Selection behavior

Overview of IBM MQ selection behavior.

• Has format MQFMT_MD_EXTENSION
• Is immediately followed by a valid MQMDE structure
• Is version one or contains the default version two fields only

It is possible for a selection string to resolve to either TRUE or FALSE before any matching against message properties takes place. For example, it might be the case if the selection string is set to TRUE <> FALSE. Such early evaluation is guaranteed to take place only when there are no message property references in the selection string.

If a selection string resolves to TRUE before any message properties are considered, all messages published to the topic subscribed to by the consumer are delivered. If a selection string resolves to FALSE before any message properties are considered, a reason code of MQRC_SELECTOR_ALWAYS_FALSE, and completion code MQCC_FAILED are returned on the function call that presented the selector.

Even if a message contains no message properties (other than header properties) then it can still be eligible for selection. If a selection string references a message property that does not exist, this property is assumed to have the value of NULL or 'Unknown'.

For example, a message might still satisfy a selection string like 'Color IS NULL', where 'Color' does not exist as a message property in the message.

Selection can be performed only on the properties that are associated with a message, not the message itself, unless an extended message selection provider is available. Selection can be performed on the message payload only if an extended message selection provider is available.

Each message property has a type associated with it. When you perform a selection, you must ensure that the values used in expressions to test message properties are of the correct type. If a type mismatch occurs, the expression in question resolves to FALSE.

It is your responsibility to ensure that the selection string and message properties use compatible types.

Selection criteria continue to be applied on behalf of inactive durable subscribers, so that only messages that match the selection string that was originally supplied are kept.

Selection strings are non-alterable when a durable subscription is resumed with alter (MQSO_ALTER). If a different selection string is presented when a durable subscriber resumes activity, then MQRC_SELECTOR_NOT_ALTERABLE is returned to the application.

Applications receive a return code of MQRC_NO_MSG_AVAILABLE if there is no message on a queue that meets the selection criteria.

If an application has specified a selection string containing property values then only those messages that contain matching properties are eligible for selection. For example, a subscriber specifies a selection string of "a = 3" and a message is published containing no properties, or properties where 'a' does not exist or is not equal to 3. The subscriber does not receive that message to its destination queue.

Messaging performance

Selecting messages from a queue requires IBM MQ to sequentially inspect each message on the queue. Messages are inspected until a message is found that matches the selection criteria or there are no more messages to examine. Therefore, messaging performance suffers if message selection is used on deep queues.

To optimize message selection on deep queues when selection is based on JMSCorrelationID or JMSMessageID, use a selection string of the form JMSCorrelationID = ... or JMSMessageID = ... and reference only one property.

This method offers a significant improvement in performance for selection on JMSCorrelationID and offers a marginal performance improvement for JMSMessageID.

Use complex selectors

Selectors can contain many components, for example: a and b or c and d or e and f or g and h or i and j... or y and z

Use of such complex selectors can have serious performance implications and excessive resource requirements. As such, IBM MQ will protect the system by failing to process overly complex selectors that could result in a system resource shortage. Protection can occur on selection strings that contain more than 100 tests, or when IBM MQ detects that the limit on the size of the operating system stack is being approached. You should thoroughly try and test the use of selection strings with many components, on the appropriate platforms, to ensure that the protection limits are not reached.

The performance and complexity of selectors can be improved by simplifying them using additional parenthesis to combine components. For example: ( a and b or c and d) or ( e and f or g and h) or ( i and j) ...