The discovery process allows OPC UA Applications to find other OPC UA Applications on the network and then discover how to connect to them. Note that this discussion builds on the discovery related concepts defined in OPC 10000-4. Discoverable applications are generally Servers; however, some Clients will support reverse connections as described in OPC 10000-6 which allows Servers to be able to discover them. OPC UA Applications can exist on hosts with a LocalDiscoveryServer (see 4.2.2) or on hosts with a dedicated Server (see 4.2.3).
Clients and Servers can be on the same host, on different hosts in the same subnet, or even on completely different locations in an administrative domain. The following clauses describe the different configurations and how discovery can be accomplished.
The mechanisms for Clients to discover Servers are specified in 4.3.
The mechanisms for Servers to make themselves discoverable are specified in 4.2.
The Discovery Services are specified in OPC 10000-4. They are implemented by individual Servers and by dedicated DiscoveryServers. The following dedicated DiscoveryServers provide a way for applications to discover registered OPC UA Application in different situations:
- A LocalDiscoveryServer (LDS) maintains discovery information for all applications that have registered with it, usually all applications available on the host that it runs on.
- A LocalDiscoveryServer with the MulticastExtension (LDS-ME) maintains discovery information for all applications that have been announced on the local MulticastSubnet.
- A GlobalDiscoveryServer (GDS) maintains discovery information for applications available in an administrative domain.
LDS and LDS-ME are specified in Clause 5. The GDS is specified in Clause 6.