Client Redundancyis supported in OPC UA by the TransferSubscriptions Serviceand by exposing Clientinformation in the Serverdiagnostic information. Since Subscriptionlifetime is not tied to the Sessionin which it was created, backup Clientsmay use standard diagnostic information available to monitor the active Client’s Sessionwith the Server. Upon detection of an active Clientfailure, a backup Clientwould then instruct the Serverto transfer the Subscriptionsto its own session. If the Subscriptionis crafted carefully, with sufficient resources to buffer data during the change-over, data loss from a Client Failovercan be prevented.

OPC UA does not provide a standardized mechanism for conveying the SessionIdand SubscriptionIdsfrom the active Clientto the backup Clients, but as long as the backup Clientsknow the Clientname of the active Client, this information is readily available using the SessionDiagnosticsand SubscriptionDiagnosticsportions of the ServerDiagnosticsdata. This information is available for authorized users and for the user active on the Session. TransferSubscriptionsrequires the same user on all redundant Clientsto succeed.