Types of Messaging

Deciding to use asynchronous messaging in your application is a big step, but it's by no means the last. There are a number of ways to implement asynchronous messaging, each involving a different Quality of Service guarantee. The U.S. Postal Service, for instance, promises to deliver a first class letter within four days, and makes their best effort to deliver the letter intact. Most letters duly arrive in the expected timeframe and in reasonable condition. However, some letters get lost, and others are damaged. Because this isn't good enough for every snail-mail enabled application, the USPS provides a set of additional delivery services: return receipts, certified mail, Priority Mail, and Express Mail. An Express Mail letter is all but certain to arrive at its destination the following day, in good condition.

Messaging middleware is the same. Depending on the sophistication and configuration of your messaging platform, you can achieve a variety of different service levels. The higher levels involve more maintenance and are generally more expensive to configure. In this chapter, we're going to talk about two different kinds of messaging. The first is guaranteed messaging, which is implemented via middleware and provides high reliability and control. We use JMS to handle guaranteed messaging in Java. The second is nonguaranteed messaging, which lacks the reliability and security of guaranteed messaging, but can be implemented with much less programmer time and less cost for external middleware.