Home

 

Usage notes

  1. Use this call only when the queue manager itself coordinates the unit of work. This can be:

    • A local unit of work, where the changes affect only MQ resources.

    • A global unit of work, where the changes can affect resources belonging to other resource managers, as well as affecting MQ resources.

    For further details about local and global units of work, see MQBEGIN - Begin unit of work.

  2. In environments where the queue manager does not coordinate the unit of work, the appropriate commit call must be used instead of MQCMIT. The environment might also support an implicit commit caused by the application terminating normally.

    • On z/OS, use the following calls:

      • Batch programs (including IMS batch DL/I programs) can use the MQCMIT call if the unit of work affects only MQ resources. However, if the unit of work affects both MQ resources and resources belonging to other resource managers (for example, DB2), use the SRRCMIT call provided by the z/OS Recoverable Resource Service (RRS). The SRRCMIT call commits changes to resources belonging to the resource managers that have been enabled for RRS coordination.

      • CICS applications must use the

        EXEC CICS SYNCPOINT command to commit the unit of work explicitly. Alternatively, ending the transaction results in an implicit commit of the unit of work. The MQCMIT call cannot be used for CICS applications.

      • IMS applications (other than batch DL/I programs) must use IMS calls such as

        GU and CHKP to commit the unit of work. The MQCMIT call cannot be used for IMS applications (other than batch DL/I programs).

    • On i5/OS, use this call for local units of work coordinated by the queue manager. This means that a commitment definition must not exist at job level, that is, the

      STRCMTCTL command with the CMTSCOPE(*JOB) parameter must not have been issued for the job.

  3. If an application ends with uncommitted changes in a unit of work, the disposition of those changes depends on whether the application ends normally or abnormally. See MQDISC usage notes for further details.

  4. When an application puts or gets messages in groups or segments of logical messages, the queue manager retains information relating to the message group and logical message for the last successful MQPUT and MQGET calls. This information is associated with the queue handle, and includes such things as:

    • The values of the GroupId, MsgSeqNumber, Offset, and MsgFlags fields in MQMD.

    • Whether the message is part of a unit of work.

    • For the MQPUT call: whether the message is persistent or nonpersistent.

    When a unit of work is committed, the queue manager retains the group and segment information, and the application can continue putting or getting messages in the current message group or logical message.

    Retaining the group and segment information when a unit of work is committed allows the application to spread a large message group or large logical message consisting of many segments across several units of work. Using several units of work is advantageous if the local queue manager has only limited queue storage. However, the application must maintain sufficient information to restart putting or getting messages at the correct point in the event that a system failure occurs. For details of how to restart at the correct point after a system failure, see MQPMO_LOGICAL_ORDER and MQGMO_LOGICAL_ORDER.

    The remaining usage notes apply only when the queue manager coordinates the units of work:

  5. A unit of work has the same scope as a connection handle; all MQ calls that affect a particular unit of work must be performed using the same connection handle. Calls issued using a different connection handle (for example, calls issued by another application) affect a different unit of work. See the Hconn parameter described in MQCONN for information about the scope of connection handles.

  6. Only messages that were put or retrieved as part of the current unit of work are affected by this call.

  7. A long-running application that issues MQGET, MQPUT, or MQPUT1 calls within a unit of work, but that never issues a commit or back-out call, can fill queues with messages that are not available to other applications. To guard against this, the administrator must set the MaxUncommittedMsgs queue-manager attribute to a value that is low enough to prevent runaway applications filling the queues, but high enough to allow the expected messaging applications to work correctly.

  8. On UNIX and Windows systems, if the Reason parameter is MQRC_CONNECTION_BROKEN (with a CompCode of MQCC_FAILED), or MQRC_UNEXPECTED_ERROR it is possible that the unit of work was successfully committed.




 

Home