Message queue overflow protection overview

About queue overflow

IMS message queues have a limit on the number of messages that they can contain. The IMS Queue Manager can detect when this limit is reached and will respond to this limit by shutting down IMS (UABEND 758). This condition is known as queue overflow.

The prevention of an IMS shutdown due to a queue overflow condition is a vital issue in a production environment. IMS Queue Control Facility has the capability to prevent a queue overflow condition.

IMS Queue Control Facility and queue overflow protection

IMS Queue Control Facility implements queue overflow protection by monitoring the message queue usage. IMS Queue Control Facility analyzes the message queue usage to determine when to send an alert or to take action to prevent the excessive queue usage from continuing.

You can configure queue overflow protection by specifying various parameters to specify partition values, set queue usage limits, and specify appropriate alerts and actions.

In this discussion, the term queue overflow protection is equivalent to the term queue space usage notification (QSUN).

Queue overflow protection methods

IMS Queue Control Facility provides three methods for implementing queue overflow protection. They are listed in the order of their development through progressive releases of IMS Queue Control Facility. Each subsequent method was designed to provide more capability and flexibility in configuring queue overflow protection.

• Type 1: Queue space notification exit (DFSQSPC0)

  Type 1 protection is the original mechanism for providing basic overflow protection and has the most limited capabilities.

  The Queue Manager DFSQSPC0 user exit provides analysis to detect excessive queue activity for a specific IMS entity.

  Actions that can be implemented when these partition boundaries are exceeded include notifications messages.

• Type 2: Queue overflow protection using threshold settings to define queue space partitions

  Type 2 protection provides more advanced overflow protection for IMS Queue Control Facility than type 1 protection.

  Type 2 protection provides analysis to detect excessive queue activity for total and specific IMS entities.

  Four analysis boundaries are defined using the IMS threshold template.

  Actions that can be implemented when these partition boundaries are exceeded include WAIT, WTO, STOP, and UNLOAD.

  Type 2 overflow protection using threshold partitions is the default protection method for a newly installed version of IMS Queue Control Facility.

• Type 3: Queue overflow protection using area and failsafe settings to define queue space partitions

  Type 3 protection provides the most advanced overflow protection for IMS Queue Control Facility.

  Type 3 protection provides analysis to detect excessive queue activity for total and specific IMS entities.

  Area protection analyzes specific queue usage. Analysis is performed on the accumulation of the queue activity due to a specific IMS process.

  Failsafe protection analyzes total queue utilization. Analysis is performed on the accumulation of the queue activity due to all IMS processes.

  A possible total of twenty analysis boundaries can be defined using AREA and FAILSAFE configuration statements.

  Area and failsafe protection can be used concurrently.

  Actions that can be implemented when these partition boundaries are exceeded include WAIT, WTO, STOP, and ABEND.

  Type 3 overflow protection is implemented by entering AREA and FAILSAFE configuration settings in the IMS Queue Control Facility extensions PROCLIB member.

  Type 2 overflow protection using threshold partitions is the default protection method for a newly installed version of IMS Queue Control Facility because there are no default type 3 AREA and FAILSAFE configuration settings specified in the extensions member.

Queue space partition methods

Implementing IMS Queue Control Facility queue overflow protection requires that the IMS queue space be logically divided into partitions.

These logical partitions allows IMS Queue Control Facility to monitor varying degrees of IMS queue usage. Partitions allow IMS Queue Control Facility to analyze the total queue usage, and identify the cause or source of the excessive queue usage. Partitions are specified as a percentage of the total queue space.

There are two methods available for defining the format of these partitions:

• IMS Queue Control Facility allows you to define partitions using threshold settings

  This partition method is used for type 2 queue overflow protection.

  You can configure threshold settings that divide the queue space into four logical partitions.

• IMS Queue Control Facility allows you to define partitions using area and failsafe settings

  This partition method is used for type 3 queue overflow protection.

  You can configure area and failsafe settings that divide the queue space into a maximum of ten logical partitions (or areas) for each area and failsafe group (for a total of 20 partitions).