Introduction to the JES Common Coupling Services

The MVS cross-system coupling facility (XCF) allows up to 32 MVS systems to communicate in a single-image environment. That is, although there are from 1 to 32 MVS systems operating, the communications and control of those systems can be accomplished as if the individual systems were a single system. This single-image environment is an MVS sysplex. Within the MVS sysplex, you can define one or more JES XCF groups. A JES XCF group is a set of JES subsystems that communicate using the JES common coupling services component (JES XCF). JES XCF uses XCF as its communication vehicle. In a JES2 environment, the JES XCF group is a multi-access spool configuration (MAS), and in a JES3 environment, the JES XCF group is a JES3 complex.

The JES common coupling services component provides macros that enable communication among JES members of a sysplex. The macros are available to either JES2 or JES3. JES2 uses the service for communication of checkpoint data set information and during checkpoint reconfiguration processing. JES3 uses the JES common coupling service component as a replacement for its channel-to-channel (CTC) communication among JES3 mains. In addition to the macros, the JES common coupling services component also provides exits that can be used to monitor or tailor messages sent among the JESes; these exits are also available and used by JES2 and JES3.

Refer to z/OS MVS Setting Up a Sysplex for an in-depth discussion of the sysplex and its management.

The JES common coupling services component:

Provides a communication mechanism to JES members (JES2 multi-access spool members or JES3 mains) to exchange data and communicate by exploiting the MVS XCF component.
Enables a JES2 exit routine, running in three of the four JES execution environments (JES2 maintask, JES2 subtask, or user) to send messages to a JES2 exit in another processor control element (PCE) running on the same JES2 or on a different JES2.
Enables a JES3 exit routine, running in any of the four JES execution environments (JES3 maintask, JES3 subtask, user, or FSS) to send messages to a dynamic support program (DSP) or function control table (FCT) running on the same JES3 or on a different JES3.
Enables a JES3 DSP to send messages to a DSP or FCT running on the same JES3 or on a different JES3.
Enables a JES dispatchable unit (JDU) to request that it be notified about XCF-monitored system events. JDU is a generic term for JES2 PCEs and JES3 DSPs and FCTs.

As each JES member is initialized, it automatically joins a JES XCF group. After all JES members have successfully joined an XCF group, JES exit routines or JES3 DSPs running on that JES member can use the JES common coupling services to communicate with the other JES members in the same JES XCF group.

IBM recommends that your JES subsystems map in a one-to-one relationship with the MVS sysplex members. That is, if you have 9 MVS systems comprising your sysplex, then each of the 9 primary JES subsystems should then form a JES XCF group as in Figure 1. Configuring your JES subsystems and your MVS systems in this way will ease your overall sysplex management task.

Figure 1. A JES XCF group and its relationship to an MVS sysplex

There are a number of restrictions that apply to JES XCF groups and their use. These include:

The JES common coupling services component and its exits and macros are not available outside either a JES2 or JES3 environment.
The JES systems in the JES XCF group must be either all JES2 or all JES3; do not mix JES2 and JES3 systems.
All JES2 multi-access spool (MAS) members or JES3 complex mains must join JES XCF if they are to communicate.
All JES2 MAS members or JES3 mains must be members of the same sysplex.
All participating JES members must be in the same JES XCF group.