In Chapter 3, “Messaging Systems,” we discussed Message Channels (60). When two applications want 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. However, by selecting a particular channel on which to send the data, 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.