Messaging Domains

The Java Message Service (JMS) specification, which Message Queue implements, supports two commonly used models of interaction between message clients and message brokers, sometimes known as messaging domains:

• In the point-to-point (or PTP) messaging model, each message is delivered from a message producer to a single message consumer. The producer delivers the message to a queue, from which it is later delivered to one of the consumers registered for the queue. Any number of producers and consumers can interact with the same queue, but each message is guaranteed to be delivered to (and be successfully consumed by) exactly one consumer and no more. If no consumers are registered for a queue, it holds the messages it receives and eventually delivers them when a consumer registers.

• In the publish/subscribe (or pub/sub) model, a single message can be delivered from a producer to any number of consumers. The producer publishes the message to a topic, from which it is then delivered to all active consumers that have subscribed to the topic. Any number of producers can publish messages to a given topic, and each message can be delivered to any number of subscribed consumers. The model also supports the notion of durable subscriptions, in which a consumer registered with a topic need not be active at the time a message is published; when the consumer subsequently becomes active, it will receive the message. If no active consumers are registered for a topic, the topic does not hold the messages it receives unless it has inactive consumers with durable subscriptions.

JMS applications are free to use either of these messaging models, or even to mix them both within the same application. Historically, the JMS API provided a separate set of domain-specific object classes for each model. While these domain-specific interfaces continue to be supported for legacy purposes, client programmers are now encouraged to use the newer unified domain interface, which supports both models indiscriminately. For this reason, the discussions and code examples in this manual focus exclusively on the unified interfaces wherever possible. Table 2-1 shows the API classes for all three domains.

Table 2-1 Interface Classes for Messaging Domains