O'Reilly - Java Message Service

Java Message Service
The acknowledge( ) method informs the JMS provider that the message has been
successfully received by the consumer. This method throws an exception to the client if a
provider failure occurs during the acknowledgment process. The provider failure results in
the message being retained by the JMS server for redelivery. Therefore, the exception
handling code should undo the results of any partially processed business logic in
preparation for receiving the message again, or it should log the message as processed so
that the redelivered message can be ignored. The acknowlege( ) method should only be
used with the CLIENT_ACKNOWLEDGE mode; if used with the AUTO_ACKNOWLEDGE or
DUPS_OK_ACKNOWLEDGE mode, the call is ignored by the JMS provider.
6.2.3.1 Grouping multiple messages
The CLIENT_ACKNOWLEDGE mode also gives you the ability to batch together multiple
message receipts and consume them in an all-or-nothing fashion. A consuming client may
receive several messages in a sequence and treat them as a group. CLIENT_ACKNOWLEDGE
does not provide the capability to consume messages selectively. A single
acknowledgment for the last message in the group implicitly acknowledges all previously
unacknowledged messages for the current session. This means that if the client application
fails before the last message is acknowledged, it may recover when it comes back up. All
of the unacknowledged messages will be resent with the JMSRedelivered flag set on each
of the unacknowledged messages. A JMS client may also call the recover( ) method on a
Session object, to force the redelivery of all previously unacknowledged messages, even if
there hasn't been a failure.
JMS also provides a transaction model for grouping multiple sends and receives. We cover
this in detail later in the Section 6.4 of this chapter.
6.3 Message Groups and Acknowledgment
When multiple messages need to be dealt with as a group, the application needs to be able
to store or cache interim messages until the entire group has been delivered. This
requirement typically means that the asynchronous invocation of the onMessage( ) handler
would result in business logic getting executed, and data would be placed temporarily in a
database table in preparation for processing the group of messages as a whole. When the
last message of the group arrives, the application can then go to the database to retrieve the
data from the previous messages to establish any context it may need.
6.3.1 Handling Redelivery of Messages in an Application
JMS provides strict rules that govern when the redelivered flag is set. In AUTO_ACKNOWLEDGE
mode, only the most recently consumed message is subject to ambiguous redelivery. In
other modes, multiple messages may have the redelivered flag set. It is up to the
application designer to isolate the conditions under which ambiguity can occur, and to
account for it in the application.
6.3.2 Message Groups in QRetailer
A message that has been redelivered should be reconciled against any application state to
resolve ambiguities related to the reason for redelivery. To see this in action, run the
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