A GDS is an OPC UA Server which allows Clients to search for Servers within the administrative domain of the GDS. It provides Methods that allow applications to search for other applications (See 6). To access the GDS, the Client uses the Call service to invoke the QueryApplications Method (see 6.6.11) to retrieve a list of Servers that meet the filter criteria provided. The QueryApplications Method is similar to the FindServers service except that it provides more advanced search and filter criteria. The discovery process is illustrated in Figure 5.

Figure 5 – The Global Discovery Process

The GDS may be coupled with any of the previous network architectures. For each MulticastSubnet, one or more LDSs may be registered with a GDS.

The Client can also be configured with the URL of the GDS using an out of band mechanism.

The complete discovery process is shown in Figure 6.