Browsing messages in logical order

Logical and physical ordering explains the difference between the logical and physical order of messages on a queue. This distinction is particularly important when browsing a queue, because, in general, messages are not being deleted and browse operations do not necessarily start at the beginning of the queue.

If an application browses through the various messages of one group (using logical order), it is important that logical order should be followed to reach the start of the next group, because the last message of one group might occur physically after the first message of the next group. The MQGMO_LOGICAL_ORDER option ensures that logical order is followed when scanning a queue.

Use MQGMO_ALL_MSGS_AVAILABLE (or MQGMO_ALL_SEGMENTS_AVAILABLE) with care for browse operations. Consider the case of logical messages with MQGMO_ALL_MSGS_AVAILABLE. The effect of this is that a logical message is available only if all the remaining messages in the group are also present. If they are not, the message is passed over. This can mean that when the missing messages arrive subsequently, they are not noticed by a browse-next operation.

For example, if the following logical messages are present,

   Logical message 1 (not last) of group 123
   Logical message 1 (not last) of group 456
   Logical message 2 (last)     of group 456

and a browse function is issued with MQGMO_ALL_MSGS_AVAILABLE, the first logical message of group 456 is returned, leaving the browse cursor on this logical message. If the second (last) message of group 123 now arrives:

   Logical message 1 (not last) of group 123
   Logical message 2 (last)     of group 123
   Logical message 1 (not last) of group 456  <=== browse cursor
   Logical message 2 (last)     of group 456

and the same browse-next function is issued, it is not noticed that group 123 is now complete, because the first message of this group is before the browse cursor.

In some cases (for example, if messages are retrieved destructively when the group is present in its entirety), we can use MQGMO_ALL_MSGS_AVAILABLE together with MQGMO_BROWSE_FIRST. Otherwise, we must repeat the browse scan to take note of newly-arrived messages that have been missed; just issuing MQGMO_WAIT together with MQGMO_BROWSE_NEXT and MQGMO_ALL_MSGS_AVAILABLE does not take account of them. (This also happens to higher-priority messages that might arrive after scanning the messages is complete.)

The next sections look at browsing examples that deal with unsegmented messages; segmented messages follow similar principles.

• Browsing messages in groups
  In this example, the application browses through each message on the queue, in logical order.
• Browsing and retrieving destructively
  In this example, the application browses each of the logical messages within a group, before deciding whether to retrieve that group destructively.

Parent topic: Browsing messages on a queue