O'Reilly - Java Message Service

Java Message Service
are delivered once-and-only-once. The mechanics of this are covered in greater detail in
Chapter 6.
The vendor-supplied client runtime and the server functionality are
collectively referred to as the JMS provider. A "provider failure"
generically describes any failure condition that is outside of the
domain of the application code. It could mean a hardware failure that
occurs while the provider is entrusted with the processing of a
message, or it could mean an unexpected exception or halting of a
process due to a software defect. It could also mean a network failure
that occurs between two processes that are part of the JMS vendor's
internal architecture.
Nonpersistent messages are not written to disk when they are received by the message
server, so if the JMS provider fails, the message will be lost. In general nonpersistent
messages perform better than persistent messages. They are delivered more quickly and
require less system resources on the message server. However, nonpersistent messages
should only be used when a loss of messages due to a JMS provider failures is not an issue.
The chat example used in Chapter 2 is a good example of a system that doesn't require
persistent delivery. It's not critical that every message be delivered to all consumers in a
chat application. In most business systems, however, messages are delivered using the
persistent mode, because it's important that they be successfully delivered.
The delivery mode can be set using the setDeliveryMode( ) method defined in both the
TopicPublisher and QueueSender message producers. The javax.jms.DeliveryMode class
defines the two constants used to declare the delivery mode: PERSISTENT and
NON_PERSISTENT :
// Publish-and-subscribe
TopicPublisher topicPublisher = topicSession.createPublisher(topic);
topicPublisher.setDeliveryMode(DeliveryMode.NON_PERSISTENT);
// Point-to-point
QueueSender queueSender = queueSession.createSender(queue);
queueSender.setDeliverMode(DeliveryMode.PERSISTENT);
Once the delivery mode has been set on the message producer, it will be applied to all the
messages delivered by that producer. The delivery mode can be changed at any time using
the setDeliveryMode( ) method; the new mode will be applied to subsequent messages.
The default delivery mode of a message producer is always PERSISTENT.
The delivery mode of a message producer can be overridden for an individual message
during the send operation, which allows a message producer to deliver a mixture of
persistent and nonpersistent messages to the same destination (topic or queue):
// Publish-and-subscribe
Message message = topicSession.createMessage( );
topicPublisher.publish(message, DeliveryMode.PERSISTENT, 5, 0);
// Point-to-point
Message message = queueSession.createMessage( );
queueSender.send(message, DeliveryMode.NON_PERSISTENT, 5, 0);
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