Multiple Buses

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Multiple Buses

A common architecture when building applications is to separate commands from queries. Commands are actions that do something and queries fetch 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 from a query bus. For example, command buses usually don't provide any results and query buses are rarely asynchronous. You can configure these buses and their rules by using middleware.

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.

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framework:
    messenger:
        # The bus that is going to be injected when injecting MessageBusInterface
        default_bus: messenger.bus.commands
        buses:
            messenger.bus.commands:
                middleware:
                    - validation
                    - doctrine_transaction
            messenger.bus.queries:
                middleware:
                    - validation
            messenger.bus.events:
                default_middleware: allow_no_handlers
                middleware:
                    - validation

This will generate the messenger.bus.commands, messenger.bus.queries and messenger.bus.events services that you can inject in your services.

Type-hints and Auto-wiring

Auto-wiring is a great feature that allows you to reduce the amount of configuration required for your service container to be created. By using MessageBusInterface as argument type-hint in your services, the default configured bus will be injected (i.e messenger.bus.commands in above examples).

When working with multiple buses, you can use the DependencyInjection component's binding capabilities to clarify which bus will be injected based on the argument's name:

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# config/services.yaml
services:
    _defaults:
        # ...

        bind:
            $commandBus: '@messenger.bus.commands'
            $queryBus: '@messenger.bus.queries'
            $eventBus: '@messenger.bus.events'

Restrict Handlers per Bus

By default, each handler will be available to handle messages on all of your buses. To prevent dispatching a message to the wrong bus without an error, you can restrict each handler to a specific bus using the messenger.message_handler tag:

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# config/services.yaml
services:
    App\MessageHandler\SomeCommandHandler:
        tags: [{ name: messenger.message_handler, bus: messenger.bus.commands }]

This way, the App\MessageHandler\SomeCommandHandler handler will only be known by the messenger.bus.commands bus.

Tip

If you manually restrict handlers be sure to have autoconfigure disabled, or not implement the Symfony\Component\Messenger\Handler\MessageHandlerInterface as this might cause your handler to be registered twice.

See service autoconfiguration for more information.

You can also automatically add this tag to a number of classes by following a naming convention and registering all of the handler services by name with the correct tag:

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# config/services.yaml

# put this after the "App\" line that registers all your services
command_handlers:
    namespace: App\MessageHandler\
    resource: '%kernel.project_dir%/src/MessageHandler/*CommandHandler.php'
    autoconfigure: false
    tags:
        - { name: messenger.message_handler, bus: messenger.bus.commands }

query_handlers:
    namespace: App\MessageHandler\
    resource: '%kernel.project_dir%/src/MessageHandler/*QueryHandler.php'
    autoconfigure: false
    tags:
        - { name: messenger.message_handler, bus: messenger.bus.queries }

Debugging the Buses

The debug:messenger command lists available messages & handlers per bus. You can also restrict the list to a specific bus by providing its name as argument.

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$ php bin/console debug:messenger

  Messenger
  =========

  messenger.bus.commands
  ----------------------

   The following messages can be dispatched:

   ---------------------------------------------------------------------------------------
    App\Message\DummyCommand
        handled by App\MessageHandler\DummyCommandHandler
    App\Message\MultipleBusesMessage
        handled by App\MessageHandler\MultipleBusesMessageHandler
   ---------------------------------------------------------------------------------------

  messenger.bus.queries
  ---------------------

   The following messages can be dispatched:

   ---------------------------------------------------------------------------------------
    App\Message\DummyQuery
        handled by App\MessageHandler\DummyQueryHandler
    App\Message\MultipleBusesMessage
        handled by App\MessageHandler\MultipleBusesMessageHandler
   ---------------------------------------------------------------------------------------
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.