The subscription mechanism enables OPC Client applications to receive multiple updates from OPC Servers. When an OPC Client application (HMI, Historian, etc) subscribes to OPC Items, it receives all updates as soon as the OPC Server detects the change.

To implement subscription, DCOM-based OPC servers use callbacks, and the call is thus asynchronous.

OPC UA Client applications implement subscription using a poll-report-by-exception mechanism. Thus, the OPC UA Client application polls the OPC Server once, and receives a result for all the item changes. This mechanism also enables OPC UA Client applications to implement a heartbeat, which enables them to know when they lose their client/server connection.

See also: Asynchronous, Callback