Figure 6 illustrates the case of a Client accessing services from an aggregating Server. An aggregating Server is a Server that provides its services by accessing services of other OPC UA Servers, referred to as lower layer-Servers.


Figure 6 – Aggregating Servers

In this case, each of the Servers receives requests and creates its own audit log entry for them. Each entry is identified by its own audit id and contains its own Auditing information. It also includes the name of the Client that issued the service request and the Client audit entry id received in the request. The Server then passes the audit id of the entry it just created to the next Server in the chain.

Using this information, an auditor can inspect the Server’s log entries and relate them back to their associated Client entries.

In most cases, the Servers will only generate Audit Events, but these Audit Events will still contain the same information as the audit log records. In the case of aggregating Servers, a Server would also be required to subscribe for Audit Events from the Servers it is aggregating. In this manner, Server “B” would be able to provide all of the Audit Events to Client “A”, including the Events generated by Server “C” and Server “D”.