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

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Figure 6– Aggregating Servers

In this case, each of the Serversreceives requests and creates its own audit log entry for them. Each entry is identified by its own audit id and contains its own Auditinginformation. It also includes the name of the Clientthat issued the service request and the Clientaudit entry id received in the request. The Serverthen passes the audit id of the entry it just created to the next Serverin the chain.

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

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