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> JMS Message Models
> JMS is based on creation and delivery of messages. Messages are structured data that one application sends to another. The creator of the message is known as the producer and the receiver of the message is known as the consumer. The TIBCO EMS server acts as an intermediary for the message and manages its delivery to the correct destination. The server also provides enterprise-class functionality such as fault-tolerance, message routing, and communication with other messaging systems, such as TIBCO Rendezvous® and TIBCO SmartSockets®.
> Figure 1 illustrates an application producing a message, sending it by way of the server, and a different application receiving the message.
> Figure 1 Message Delivery
> JMS supports these messaging models:

> • | Point-to-Point (queues)

> • | Publish and Subscribe (topics)

> • | Multicast (topics)

> Point-to-Point
> Point-to-point messaging has one producer and one consumer per message. This style of messaging uses a queue to store messages until they are received. The message producer sends the message to the queue; the message consumer retrieves messages from the queue and sends acknowledgement that the message was received.
> More than one producer can send messages to the same queue, and more than one consumer can retrieve messages from the same queue. The queue can be configured to be exclusive, if desired. If the queue is exclusive, then all queue messages can only be retrieved by the first consumer specified for the queue. Exclusive queues are useful when you want only one application to receive messages for a specific queue. If the queue is not exclusive, any number of receivers can retrieve messages from the queue. Non-exclusive queues are useful for balancing the load of incoming messages across multiple receivers. Regardless of whether the queue is exclusive or not, only one consumer can ever consume each message that is placed on the queue.
> Figure 2 illustrates point-to-point messaging using a non-exclusive queue. Each message consumer receives a message from the queue and acknowledges receipt of the message. The message is taken off the queue so that no other consumer can receive it.
> Figure 2 Point-to-point messages
> Publish and Subscribe
> In a publish and subscribe message system, producers address messages to a topic. In this model, the producer is known as a publisher and the consumer is known as a subscriber.
> Many publishers can publish to the same topic, and a message from a single publisher can be received by many subscribers. Subscribers subscribe to topics, and all messages published to the topic are received by all subscribers to the topic. This type of message protocol is also known as broadcast messaging because messages are sent over the network and received by all interested subscribers, similar to how radio or television signals are broadcast and received.
> Figure 3 illustrates publish and subscribe messaging. Each message consumer subscribes to a topic. When a message is published to that topic, all subscribed consumers receive the message.
> Figure 3 Publish and subscribe messages
> Durable Subscribers for Topics
> By default, subscribers only receive messages when they are active. If messages arrive on the topic when the subscriber is not available, the subscriber does not receive those messages.
> The EMS APIs allow you to create durable subscribers to ensure that messages are received, even if the message consumer is not currently running. Messages for durable subscriptions are stored on the server as long as durable subscribers exist for the topic, or until the message expiration time for the message has been reached, or until the storage limit has been reached for the topic. Durable subscribers can receive messages from a durable subscription even if the subscriber was not available when the message was originally delivered.
> When an application restarts and recreates a durable subscriber with the same ID, all messages stored on the server for that topic are delivered to the durable subscriber.
> See Creating a Message Consumer for details on how to create durable subscribers.
> Multicast
> Multicast messaging allows one message producer to send a message to multiple subscribed consumers simultaneously. As in the publish and subscribe messaging models, the message producer addresses the message to a topic. Instead of delivering a copy of the message to each individual subscriber over TCP, however, the EMS server broadcasts the message over Pragmatic General Multicast (PGM). A daemon running on the machine with the subscribed EMS client receives the multicast message and delivers it to the message consumer.
> Multicast is highly scalable because of the reduction in bandwidth used to broadcast messages, and because of reduced EMS server resources used. However, multicast does not guarantee message delivery to all subscribers.
> Figure 4 illustrates the multicast messaging model. Each message consumer subscribes to a multicast-enabled topic. When a message is sent to that topic, the EMS server broadcasts the message. Listening multicast daemons receive the message and deliver it to subscribed clients.
> Figure 4 Multicast messages
> For more information about multicast, see Chapter 13, Using Multicast.

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