By using named destinations, you can support fan-in and fan-out use cases. Fan-in use cases are when multiple sources all send data to the same named destination, as shown in the following example:
s3 > :data ftp > :data http > :data
The preceding example directs the data payloads from the Amazon S3, FTP, and HTTP sources to the same named destination called
Then an additional stream created with the following DSL expression would have all the data from those three sources sent to the file sink:
:data > file
A graphical representation can be seen as follows:
The fan-out use case is when you determine the destination of a stream based on some information that is only known at runtime. In this case, the Router Application can be used to specify how to direct the incoming message to one of N named destinations.
A nice Video showing Fan-in and Fan-out behavior is also available.