Request-reply
The simplest pattern you'll see with messaging is the request-reply pattern. Also known as request-response, this is a method of retrieving data that is owned by another part of the application.
In many cases the sending of a command is an asynchronous operation. A command is fired and the application flow continues on. Because of this, there is no easy way to do things like lookup a record by ID. Instead one needs to send a command to retrieve a record and then wait for the associated event to be returned. A normal workflow looks like the following diagram:
Most events can be subscribed to by any number listeners. While it is possible ...
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