Week 3: Enterprise Integration Patterns

In the Enterprise Integration Patterns the author talks about application coupling and that “applications should minimize their dependencies on each other so that each can evolve without causing problems for the others because when the applications change and break those assumptions, the integration breaks.” There’s more than one approach for integrating applications. Each approach addresses some of the integration criteria better than others. The various approaches can be summed up in four main integration styles:
File Transfer — Have each application produce files of shared data for others to consume, and consume files that others have produced.
Shared Database — Have the applications store the data they wish to share in a common database.
Remote Procedure Invocation — Have each application expose some of its procedures so that they can be invoked remotely, and have applications invoke those to run behavior and exchange data.
Messaging — Have each application connect to a common messaging system, and exchange data and invoke behavior using messages.

Messaging involves more than just connecting them to the messaging system so that they can send messages. The messages must have Message Channels to transmit on. Slapping in some channels doesn’t get the job done either. They have to be designed with a purpose, based on the data type being shared, the sort of application making the data available, and the sort of application receiving the data.Messaging makes applications loosely coupled by communicating asynchronously, which also makes the communication more reliable because the two applications do not have to be running at the same time. Messaging makes the messaging system responsible for transferring data from one application to another, so the applications can focus on what data they need to share but not worry so much about how to share it.

When an application has information to communicate, it doesn't just fling the information into the messaging system, it adds the information to a particular Message Channel. An application receiving information doesn't just pick it up at random from the messaging system; it retrieves the information from a particular Message Channel. When two applications wish to exchange data, they do so by sending the data through a channel that connects the two. The application sending the data may not know which application will receive the data, but by selecting a particular channel to send the data on, the sender knows that the receiver will be one that is looking for that sort of data by looking for it on that channel. In this way, the applications that produce shared data have a way to communicate with those that wish to consume it.