Java Message Service (JMS) Question:

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Answer #1
Messaging systems provide a host of powerful advantages over other conventional distributed computing models. Primarily, they encourage "loose coupling" between message consumers and message producers. There is a high degree of anonymity between producer and consumer: to the message consumer, it doesn't matter who produced the message, where the producer lives on the network, or when the message was produced.

Answer #2
In Publish/Subscribe model:
A publish/subscribe (pub/sub) messaging system supports an event driven model where information consumers and producers participate in the transmission of messages. Producers "publish" events, while consumers "subscribe" to events of interest, and consume the events. Producers associate messages with a specific topic, and the messaging system routes messages to consumers based on the topics the consumers register interest in.
In Point-To-Point model:
In point to point messaging systems, messages are routed to an individual consumer which maintains a queue of "incoming" messages. Messaging applications send messages to a specified queue, and clients retrieve messages from a queue.

Answer #3
In Point-To-Point model, one client can send message to the another client through the Destination.
There is a guarantee to receive the message whenever receiver is connected.
example:your telephone answering machine ,outer send a message to u,but you can receive those msg whenever u connected to answering machine.
In pub/sub model. one publisher,many no.of clients will be there,publisher publish the message,subscriber or consumer can receive those messages when he got subscription through the topic.There is no guarantee consumer can receive the messages send by the publisher.