O'Reilly - Java Message Service

Java Message Service
already use for electronic communication. An Electronic Data Interchange (EDI) system
may have nightly batch jobs that export flat files to an FTP site. A trading partner may
expect to send and receive HTTP transmissions as its way of communicating with the
outside world. A supply chain portal may require that you install one of their clients on
your site in order to communicate with them through whatever protocol they dictate.
Sometimes email is required as a way of sending a "Thank you for your order" message.
Ideally each of these outside entities would have a close working relationship with you,
and would allow you to install a JMS client at each site. That would make communication
very easy - but it's not how the world works. These other communication mechanisms may
have been in place for a number of years, and their users aren't about to rip them out just
because you want them to. They may not be capable of changing the way their systems
work just to accommodate your JMS provider. These are "legacy systems"; in the future,
they may gradually disappear, but for the time being, we have to figure out how to work
with them.
Someday JMS may provide on-the-wire interoperability, and be ubiquitous. Until then, we
are left to building bridges, or connectors to those other protocols. As illustrated in Figure
7.1, a connector is simply a JMS client. Its sole purpose is to receive data using the foreign
protocol, create a JMS message, and send it along through your JMS-based system.
Likewise an outbound connector would listen for messages from your JMS-based system
and transmit the message out into the world using the protocol expected by the entity at the
other end.
Figure 7.1. JMS clients can be dedicated as protocol connectors to the outside world
The JMS specification does not suggest this notion of connectors. [7] However, legacy
systems are a fact of life. In recognition of this, most JMS vendors are starting to provide
connectors to legacy systems as a way to provide added value. If your JMS provider does
not support the connector you are looking for, it is typically easy enough to write your
own. In fact, this is an ideal situation for using CLIENT_ACKNOWLEDGE mode. As illustrated in
Figure 7.2, a JMS consumer can explicitly acknowledge the receipt of the message once its
data transmission has been successfully completed.
[7] The use of the term "connector" in this discussion should not be confused with "connectors" as
defined by the J2EE connector specification - a different thing altogether.
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