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sending messages. Such messages, which are delivered asynchronously, typically

consist of two parts: one part the message body contains the business data and

the other part the message header contains routing information. Since messages

are sent asynchronously, the sender does not have to wait for the message to be

delivered to the receiver. 

There are two common messaging styles: point to point, and publish and sub 

scribe. A point to point messaging style is used when messages are sent to only

one receiver. The recipient receives messages sent to it through a queue specifi 

cally set up for the receiver. A message sender sends messages to this queue, and

the recipient retrieves (and removes) its messages from the queue. Publish and

subscribe, on the other hand, is intended to be used when there can be multiple

recipients of a message. Rather than a queue, this style uses a topic. Messages are

sent or, more correctly, published to the topic, and all receivers interested in

these messages subscribe to the topic. Any message published to a topic can be

received by any receiver that has subscribed to the topic. See Figure 6.2.

In addition, a typical MOM system has a message router that is responsible

for ensuring message delivery (according to the agreed upon quality of service) to

the receiver. The message router uses the message header information to deter 

mine where and how to route the message contents.

message A

Topic

 Client

m

Publish Subscribe

essag

Messaging System

e A

publish

message A

Client

Client

sen

m

d

essage B

message B

Client

Queue

Figure 6.2

Messaging System Queues and Topics