Processing overview

An overview of how MCAs use channel-exit programs.

On startup, the MCAs exchange a startup dialog to synchronize processing. Then they switch to a data exchange that includes the security exits. These exits must end successfully for the startup phase to complete and to allow messages to be transferred.

The security check phase is a loop, as shown in Figure 1.
Figure 1. Security exit loop

During the message transfer phase, the sending MCA gets messages from a transmission queue, calls the message exit, calls the send exit, and then sends the message to the receiving MCA, as shown in Figure 2.
Figure 2. Example of a send exit at the sender end of message channel

Figure 3. Example of a receive exit at the receiver end of message channel

The receiving MCA receives a message from the communications link, calls the receive exit, calls the message exit, and then puts the message on the local queue, as shown in Figure 3. (The receive exit can be called more than once before the message exit is called.)