The LocalDiscoveryServeris useful for networks where the host names can be discovered. However, this is typically not the case in large systems with multiple servers on multiple subnets. For this reason there is a need for an enterprise wide DiscoveryServercalled a GlobalDiscoveryServer. The GlobalDiscoveryServer (GDS) is an OPC UA Serverwhich allows Clientsto search for Serversin the administrative domain. It provides methods that allow applications to register themselves and to search for other applications.

The essential element of a GlobalDiscoveryServer(GDS) is that it can provide the Certificatemanagement services defined in Clause 7. These services can simplify Certificatemanagement even in medium to small systems, therefore, a GDS can be deployed in smaller systems. Different implementations are expected. Some of them will likely provide a front-end to an existing DirectoryServicesuch as LDAP (See Annex E). By standardizing on an OPC UA based interface, OPC UA Clientsdo not need to have knowledge of different DirectoryServices.

If an administrator registers a LocalDiscoveryServerwith the GDS, then the GDS shall periodically update its database by calling FindServersOnNetworkor FindServerson the LDS. Figure 7shows the relationship between a GDS and the LDS-ME or LDS.


Figure 7– The Relationship Between GDS and other components

The steps shown in Figure 7are:


The Server calls RegisterServer2on the LDS running on the same machine.


The administrator registers LDS-ME installations with the GDS.


The GDS calls FindServersOnNetworkon the LDS-ME to find all applications on the same MulticastSubnet.


The GDS creates a record for each application returned by the LDS-ME. These records shall be approved before they are made available to Clientsof the GDS. This approval can be obtained from an Administrator orthe GDS can connect to the Serverand verifies that it has a trusted Certfiicate.


The Clientcalls QueryServers Methodon the GDS to discover applications.

TheInformation Modelused for registration and discovery is shown in clause 6.2. Any Clientshall be able to call the QueryServers Methodto find applications known to GDS. The complete definitions for each of the types used are described in clause 7.5.