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:

1. 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),
2. are chained in a layered architecture of Serversto provide:
3. aggregation of data from lower-layer Servers,
4. higher-layer data constructs to Clients, and
5. 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