IMS Queue Manager

The IMS Queue Manager manages the IMS message queues. The Queue Manager data sets, SHMSG and LMSG, are not used to back up messages in a shared-queues environment.

However, in order to reduce necessary changes during IMS system definition, the MSGQUEUE macro is still required; the data sets are defined, but they are never allocated or opened.

The Queue Manager uses a number of real storage queue buffers to store the messages prior to placing them in the shared queues on the coupling facility. However, if the message needs to remain in the local IMS rather than being placed on the shared queues, the message is placed on a local queue. The local queues are also managed by the Queue Manager.

Messages that are placed on the shared queues are recoverable. Because the message queue data sets are not used, a message on a local queue is only recoverable if the log record for the message was written to the IMS log after the oldest of the last two checkpoints was taken.

In an XRF environment, the local queue manager data sets (QBLKSL, SHMSGL, and LGMSGL) are used. While the XRF alternate subsystem is tracking, local messages are placed in the local message queue data sets (messages for the active subsystem are on the shared queues). When the XRF alternate subsystem takes over the active subsystem, any messages in the local message queue data sets are merged with those on the shared queues.