Exploring Messaging Models
In general, there are two main types of messaging models: point-to-point (PTP) and publish-and-subscribe (Pub/Sub). JMS refers to these models as messaging domains. A single JMS application can use both models; however, JMS focuses on applications that use one or the other.
A JMS client that sends (or publishes) a message is called a producer, whereas a JMS client that receives (or subscribes) to a message is called a consumer.
Point-to-Point Messaging Model
The PTP messaging model is intended for one-to-one delivery of messages, and is built around the concept of a message queue. Each message is sent to a specific queue; clients receive messages from the queue(s) established to hold their messages. They are point-to-point in the sense that a client sends a message to a specific queue. Receivers must either poll a queue periodically or listen to incoming events in order to extract their messages. Each message is consumed once and only once by a receiver. A message is automatically removed from the queue when it's consumed by a receiver. In PTP, a producer is called a sender, and a consumer is called a receiver. Figure 13.2 illustrates a few scenarios of PTP messaging.
Figure 13.2. Point-to-point messaging model.
As you can see in Figure 13.2, a queue might have more than one sender and more than one receiver, but only one receiver may consume each message. In the PTP model, a JMS client can choose to be a message browser that is allowed to peek into a message without consuming it.
Publish-and-Subscribe Messaging Model
The Pub/Sub messaging model is intended for one-to-many broadcast of messages and is built around the concept of a message topic. Each message published to a topic is broadcast to all the clients that subscribe to this topic. A Topic object encapsulates a provider-specific topic name. It is the way a client specifies the identity of a topic to JMS methods. Many Pub/Sub providers group topics into hierarchies and provide various options for subscribing to parts of the hierarchy. This is similar to the concept of newsgroups, in which a user subscribes to the newsgroup topic of interest. Each consumer receives a copy of each message, which is pushed to the clients subscribed to it. In the Pub/Sub model, a producer is called a publisher, and a consumer is called a subscriber. Figure 13.3 illustrates a few scenarios of Pub/Sub messaging.
Figure 13.3. Publish-and-subscribe messaging model.
In the Pub/Sub model, a JMS client can choose to be a durable subscriber that can disconnect and later connect to get its messages that it subscribed to.