In the Client Servercommunication model the application exposing information consisting of physical and software objects is the OPC UA Serverand the application operationg upon this information is the OPC UA Client.

The information provided by an OPC UA Serveris organized in the Server Address Space. Serviceslike Read, Writeand Browseare available with a request/response pattern used by OPC UA Clientsto access information provided by an OPC UA Server.

Every Clientcreates individual Sessions, Subscriptionsand MonitoredItemswhich are not shared with other Clients. In other words, the data that is published only goes to the Clientthat created the Subscription.

Sessionsare used to manage the communication relationship between Clientand Server. MonitoredItemsrepresent the settings used to subscribe to Eventsand Variable Valuedata changes from the OPC UA Server Address Space. MonitoredItemsare grouped in Subscriptions.

The entities used by OPC UA Clientsto subscribe to information from an OPC UA Serverare illustrated in Figure B.1.

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Figure B.1– Subscriptions in OPC UA Client Server model

In this model the Clientis the active entity. It chooses what Nodesof the Server AddressSpaceand what Servicesto use. Subscriptionsare created or deleted on the fly. The published data only goes to the Clientthat created a Subscription.

The Client Server Subscriptionmodel provides reliable delivery using buffering, acknowledgements, and retransmissions. This requires resources in the Serverfor each connectedClient.

Resource-constrained Serverslimit the number of parallel Clientconnections, Subscriptions, and MonitoredItems. Similar limitations can also occur in the Client. Clientsthat continuously need data from a larger number of Serversalso consume significant resources.