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.
- YAML
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framework: messenger: # The bus that is going to be injected when injecting MessageBusInterface default_bus: command.bus buses: command.bus: middleware: - validation - doctrine_transaction query.bus: middleware: - validation event.bus: default_middleware: allow_no_handlers middleware: - validation
- XML
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<!-- config/packages/messenger.xml --> <?xml version="1.0" encoding="UTF-8" ?> <container xmlns="http://symfony.com/schema/dic/services" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:framework="http://symfony.com/schema/dic/symfony" xsi:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd http://symfony.com/schema/dic/symfony https://symfony.com/schema/dic/symfony/symfony-1.0.xsd"> <framework:config> <!-- The bus that is going to be injected when injecting MessageBusInterface --> <framework:messenger default-bus="command.bus"> <framework:bus name="command.bus"> <framework:middleware id="validation"/> <framework:middleware id="doctrine_transaction"/> <framework:bus> <framework:bus name="query.bus"> <framework:middleware id="validation"/> <framework:bus> <framework:bus name="event.bus" default-middleware="allow_no_handlers"> <framework:middleware id="validation"/> <framework:bus> </framework:messenger> </framework:config> </container>
- PHP
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// config/packages/messenger.php $container->loadFromExtension('framework', [ 'messenger' => [ // The bus that is going to be injected when injecting MessageBusInterface 'default_bus' => 'command.bus', 'buses' => [ 'command.bus' => [ 'middleware' => [ 'validation', 'doctrine_transaction', ], ], 'query.bus' => [ 'middleware' => [ 'validation', ], ], 'event.bus' => [ 'default_middleware' => 'allow_no_handlers', 'middleware' => [ 'validation', ], ], ], ], ]);
This will create three new services:
command.bus
: autowireable with theMessageBusInterface
type-hint (because this is thedefault_bus
);query.bus
: autowireable withMessageBusInterface $queryBus
;event.bus
: autowireable withMessageBusInterface $eventBus
.
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:
- YAML
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# config/services.yaml services: App\MessageHandler\SomeCommandHandler: tags: [{ name: messenger.message_handler, bus: command.bus }] # prevent handlers from being registered twice (or you can remove # the MessageHandlerInterface that autoconfigure uses to find handlers) autoconfigure: false
- XML
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<!-- config/services.xml --> <?xml version="1.0" encoding="UTF-8" ?> <container xmlns="http://symfony.com/schema/dic/services" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd"> <services> <service id="App\MessageHandler\SomeCommandHandler"> <tag name="messenger.message_handler" bus="command.bus"/> </service> </services> </container>
- PHP
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// config/services.php $container->services() ->set(App\MessageHandler\SomeCommandHandler::class) ->tag('messenger.message_handler', ['bus' => 'command.bus']);
This way, the App\MessageHandler\SomeCommandHandler
handler will only be
known by the command.bus
bus.
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:
- YAML
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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: command.bus } query_handlers: namespace: App\MessageHandler\ resource: '%kernel.project_dir%/src/MessageHandler/*QueryHandler.php' autoconfigure: false tags: - { name: messenger.message_handler, bus: query.bus }
- XML
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<!-- config/services.xml --> <?xml version="1.0" encoding="UTF-8" ?> <container xmlns="http://symfony.com/schema/dic/services" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd"> <services> <!-- command handlers --> <prototype namespace="App\MessageHandler\" resource="%kernel.project_dir%/src/MessageHandler/*CommandHandler.php" autoconfigure="false"> <tag name="messenger.message_handler" bus="command.bus"/> </prototype> <!-- query handlers --> <prototype namespace="App\MessageHandler\" resource="%kernel.project_dir%/src/MessageHandler/*QueryHandler.php" autoconfigure="false"> <tag name="messenger.message_handler" bus="query.bus"/> </prototype> </services> </container>
- PHP
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// config/services.php // Command handlers $container->services() ->load('App\MessageHandler\\', '%kernel.project_dir%/src/MessageHandler/*CommandHandler.php') ->autoconfigure(false) ->tag('messenger.message_handler', ['bus' => 'command.bus']); // Query handlers $container->services() ->load('App\MessageHandler\\', '%kernel.project_dir%/src/MessageHandler/*QueryHandler.php') ->autoconfigure(false) ->tag('messenger.message_handler', ['bus' => 'query.bus']);
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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | $ php bin/console debug:messenger
Messenger
=========
command.bus
-----------
The following messages can be dispatched:
---------------------------------------------------------------------------------------
App\Message\DummyCommand
handled by App\MessageHandler\DummyCommandHandler
App\Message\MultipleBusesMessage
handled by App\MessageHandler\MultipleBusesMessageHandler
---------------------------------------------------------------------------------------
query.bus
---------
The following messages can be dispatched:
---------------------------------------------------------------------------------------
App\Message\DummyQuery
handled by App\MessageHandler\DummyQueryHandler
App\Message\MultipleBusesMessage
handled by App\MessageHandler\MultipleBusesMessageHandler
---------------------------------------------------------------------------------------
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