Symfony Messenger component documentation

components/messenger.rst

@@ -0,0 +1,189 @@
+.. index::
+   single: Messenger
+   single: Components; Messenger
+
+The Messenger Component
+=======================
+
+    The Messenger component helps applications send and receive messages to/from other applications or via message queues.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/messenger
+
+Alternatively, you can clone the `<https://github.com/symfony/messenger>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Concepts
+--------
+
+.. image:: /_images/components/messenger/overview.png
+
+**Sender**:
+   Responsible for serializing and sending the message to _something_. This
+   something can be a message broker or a third party API for example.
+
+**Receiver**:
+   Responsible for deserializing and forwarding the messages to handler(s). This
+   can be a message queue puller or an API endpoint for example.
+
+**Handler**:
+   Given a received message, contains the user business logic related to the
+   message. In practice, that is just a PHP callable.
+
+Bus
+---
+
+The bus is used to dispatch messages. The behaviour of the bus is in its ordered
+middleware stack. The component comes with a set of middlewares that you can use.
+
+When using the message bus with Symfony's FrameworkBundle, the following middlewares
+are configured for you:
+
+#. :code:`LoggingMiddleware` (logs the processing of your messages)
+#. :code:`SendMessageMiddleware` (enables asynchronous processing)
+#. :code:`HandleMessageMiddleware` (calls the registered handle)
+
+Example::
+
+    use App\Message\MyMessage;
+    use Symfony\Component\Messenger\MessageBus;
+    use Symfony\Component\Messenger\HandlerLocator;
+    use Symfony\Component\Messenger\Middleware\HandleMessageMiddleware;
+
+    $bus = new MessageBus([
+        new HandleMessageMiddleware(new HandlerLocator([
+            MyMessage::class => $handler,
+        ])),
+    ]);
+
+    $result = $bus->handle(new MyMessage(/* ... */));
+
+.. note:
+
+    Every middleware need to implement the :code:`MiddlewareInterface` interface.
+
+Handlers
+--------
+
+Once dispatched to the bus, messages will be handled by a "message handler". A
+message handler is a PHP callable (i.e. a function or an instance of a class)
+that will do the required processing for your message. It _might_ return a
+result::
+
+    namespace App\MessageHandler;
+
+    use App\Message\MyMessage;
+
+    class MyMessageHandler
+    {
+       public function __invoke(MyMessage $message)
+       {
+           // Message processing...
+       }
+    }
+
+Adapters
+--------
+
+The communication with queuing system or third parties is delegated to
+libraries for now.
+
+Your own sender
+~~~~~~~~~~~~~~~
+
+Using the ``SenderInterface``, you can easily create your own message sender.
+Let's say you already have an ``ImportantAction`` message going through the
+message bus and handled by a handler. Now, you also want to send this message as
+an email.
+
+First, create your sender::
+
+    namespace App\MessageSender;
+
+    use App\Message\ImportantAction;
+    use Symfony\Component\Message\SenderInterface;
+
+    class ImportantActionToEmailSender implements SenderInterface
+    {
+       private $toEmail;
+       private $mailer;
+
+       public function __construct(\Swift_Mailer $mailer, string $toEmail)
+       {
+           $this->mailer = $mailer;
+           $this->toEmail = $toEmail;
+       }
+
+       public function send($message)
+       {
+           if (!$message instanceof ImportantAction) {
+               throw new \InvalidArgumentException(sprintf('Producer only supports "%s" messages.', ImportantAction::class));
+           }
+
+           $this->mailer->send(
+               (new \Swift_Message('Important action made'))
+                   ->setTo($this->toEmail)
+                   ->setBody(
+                       '<h1>Important action</h1><p>Made by '.$message->getUsername().'</p>',
+                       'text/html'
+                   )
+           );
+       }
+    }
+
+Your own receiver
+~~~~~~~~~~~~~~~~~
+
+A receiver is responsible for receiving messages from a source and dispatching
+them to the application.
+
+Let's say you already processed some "orders" in your application using a
+``NewOrder`` message. Now you want to integrate with a 3rd party or a legacy
+application but you can't use an API and need to use a shared CSV file with new
+orders.
+
+You will read this CSV file and dispatch a ``NewOrder`` message. All you need to
+do is to write your custom CSV receiver and Symfony will do the rest.
+
+First, create your receiver::
+
+    namespace App\MessageReceiver;
+
+    use App\Message\NewOrder;
+    use Symfony\Component\Message\ReceiverInterface;
+    use Symfony\Component\Serializer\SerializerInterface;
+
+    class NewOrdersFromCsvFile implements ReceiverInterface
+    {
+       private $serializer;
+       private $filePath;
+
+       public function __construct(SerializerInteface $serializer, string $filePath)
+       {
+           $this->serializer = $serializer;
+           $this->filePath = $filePath;
+       }
+
+       public function receive() : \Generator
+       {
+           $ordersFromCsv = $this->serializer->deserialize(file_get_contents($this->filePath), 'csv');
+
+           foreach ($ordersFromCsv as $orderFromCsv) {
+               yield new NewOrder($orderFromCsv['id'], $orderFromCsv['account_id'], $orderFromCsv['amount']);
+           }
+       }
+    }
+
+Same bus received and sender
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To allow us to receive and send messages on the same bus and prevent an infinite
+loop, the message bus is equipped with the :code:`WrapIntoReceivedMessage` middleware.
+It will wrap the received messages into :code:`ReceivedMessage` objects and the
+:code:`SendMessageMiddleware` middleware will know it should not route these
+messages again to an adapter.

index.rst

@@ -41,6 +41,7 @@ Topics
     frontend
     http_cache
     logging
+    messenger
     performance
     profiler
     routing