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Shared queue recovery

Use this topic to understand IBM MQ recovery and resilience of various components in the queue sharing group environment.


Transactional recovery

When an application issues a MQBACK call or terminates abnormally (for example, because of an EXEC CICS ROLLBACK or an IMS abend) thread-level information stored in the queue manager ensures that the in-flight unit of work is rolled back. MQPUT and MQGET operations within syncpoint on shared queues are rolled back in the same way as updates to non-shared queues.


Peer recovery

If a queue manager fails, it disconnects abnormally from the coupling facility structures that it is currently connected to. If the connection between the z/OS instance and the coupling facility fails (for example, physical link failure or power-off of a coupling facility or partition) this is also detected as an abnormal termination of the connection between the queue manager and the coupling facility structures involved. Other queue managers in the same queue sharing group that remain connected to that structure detect the abnormal disconnection and all attempt to initiate peer recovery for the failed queue manager on that structure. Only one of these queue managers initiates peer recovery successfully, but all the other queue managers cooperate in the recovery of units of work that were owned by the queue manager that failed.

If a queue manager fails when there are no peers connected to a structure, recovery is performed when another queue manager connects to that structure, or when the queue manager that failed restarts.

Peer recovery, often referred to as Peer Level Recovery (PLR), is performed on a structure by structure basis and it is possible for a single queue manager to participate in the recovery of more than one structure at the same time. However, the set of peers cooperating in the recovery of different structures might vary depending on which queue managers were connected to the different structures at the time of failure.

When the failed queue manager restarts, it reconnects to the structures that it was connected to at the time of failure, and recovers any remaining unresolved units of work that were not recovered by peer recovery.

Peer recovery is a multi-phase process. During the first phase, units of work that had progressed beyond the in-flight phase are recovered; this might involve committing messages for units of work that are in-commit and locking messages for units of work that are in-doubt. During the second phase, queues that had threads active against them in the failing queue manager are checked, uncommitted messages related to in-flight units of work are rolled back, and information about active handles on shared queues in the failed queue manager are reset. This means that IBM MQ resets any indicators that the failing queue manager had a shared queue open for input-exclusive, allowing other active queue managers to open the queue for input.


Shared queue definitions

The queue objects that represent the attributes of a shared queue are held in the shared Db2 repository used by the queue sharing group. Ensure that adequate procedures are in place for the backup and recovery of the Db2 tables used to hold IBM MQ objects. We can also use the IBM MQ CSQUTIL utility to create MQSC commands for replay into a queue manager to redefine IBM MQ objects, including shared queue and group definitions stored in Db2.


Logging

Queue sharing groups can support persistent messages, because the messages on shared queues can be logged in the queue manager logs.


Coupling facility and structure failures

There are two types of failure that can be reported for a coupling facility (CF) structure: structure failure and loss of connectivity. Sysplex services for data sharing (XES) inform IBM MQ of a CF structure failure or a CF failure with a structure failure event. If XES creates a loss of connectivity event this does not necessarily indicate that there is a problem with the structure, it might be that there is no connection available to communicate with the structure. It is possible that not all of the queue managers receive a loss of connectivity event for the structure; it depends on the configuration of connections to the CF. A loss of connectivity event can also be received because of operator commands, for example VARY PATH OFFLINE or CONFIG CHP OFFLINE.

The CF structures that are used by IBM MQ can be configured to use system-managed duplexing. This means that if there is a single failure, system-managed failover processing hides the failure of a structure or the loss of connectivity, and the queue manager is not informed of the failure. If there is a failure of both instances of a duplexed structure or connection, the queue manager receives the appropriate event and handles it in the same way as a failure event for a simplex structure. Details of how the queue manager handles the events are described in Scenarios.

In the unlikely event of a CF or structure failure, any nonpersistent messages stored in the affected application structures are lost. We can recover persistent messages using the RECOVER CFSTRUCT command. If a recoverable application structure has failed, any further application activity to this structure is prevented until the structure has been recovered.

To ensure that we can recover a CF structure in a reasonable period of time, take frequent backups, using the BACKUP CFSTRUCT command. We can choose to perform the backups on any queue managers in the queue sharing group or dedicate one queue manager to perform all the backups. Automate the process of taking backups to ensure that they are taken on a regular basis.

Each backup is written to the active log data set of the queue manager taking the backup. The shared queue Db2 repository records the name of the CF structure being backed up, the name of the queue manager doing the backup, the RBA range for this backup on that queue manager's log, and the backup time.

The administration structure contains information about incomplete units of work on shared queues at the time of any application structure failure so the administration structure must be available during RECOVER CFSTRUCT processing. If the administration structure has failed, all the queue managers in the queue sharing group must have rebuilt their administration structure entries before we can issue the RECOVER CFSTRUCT command.

Queue managers rebuild their administration structure entries automatically and without terminating. If a queue manager is not running at the time of the failure, its administration structure entries can be rebuilt by another queue manager in the queue sharing group that is running at the same or higher level.

To recover an application structure, issue a RECOVER CFSTRUCT command to the queue manager that we want to perform the recovery. We can recover a single CF structure or we can recover several CF structures simultaneously. We can recover using any queue manager in the queue sharing group, it does not have to be the one that performed the backup, or one that has been connected previously to the failed structure.

The RECOVER CFSTRUCT command uses the backup, located through the Db2 repository information ( Db2 must therefore be available on the queue manager where recovery is being carried out), and recovers this to the point of failure.

The RECOVER CFSTRUCT command does this by applying log records from every queue manager in the queue sharing group that has performed an MQPUT or MQGET between the start of the backup and the time of failure, to any shared queue that maps to the CF structure. The resulting merging of the logs might require reading a considerable amount of log data because all the log data written by participating queue managers since the backup is read. You are strongly recommended to make frequent (for example, hourly) backups, especially if there are large messages within the backup.


Structure failure scenarios

    Scenarios
    If a failure is reported for a CF structure, the action taken by connected queue managers depends on the following:

    • The type of failure reported by the XES component of z/OS to IBM MQ.
    • The structure type (application or administration)
    • The queue manager level
    • The CFLEVEL of the IBM MQ CFSTRUCT object (2, 3, 4 or 5. This is not the CFLEVEL of the CFCC microcode)
    • The RECAUTO attribute of an IBM MQ CFSTRUCT object at CFLEVEL(5)

    The following scenarios describe what happens when a failure is reported for the administration structure:

    • If a structure failure event is received for the administration structure, the structure is reallocated and rebuilt automatically without the queue manager terminating. A new instance of the structure is allocated by XES when a queue manager attempts to connect to it.

      When the queue manager has connected to the new instance of the structure, the queue manager writes the entries for itself into the structure. This processing is carried out by the queue manager and is not part of XES rebuild processing.

      If a queue manager was not running at the time of the failure, or terminates before recovery of its part of the administration structure has been completed, its administration structure entries can be rebuilt by another queue manager in the queue sharing group.

      Administration structure entries of a queue manager can only be rebuilt by another queue manager running at the same level or higher. If administration structure entries of a queue manager cannot be rebuilt by another queue manager in the queue sharing group, restart the queue manager so that it can complete the rebuild of its part of the structure.

      Certain actions are suspended until the administration structure entries for all queue managers have been rebuilt. The suspended actions include the following:

      • Opening and closing of shared queues.
      • Committing or backing out units of recovery.
      • Serialized applications connecting to or disconnecting from the queue manager.
      • Backing up or recovering an application structure.

      Any serialized applications that have already connected to the queue manager can continue processing. Any serialized application attempting to connect with the MQCNO_SERIALIZE_CONN_TAG_QSG or MQCNO_RESTRICT_CONN_TAG_QSG parameters receive the MQRC_CONN_TAG_NOT_USABLE return code.

      When the administration structure entries for the queue manager have been rebuilt, the suspended actions are resumed.

    The following scenarios describe what happens when a failure is reported for an application structure:

    • If a structure failure event is received for an application structure, and the CFLEVEL is 1 or 2, the queue manager terminates. Restart the queue manager. The first queue manager to attempt to connect to the structure again causes XES to allocate a new instance of the structure.
    • If a structure failure event is received for an application structure, and the CFLEVEL is 3, 4, or 5 the queue managers connected to the structure continue to run. Applications that do not use the queues in the failed structure can continue normal processing.

      However, applications that attempt operations on queues in the failed structure receive an MQRC_CF_STRUC_FAILED error until the structure has been successfully rebuilt, at which point the application can open the queues again.

      Structure rebuild is started automatically for CFLEVEL(5) application structures defined with RECAUTO(YES). Otherwise, the structure will be rebuilt when the RECOVER CFSTRUCT command is issued.


Resilience to coupling facility connectivity failures

    What is resilience to coupling facility connectivity failures?

    Resilience to coupling facility connectivity failures refers to the ability of queue managers in a queue sharing group to tolerate loss of connectivity to a coupling facility structure without terminating. This function also attempts to rebuild the structure in another coupling facility with better connectivity in order to regain access to shared queues as soon as possible.

    What is partial loss of connectivity?

    IBM MQ defines partial loss of connectivity as a situation where one or more systems in the sysplex lose connectivity to the coupling facility where the structure being accessed by the system is allocated, but at least one system in the sysplex maintains connectivity to the same coupling facility.

    What is total loss of connectivity?

    IBM MQ defines a total loss of connectivity as a situation where no systems in the sysplex have connectivity to the coupling facility and the structure allocated within it.

    Why would you enable this function?

    Resilience to coupling facility connectivity failures improves the availability of IBM MQ, allowing non-shared queues to remain available after a queue manager has lost connectivity to one or more coupling facility structures. Additionally, queue managers that lose connectivity to a coupling facility structure automatically attempt to rebuild the structure in another available coupling facility, improving the availability of the shared queues within the queue sharing group.

    Considerations when enabling this function

    A queue manager that tolerates loss of connectivity to coupling facility structures without terminating may not be able to reconnect to a coupling facility structure for some time if there is no alternative coupling facility available. Shared queues defined on a structure that has suffered loss of connectivity remain unavailable until connectivity to the structure is restored. In this situation, applications that connect into members of the queue sharing group in order to perform shared queue work may find that the shared queues they need to access are not available. To avoid this situation it is recommended that queue managers should be configured to terminate when connectivity to a coupling facility structure is lost. This termination forces applications to connect to another member of the queue sharing group that has connectivity to the coupling facility structures where the shared queues the application requires are defined.


Manage Resilience to coupling facility connectivity failures

    How do I enable this functionality?
    The following steps must be performed in order to enable resilience to coupling facility connectivity
    1. Ensure that the CFRM couple data set has been formatted to support system-managed rebuild. This allows queue managers to initiate a system-managed rebuild to re-create a structure into an available coupling facility. Use the DISPLAY XCF,COUPLE,TYPE=CFRM command to determine the format of the CFRM couple data set. To support system-managed rebuild, the CFRM couple data set should be formatted by specifying:
      "ITEM NAME(SMREBLD) NUMBER(1)"
      Refer to the z/OS MVS™ Setting Up a Sysplex documentation for more information on formatting a CFRM couple data set.
    2. Ensure that an alternative coupling facility is available and is in the CFRM preference list for all IBM MQ coupling facility structures. This enables the queue managers to attempt to rebuild structures into an alternative available coupling facility to restore access to the structures as soon as possible.

      IBM MQ structures must be defined with ENFORCEORDER(NO) in CFRM policy, so that XCF is able to choose the optimum CF in the configuration if IBM MQ needs to reallocate the structure.

      Refer to the z/OS MVS Setting Up a Sysplex documentation for more information about structure preference lists.

    3. Alter all application coupling facility structures that need to tolerate loss of connectivity to CFLEVEL(5). This is the minimum level that can tolerate a loss of connectivity.
    4. Determine the values required for the QMGR CFCONLOS and the CFSTRUCT CFCONLOS attributes and alter these accordingly. The QMGR CFCONLOS attribute controls whether loss of connectivity to the administration structure is tolerated, and the CFSTRUCT CFCONLOS attribute controls whether loss of connectivity is tolerated by each application coupling facility structure. If the default values for these attributes are retained, the queue manager terminates following loss of connectivity to any coupling facility structure.
    5. Determine the values required for the CFSTRUCT RECAUTO attribute for each application coupling facility structure, and alter these accordingly. This attribute controls whether coupling facility structures should be automatically recovered using logged data following total loss of connectivity. If the default value for this attribute is retained, no automatic recovery is performed for application structures following total loss of connectivity.

    Scenario 1 - Loss of connectivity to the administration structure

    Queue managers can tolerate loss of connectivity to the administration structure without terminating if all queue managers in the queue sharing group are at Version 7.1 or later. If there are queue managers at a level lower then Version 7.1 in the queue sharing group, all queue managers in the queue sharing group abend with reason code 00C510AB when connectivity to the administration structure is lost.

    When connectivity to the administration structure is lost by any queue manager that has been configured to tolerate loss of connectivity to the administration structure, all members of the queue sharing group disconnect from the administration structure. All active queue managers in the queue sharing group then attempt to reconnect to the administration structure, causing it to be reallocated in the coupling facility with the best connectivity to all systems in the sysplex, and rebuild the administration structure data. Note: This may not necessarily be the coupling facility which has the best connectivity to all systems that have active queue managers.

    If a queue manager cannot reconnect to the administration structure, for example because none of the coupling facilities in the CFRM preference list for the administration structure are available, some shared queue operations remain unavailable until the queue manager can successfully reconnect to the administration structure and rebuild its administration structure data. Reconnection occurs automatically when a suitable coupling facility becomes available on the system.

    Failure to connect to the administration structure during queue manager startup as a result of a lack of connectivity to the coupling facility, or no suitable coupling facility available to allocate the structure, is not tolerated. All active queue managers in the queue sharing group then attempt to reconnect to the administration structure, causing it to be reallocated in another coupling facility if one is available, and rebuild the administration structure data.

    Scenario 2- Loss of connectivity to the application structure

    Loss of connectivity to application structures at CFLEVEL(5) or higher can be tolerated without the queue manager terminating. Queue managers connected to application structures at CFLEVEL(4) or lower, or structures at CFLEVEL(5) that have not been configured to tolerate loss of connectivity, abend with reason code 00C510AB when connectivity to the structure is lost.

    When connectivity is lost to an application structure that has been configured to tolerate loss of connectivity, all queue managers that lost connectivity to the structure disconnect. The subsequent behavior of the queue manager depends on whether the loss of connectivity is partial or total.

    Partial loss of connectivity to an application structure

    If the loss of connectivity is determined to be partial, queue managers that have lost connectivity to the structure attempt to initiate a system-managed rebuild in order to move the structure to another coupling facility with improved connectivity. If this rebuild is successful, both persistent and non-persistent messages in the structure are copied to the other coupling facility, and access to queues on the structure is restored. Queue managers that did not lose connectivity remain connected to the structure, however, operations that access the structure are delayed during the system-managed rebuild process.

    If an application structure cannot be rebuilt to another coupling facility with improved connectivity, or some queue managers still do not have connectivity to the structure after it has been rebuilt in another coupling facility, queues defined on the structure remain unavailable on the queue managers that do not have connectivity to the structure until connectivity is restored to the coupling facility. Queue managers automatically reconnect to the structure when it becomes available and access to the shared queues defined on the structure are restored.

    Total loss of connectivity to an application structure

    If all MVS systems in the sysplex have lost connectivity to the coupling facility that the application structure is allocated in, z/OS deallocates the structure from the coupling facility whenever an attempt is made to reconnect to the structure. It is possible for the queue manager to attempt to reconnect to the structure for several reasons, such as an attempt by an application to open a shared queue, or a notification from the system that new coupling facility resources may have become available. It is therefore likely that all non-persistent messages in the affected structure are lost following total loss of connectivity to an application structure.

    Recoverable application structures are automatically recovered following total loss of connectivity, if they have been defined with RECAUTO(YES). The recovery starts almost immediately if an alternative coupling facility is available to allocate the structure in, or whenever such a coupling facility becomes available. If a structure has not been defined with RECAUTO(YES), recovery can be started by issuing the RECOVER CFSTRUCT command. This recovers all persistent messages in the structure, but all non-persistent messages are lost. As this process involves reading the queue manager log it can take some time to complete, therefore it is recommended that structure backups be taken regularly to reduce the time until access to the shared queues on the structure is restored.

    Queue managers attempt to reconnect to non-recoverable application structures as soon as an application attempts to open a shared queue that is defined on the structure or a notification is received from the system that new coupling facility resources have become available. If a suitable coupling facility is available to allocate the structure in, a new structure is allocated and access to the shared queues defined on the structure is restored. As persistent messages cannot be put to queues defined in non-recoverable structures, all messages on the shared queues are lost.


Operational behavior

If a Version 7.1, or later, queue manager, configured to tolerate loss of connectivity to a particular coupling facility structure loses connectivity, the members of the queue sharing group attempt to automatically recover from the failure and reconnect to the structure. This activity may involve reallocating the structure in another coupling facility with better connectivity if one is available. However, operator intervention may still be required to recover from the loss of connectivity.

Typically the required operator action is to:
  1. Resolve the cause of the failure that resulting in the loss of connectivity.
  2. Ensure that a coupling facility where the IBM MQ structures can be allocated is available on all systems in the sysplex

Any structures that have been automatically reallocated in another coupling facility after the loss of connectivity event, can be moved to the coupling facility with the optimal connectivity to all queue managers in the queue sharing group. If required, this can be done by initiating the system-managed rebuild command SETXCF START,REBUILD as documented in z/OS MVS System Commands Reference. In the case of a partial loss of connectivity to an application structure, the queue managers that lost connectivity to the structure attempt to initiate a system-managed rebuild. This process only allocates the structure in another coupling facility if that coupling facility has connectivity to all active queue managers currently connected to the structure. Therefore, it is possible that where the majority of queue managers in a queue sharing group have lost connectivity to an application structure, they are unable to rebuild the structure into another coupling facility due to the queue managers that are still connected to the original structure. In this situation the queue managers that are still connected to the original structure can be shut down to allow the structure to be rebuilt, or the RESET CFSTRUCT ACTION(FAIL) command can be issued to fail the structure. Recovery can be initiated on applicable structures by issuing the RECOVER CFSTRUCT command. Note: When failing and recovering the structure, all non-persistent messages on the structure are lost. Parent topic: Recovery and restart on z/OS

Last updated: 2020-10-04