Before discussing the specifics of how you can affect JES message processing, you need a basic understanding of the terminology used in this document and of the underlying concepts of JES XCF.

In general, the JES common coupling services allow JES members to communicate, but they also allow you to affect that communication. When each JES member of either your JES2 MAS or JES3 complex initialize, it is automatically attached to the JES XCF group. The members of the group will be the same set of members in your MAS or complex. When using the macros, you need to use the group token that JES XCF provides during attach processing to identify the member and group. The group token is the unique identifier for a particular member of a group.

The term message refers to data that is transported between members of the JES XCF group. Messages also include XCF events. An XCF event includes any XCF events on any JES member of the JES XCF group and the MVS system under which it runs. The messages are transported by XCF and JES XCF from one JDU to another. If the quantity of data being sent is larger than 60 kilobytes, you have the ability to break the entire set of data into parts and transport it as a multi-segment message.

Acknowledgements are required by JES XCF for every message that is received. This is required so that JES XCF knows when it can free whatever resources it held for that message. Also, the message sender can request that an acknowledgement be returned by the receiver. The sender can then base further processing on that acknowledgement. This can be valuable if you chose to send messages in an asynchronous manner such that the acknowledgement is used to confirm the future delivery or reception of the message at the destination member without causing the sending member to wait for the response.

Messages are packaged in an envelope that contains the source and destination address as well as other data concerning the message data. The entire package is then sent to and placed in the designated mailbox. A mailbox is a named, logical queue of ordered messages that is maintained by JES XCF to hold messages that have been sent to a JDU but have not yet been received by that JDU. Should the message not be delivered or received as expected, the original sender will receive return and reason codes from several sources, including JES XCF and the receiver.

Using the JES XCF exits and macros details the use of the JES XCF exits and macros to performs specific tasks, and Coding the JES XCF exits details the use of the JES XCF exits to monitor and modify JES XCF message processing.