List structures

CQS manages message queues that are shared within an IMSplex by using coupling facility list structures. A primary list structure holds the message queues. An overflow list structure, if you define one, holds additional queues after the primary list structure reaches a predefined threshold.

For IMS systems that do not share Fast Path transactions, define only message queue primary structures and optional message queue overflow structures. For IMS systems that do share Fast Path transactions, define both message queue primary (and optional overflow) structures and expedited message handler queue (EMHQ) primary (and optional overflow) structures.

Figure 1 and Figure 2 show shared queues for IMS clients A and B.

In the following figure, only message queue structures, message queue structure recovery data sets (SRDSs), message queue checkpoint data sets, and a message queue z/OS® log stream are used.

Figure 1. Shared-queues environment with its message queue (MSGQ) list structures

The configuration shown in the following figure shows EMHQ structures, along with their corresponding SRDSs, checkpoint data sets, and a z/OS log stream. A configuration such as this one would also include message queues, but these are not shown in the figure.

Figure 2. Shared-queues environment with its EMHQ list structures