Serverto Serverinteractions in the Client Servermodel are interactions in which one Serveracts as a Clientof another Server. Serverto Serverinteractions allow for the development of servers that:
exchange information with each other on a peer-to-peer basis, this could include redundancy or remote Serversthat are used for maintaining system wide type definitions (see Figure 5),
are chained in a layered architecture of Serversto provide:
aggregation of data from lower-layer Servers,
higher-layer data constructs to Clients, and
concentrator interfaces to Clientsfor single points of access to multiple underlying Servers.
Figure 5illustrates interactions between Servers.
Figure 5– Peer-to-peer interactions between Servers
Similar peer-to-peer interactions can also be accomplished using the OPC UA PubSubmodel where each peer Applicationis both a Publisherand a Subscriber.
Figure 6extends the previous example and illustrates the chaining of Serverstogether for vertical access to data in an enterprise.
Figure 6– Chained Server example