O'Reilly - Java Message Service

Java Message Service
JMS doesn't attempt to define a set of APIs for all the possible options on a queue. It
should be possible to set these options administratively, using the vendor-specific
administration capabilities. Figure 5.1 shows what a graphical administration tool for
queues would look like. Most vendors supply a command-line administration tool, a
graphical administration tool, or an API for administering queues at runtime. Some
vendors supply all three.
Figure 5.1. Queues are created and administered using vendor-specific administration tools
Using vendor-specific administration APIs to create and configure a queue may be
convenient at times. However, it is not very portable, and may require that the application
have administrator privileges.
JMS provides a QueueSession.createQueue(String queueName) method, but this is not
intended to define a new queue in the messaging system. It is intended to return a Queue
object that represents an existing queue. There is also a JMS-defined method for creating a
temporary queue that can only be consumed by the JMS client that created it:
QueueSession.createTemporaryQueue( ). A temporary queue is similar to a temporary
topic, and could have been used in our example just like the TemporaryTopic was used in
Chapter 4.
5.4 Load Balancing Using Multiple QueueSessions
A queue may have multiple receivers attached to it for the purpose of distributing the
workload of message processing. The JMS specification states that this capability must be
implemented by a JMS provider, although it does not define the rules for how the
messages are distributed among consumers. A sender could use this feature to distribute
messages to multiple instances of an application, each of which would provide its own
receiver.
When multiple receivers are attached to a queue, each message in the queue is delivered to
one receiver. The absolute order of messages cannot be guaranteed, since one receiver may
process messages faster than another. From the receiver's perspective, the messages it
consumes should be in relative order; messages delivered to the queue earlier are
consumed first. However, if a message needs to be redelivered due to an acknowledgment
failure, it is possible that it could be delivered to another receiver. The other receiver may
79