Message sending and receiving

Figure 28 shows the relationships between entities when messages are transmitted, and shows the flow of control.

Figure 28. Distributed queue management model
The figure shows the distributed queue management model. User exits, Message Channel Agents (MCAs), and the Channel control function, are the starred components, and can use the MQI, as described in the text following the diagram. Messages are operated on according to user exits by the receiving MCA, and received to the local queues. The MCA receives commands from the channel control function, which also sends instructions to the sending MCA. Messages are received from and sent to the communications network.

Notes:
  1. There is one MCA per channel, depending on the platform. There may be one or more channel control functions for a given queue manager.
  2. The implementation of MCAs and channel control functions is highly platform dependent; they may be programs or processes or threads, and they may be a single entity or many comprising several independent or linked parts.
  3. All components marked with a star can use the MQI.