When two or more applications want to exchange data, they do so by sending the data through a channel that connects to the others. 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.

When designing an application, a developer has to know where to put what types of data to share that data with other applications, and likewise where to look for what types of data coming from other applications. These paths of communication cannot be dynamically created and discovered at runtime; they need to be agreed upon at design time so that the application knows where its data is coming from and where the data is going to. One exception is the reply channel in Request-Reply. The requestor can create or obtain a new channel the replier knows nothing about, specify it as the Return Address of a request message, and then the replier can make use of it. Another exception is messaging system implementations that support hierarchical channels. A receiver can subscribe to a parent in the hierarchy, then a sender can publish to a new child channel the receiver knows nothing about, and the subscriber will still receive the message.

First, the applications determine the channels the messaging system will need to provide. Subsequent applications will try to design their communication around the channels that are available, but when this is not practical, they will require that additional channels be added. When a set of applications already use a certain set of channels, and new applications wish to join in, they too will use the existing set of channels. When existing applications add new functionality, they may require new channels.

Another common source of confusion is whether a Message Channel is unidirectional or bidirectional. Technically, it’s neither; a channel is more like a bucket that some applications add data to and other applications take data from. But because the data is in messages that travel from one application to another, that gives the channel direction, making it unidirectional. If a channel were bidirectional, that would mean that an application would both send messages to and receive messages from the same channel because the application would tend to keep consuming its own messages, the messages it’s supposed to be sending to other applications. So for all practical purposes, channels are unidirectional. As a consequence, for two applications to have a two-way conversation, they will need two channels, one in each direction.

Therefore different types of channels used in a messaging system. A Message channel is a basic architectural pattern of a messaging system and is used fundamentally for exchanging data between applications.

One-to-One or One-to-Many

When application shares data with just one other application or any other application that is interest on that data. Then you can use a Point-to-Point Channel. This does not guarantee that every piece of data sent on that channel will necessarily go to the same receiver, because the channel might have multiple receivers; but it does ensure that any one piece of data will only be received by one of the applications.

If all of the receiver need to be receive the data, use a Publish-Subscribe Channel. Then piece of data is copied effectively by the channel and pass it to each of the receivers. Simple the sender broadcasts an event to all interested receivers.

Type of Data (Datatype Channel)

The message contents must conform to some type so that the receiver understands the data’s structure. Datatype Channel is the principle that all of the data on a channel has to be of the same type. This is the main reason why messaging systems need lots of channels; if the data could be of any type, the messaging system would only need one channel (in each direction) between any two applications.

Invalid and Dead Messages

The message system can ensure that a message is delivered properly, but it cannot guarantee that the receiver will know what to do with it. The receiver has expectations about the data’s type and meaning. What it can do, though, is put the strange message on a specially designated Invalid Message Channel, in hopes that some utility monitoring the channel will pick up the message and figure out what to do with it.

Many messaging systems have a similar built-in feature, a Dead Letter Channel for messages which are successfully sent but ultimately cannot be successfully delivered. Again, hopefully some utility monitoring the channel will know what to do with the messages that could not be delivered.

Crash Proof

If the messaging system crashes or is shut down for maintence. When it is back up and running, those messages will still be in its channels. By default, no; channels store their messages in memory. However, Guaranteed Delivery makes channels persistent so that their messages are stored on disk. This hurts performance but makes messaging more reliable.

Non-Messaging Clients

If an application cannot connect to a messaging system but still wants to participate in messaging. But if the messaging system can connect to the application somehow—through its user interface, its business services API, its database, or through a network connection such as TCP/IP or HTTP—then a Channel Adapter on the messaging system can be used to connect a channel (or set of channels) to the application without having to modify the application and perhaps without having to have a messaging client running on the same machine as the application.

Sometimes the "non-messaging client" really is a messaging client, just for a different messaging system. In that case, an application that is a client on both messaging systems can build a Messaging Bridge between the two, effectively connecting them into one composite messaging system.