Client Redundancy is supported in OPC UA by the TransferSubscriptions Service and by exposing Client information in the Server diagnostic information. Since Subscription lifetime is not tied to the Session in which it was created, backup Clients may use standard diagnostic information available to monitor the active Client’s Session with the Server. Upon detection of an active Client failure, a backup Client would then instruct the Server to transfer the Subscriptions to its own session. If the Subscription is crafted carefully, with sufficient resources to buffer data during the change-over, data loss from a Client Failover can be prevented.
OPC UA does not provide a standardized mechanism for conveying the SessionId and SubscriptionIds from the active Client to the backup Clients, but as long as the backup Clients know the Client name of the active Client, this information is readily available using the SessionDiagnostics and SubscriptionDiagnostics portions of the ServerDiagnostics data. This information is available for authorized users and for the user active on the Session. TransferSubscriptions requires the same user on all redundant Clients to succeed.