IMS Shared Message Queue Help

IMS uses Coupling Facility list structures for shared queues. There are two types of IMS list structures: the full-function (MSGQ) structure and the Fast Path (EMHQ) structure. The MSGQ is always required. The EMHQ is only required if IMS has been generated as a Fast Path system (includes the FPCTRL macro).

The primary list structure contains the shared queues. The overflow list structure, if defined, contains shared queues that overflow after the primary reaches a predefined (use specified) threshold.

The IMS shared message queue (SMQ) solution allows IMS customers to take advantage of the coupling facility to store and share the IMS message queues among multiple IMS Transaction Management systems. Incoming messages from an IMS TM on one central processor complex (CPC) in the sysplex could be placed on a shared queue in the coupling facility by IMS Common Queue Server (CQS) for processing by an IMS TM on another in the sysplex. This can provide increased capacity and availability their IMS systems.

The IMS SMQ solution allows IMS customers to take advantage of the sysplex environment, enabling increased capacity and incremental horizontal growth.

SMQ(Shared-queued) structures are persistent. They remain allocated even if all CQSs have disconnected, or all IMSs and their associated CQSs cold start.

All logical records include the full message prefix. The size of the prefix depends on the IMS release and the functions available within this IMS. It is recommended that the sizings used for this tool come from an IMS Version 6 system. Otherwise, you must take into account prefixes change size from version to version and can influence the stored length of the message.

One other consideration in sizing the IMS SMQ structure is conversational messages. For IMS conversations, the SPA is stored as the first segment of the message on the message queue when destined for a transaction program. This may affect the structure size for installations with many conversations since the SPA can be very large and remains in the structure during interactions of the conversation. A conversation can hang around days at a time in some cases.

Event Monitor Controls - an EMC entry is a control block in the CF list structure that is used to represent a CQS that has a registered interest in a queue. Each EMC entry occupies 64 bytes of storage. CQS reserves 20% of the total structure size for EMCs. If CFCC level 4 or higher is being used (CFSIZER uses CFCC level 8), the area dynamically increases in 5% increments up to a maximum of 50%, if required.

Available data elements are used to create additional storage for EMCs. If the EMC area is increased, the amount of structure space available for list entries decreases. If the EMC area runs out of space, any attempt to register an interest in a queue fails.

The size of the IMS SMQ list structure is therefore a function of the number of list headers, list elements, data elements, message size, conversational messages, SPA and EMCs.

The space you want to allocate for the storage of the data elements is calculated as the amount that would be occupied by the data content of the transactions and messages in the coupling facility list structure.

The IMSPARS Internal Resource Usage Report can be used to find the short-message and long-message high water marks.

Short Message High Water Mark

The highest number of messages that was placed on the short message queue during the time period the data was captured. The IMS Performance Analyzer (IMSPA) Message Queue Pool Statics report can be used to find the short message high water mark (the “Highest SMSG Block Number ever used” field). If IMSPA is not available, the short message high water mark can be derived from the IMS system checkpoint record 4502.

Long Message High Water Mark

The highest number of messages that was placed on the long message queue during the time period the data was captured. The IMS Performance Analyzer (IMSPA) Message Queue Pool Statics report can be used to find the long message high water mark (the “Highest LMSG Block Number ever used” field). If IMSPA is not available, the long message high water mark can be derived from the IMS system checkpoint record 4502.

SHMSG LRECL

Short message logical record length. This can be obtained from the logical record length of the short-message dataset.

LGMSG LRECL

Long message logical record length. This can be obtained from the logical record length of the long-message dataset.