Each JES member is in constant communication with every other JES member of its JES2 MAS or JES3 complex. In JES2, the checkpoint data set is being continuously updated for job and output data. In JES3, there is a continuous flow of allocation requests, system status, job status, work requests, and so forth that is needed to properly coordinate the work JES3 is putting into and taking out of the MVS system. These pieces of communicated data are considered messages, and each causes the invocation of two JES XCF exits: the transport exit and the receive exit.

Exit IXZXIT01, called the transport exit, gets control whenever JES or installation code sends a message. JES XCF packages the message data in an envelope that contains the address information and other related data, and sends it to a specified mailbox.

Refer to Figure 1 for a graphic view of message processing.

Figure 1. JES message processing and its relationship to JES XCF processing

You have the option of adding code to exit IXZXIT01 to alter the JES message processing. You can change the message text, add to the message text, or reroute the message to another member. This ability can be an extremely powerful tool that you can use to monitor member activity.

In addition, you can use standard JES exits and JES3 DSPs to invoke the JES XCF macros to build, clear, and delete mailboxes; send, receive, and acknowledge messages; and request member information during normal JES processing.

Once a message is sent to a mailbox (on the same member or another), the receiving member is informed that mail is waiting. After the receiving member issues the receive macro, IXZXIXRM, the receive message exit (IXZXIT02) gets control. This exit allows the receiving member to view or modify the message and to receive any extents that have been added to it by the transport exit. Once the message is received, the receiving member acknowledges the mail, informs JES XCF of its receipt and, if requested by the sending member, also returns an acknowledgement to the sender.