Actions allow a request response message exchange pattern to be used via a Message Oriented Middleware. The entities involved in Actions are shown in Figure 9.

Actions are operations executed by a Responder when it receives a request message sent by a Requestor. An Action could be a Method in an OPC UA Address Space or business logic in an OPC UA PubSub application.

The ActionMetaData consists of the DataSetMetaData for the request message, the DataSetMetaData for the response message and a list of Action targets that share the same request and response parameters. An example is a Method defined on an ObjectType that can be called on different Object instances. In this case the Method on the ObjectType is the Action and the Methods on the different Object instances are the Action targets. The ActionMetaData as a contract is provided by the Responder and used by the Requestor to execute Actions on the Responder.

Figure 9 – Action execution sequence

The content of the ActionMetaData and the mapping to Methods is defined in 6.2.3.10. The ActionMetaData message for UADP message mapping is defined in 7.2.4.6.11. The ActionMetaData message for JSON message mapping is definied in 7.2.5.5.7.

The information flow, sequence diagrams and status handling for Actions is defined in 6.2.11.2.

The Action request and response messages for UADP message mapping are definied in 7.2.4.5.9 and 7.2.4.5.10. The Action NetworkMessage, request and response messages for JSON message mapping are defined in 7.2.5.6.

The MQTT Topic levels for Action messages are defined in 7.3.5.7.9, 7.3.5.7.10, 7.3.5.7.11 and 7.3.5.7.12.