup processing may be activated periodically based on a predetermined time interval. Alternatively, it may be activated when the primary server's memory for storing the primary server state information is filled to a predetermined amount.