Data Transformation Engine IBM WebSphere MQ Adapter Reference Guide

Chapter 3 - IBM WebSphere MQ Commands List of Commands
IBM WebSphere MQ Adapter Reference Guide
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After T1 occurs, both messages have already been placed on the queue (with
M1 in front of M2). However, they are not being recognized by the message
cursor. After T2 occurs, the message cursor recognizes message M2 and it
positions itself on that message. After T3 occurs, message M1 is also
committed, but it is now beyond the message cursor. The message cursor will
never be able to recognize that message unless it is repositioned to the
beginning of the queue. Use the Refresh Message Cursor adapter command
(
-REFRESH) to accomplish this repositioning.
Note Using the Refresh Message Cursor adapter command (-REFRESH) may degrade
event notification and mapping performance and should only be used when
necessary. The adapter must be careful not to trigger the map more than once for
the same input message. To ensure this, the adapter maintains a list of messages
that have already been seen by the cursor. Each time the cursor is repositioned to
the beginning of the queue, each message that the cursor finds on the queue is
compared with all of the messages in this list. Only in situations in which this
message is not in the adapter list is that message considered to be a new
message to be used for the event notification. As part of this process, this new
message will be added to the adapter list. To ensure that this list does not grow
infinitely, there are two important limitations for the input cards that use the
-REFRESH adapter command:
♦ The Rollback option for the OnFailure setting should not be used.
♦ The Keep option for the OnSuccess setting should also not be used.
If either of these two options is specified, running the Event Server for a long
period of time with many messages being processed may cause excessive memory
usage and may slow down processing time significantly, and eventually may cause
the system to stop.
The reason for this is that messages once seen by the cursor are added to the
internal list, and as long as they are not removed from the source queue, they are
also not removed from the internal adapter's list so that they are not processed
more than once. Messages are freed from the internal list only after they have
been removed from the source queue.
In situations in which it is necessary to use the Refresh Message Cursor adapter
command (
-REFRESH) and to preserve the messages for the failed map, it is
recommended that you use Error Queue Name adapter command (
-EQN) in
combination with
-REFRESH adapter command for the input card. In this case, the
Source > Transaction > OnSuccess setting should be Delete and the
Source > Transaction > OnFailure setting should be Rollback. The messages
for the map failure will be placed on the error queue and removed from the source
queue. Thus, messages will be preserved (on the error queue), and no memory
leak will occur because it will be possible to remove messages from the internal