O'Reilly - Java Message Service

Java Message Service
Chapter 1. Understanding the Messaging Paradigm
Computers and people can communicate by using messaging systems to exchange
messages over electronic networks. The most ubiquitous messaging system today is email,
which facilitates communication among people. While email is an important human-tohuman
messaging system, this book is not about email. Instead, this book is concerned
with messaging systems that allow different software applications to communicate with
each other. These application-to-application messaging systems, when used in business
systems, are generically referred to as enterprise messaging systems, or Message-Oriented
Middleware (MOM).
Enterprise messaging systems allow two or more applications to exchange information in
the form of messages. A message, in this case, is a self-contained package of business data
and network routing headers. The business data contained in a message can be anything -
depending on the business scenario - and usually contains information about some
business transaction. In enterprise messaging systems, messages inform an application of
some event or occurrence in another system.
Using Message-Oriented Middleware, messages are transmitted from one application to
another across a network. MOM products ensure that messages are properly distributed
among applications. In addition, MOMs usually provide fault tolerance, load balancing,
scalability, and transactional support for enterprises that need to reliably exchange large
quantities of messages.
MOM vendors use different message formats and network protocols for exchanging
messages, but the basic semantics are the same. An API is used to create a message, give it
a payload (application data), assign it routing information, and then send the message. The
same API is used to receive messages produced by other applications.
In all modern enterprise messaging systems, applications exchange messages through
virtual channels called destinations. When a message is sent, it's addressed to a destination,
not a specific application. Any application that subscribes or registers an interest in that
destination may receive that message. In this way, the applications that receive messages
and those that send messages are decoupled. Senders and receivers are not bound to each
other in any way and may send and receive messages as they see fit.
All MOM vendors provide application developers with an API for sending and receiving
messages. While a MOM vendor implements its own networking protocols, routing, and
administration facilities, the basic semantics of the developer API provided by different
MOMs are the same. This similarity in APIs makes the Java Message Service possible.
The Java Message Service (JMS) is a vendor-agnostic Java API that can be used with
many different MOM vendors. JMS is analogous to JDBC in that application developers
reuse the same API to access many different systems. If a vendor provides a compliant
service provider for JMS, then the JMS API can be used to send and receive messages to
that vendor. For example, you can use the same JMS API to send messages using Progress'
SonicMQ as you do IBM's MQSeries. It is the purpose of this book to explain how
enterprise messaging systems work and in particular how the Java Message Service is used
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