OPC UA Client Server interaction 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.
Sessions are defined as logical connections between Clients and Servers. Servers may limit the number of concurrent Sessions based on resource availability, licensing restrictions, or other constraints. Each Session is independent of the underlying communications protocols. Failures of these protocols do not automatically cause the Session to terminate. Sessions terminate based on Client or Server request, or based on inactivity of the Client. The inactivity time interval is negotiated during Session establishment.