Added configuration for multiple buses

Author: Nyholm

messenger/multiple-buses.rst

@@ -0,0 +1,41 @@
+.. index::
+    single: Messenger; Multiple buses
+
+Multiple Buses
+==============
+
+A common architecture when building application is to separate commands from
+queries. Commands are actions that do something and queries fetches data. This
+is called CQRS (Command Query Responsibility Segregation). See Martin Fowler's
+`article about CQRS`_ to learn more. This architecture could be used together
+with the messenger component by defining multiple buses.
+
+A **command bus** is a little different form a **query bus**. As example,
+command buses must not return anything and query buses are rarely asynchronous.
+You can configure these buses and their rules by using middlewares.
+
+It might also be a good idea to separate actions from reactions by introducing
+an **event bus**. The event bus could have zero or more subscribers.
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        framework:
+            messenger:
+                default_bus: messenger.bus.command
+                buses:
+                    messenger.bus.command:
+                        middleware:
+                            - messenger.middleware.validation
+                            - messenger.middleware.handles_recorded_messages: ['@messenger.bus.event']
+                            - doctrine_transaction_middleware: ['default']
+                    messenger.bus.query:
+                        middleware:
+                            - messenger.middleware.validation
+                    messenger.bus.event:
+                        middleware:
+                            - messenger.middleware.allow_no_handler
+                            - messenger.middleware.validation
+
+.. _article about CQRS: https://martinfowler.com/bliki/CQRS.html