J2EE Design Patterns

Messaging and Integration

Back around the dawn of computing, sending a message between systems was easy: you wrote the data onto a magnetic tape, took the tape out of the computer, and carried it over to the other computer. Assuming the two computers shared a common data format, you were all set. When networks were invented, programmers devised integration techniques based on direct network connections. An application opened a connection to another application, and if the network was running and the target application was responding, the two systems could interact with each other, as long as they shared a common format for data. The problems with this approach should be familiar: the system wouldn’t work if one application was down, or if there were intermittent network failures, or if the systems had different data formats. Building direct linkages and custom protocols were also a time-intensive and nonstandard approach, which had to be revisited with each new application or new version. And did we mention the data formats?

There are other approaches; many systems communicate with each other by reading and writing shared files, or by accessing shared tables within a database. The former method works well when two applications are running on the same physical machine or connected by a reliable network that supports file sharing. The latter works well for multiple applications and applications spread across a variety of systems. Both approaches have the benefit of being asynchronous: ...