OPC UA Client Serverinteraction requires a stateful model. The state information is maintained inside an application Session. Examples of state-information are Subscriptions, user credentials and continuation points for operations that span multiple requests.

Sessionsare defined as logical connections between Clientsand Servers. Serversmay limit the number of concurrent Sessionsbased on resource availability, licensing restrictions, or other constraints. Each Sessionis independent of the underlying communications protocols. Failures of these protocols do not automatically cause the Sessionto terminate. Sessionsterminate based on Clientor Serverrequest, or based on inactivity of the Client. The inactivity time interval is negotiated during Sessionestablishment.