MsgDeliverySequence (MQLONG)

Local	Model	Alias	Remote	Cluster
X	X

This determines the order in which the MQGET call returns messages to the application :

MQMDS_FIFO

Messages are returned in FIFO order (first in, first out).

An MQGET call returns the first message that satisfies the selection criteria specified on the call, regardless of the priority of the message.

MQMDS_PRIORITY

Messages are returned in priority order.

An MQGET call returns the highest-priority message that satisfies the selection criteria specified on the call. Within each priority level, messages are returned in FIFO order (first in, first out).

• On z/OS®, if the queue has an IndexType of MQIT_GROUP_ID, the MsgDeliverySequence attribute specifies the order in which message groups are returned to the application. The particular sequence in which the groups are returned is determined by the position or priority of the first message in each group. The physical order of messages on the queue is not defined, as the queue is optimized for efficient retrieval of messages using the MQGMO_LOGICAL_ORDER option on the MQGET call.
• On z/OS, if IndexType is MQIT_GROUP_ID and MsgDeliverySequence is MQMDS_PRIORITY, the queue manager uses message priorities zero and one to optimize the retrieval of messages in logical order. As a result, the first message in a group must not have a priority of zero or one; if it does, the message is processed as though it had a priority of two. The Priority field in the MQMD structure is not changed.

If the relevant attributes are changed while there are messages on the queue, the delivery sequence is as follows:

• The order in which messages are returned by the MQGET call is determined by the values of the MsgDeliverySequence and DefPriority attributes in force for the queue at the time that the message arrives on the queue:
  • If MsgDeliverySequence is MQMDS_FIFO when the message arrives, the message is placed on the queue as though its priority were DefPriority. This does not affect the value of the Priority field in the message descriptor of the message; that field retains the value it had when the message was first put.
  • If MsgDeliverySequence is MQMDS_PRIORITY when the message arrives, the message is placed on the queue at the place appropriate to the priority given by the Priority field in the message descriptor.

  If the value of the MsgDeliverySequence attribute is changed while there are messages on the queue, the order of the messages on the queue is not changed.

  If the value of the DefPriority attribute is changed while there are messages on the queue, the messages are not necessarily delivered in FIFO order, even though the MsgDeliverySequence attribute is set to MQMDS_FIFO; those that were placed on the queue at the higher priority are delivered first.

To determine the value of this attribute, use the MQIA_MSG_DELIVERY_SEQUENCE selector with the MQINQ call.