Symfony Messenger component documentation

components/message.rst

@@ -0,0 +1,284 @@
+.. index::
+   single: Message
+   single: Components; Message
+
+The Message Component
+=====================
+
+    The Message component helps application to send and receive messages
+    to/from other applications or via
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/message
+
+Alternatively, you can clone the `<https://github.com/symfony/message>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Concepts
+--------
+
+.. image:: /_images/components/message/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. MessageBus' behavior is in its ordered
+middleware stack. When using the message bus with Symfony's FrameworkBundle, the
+following middlewares are configured for you:
+
+#. ``LoggingMiddleware`` (logs the processing of your messages)
+#. ``SendMessageMiddleware`` (enables asynchronous processing)
+#. ``HandleMessageMiddleware`` (calls the registered handle)
+
+Example::
+
+    use App\Message\MyMessage;
+
+    $result = $this->get('message_bus')->handle(new MyMessage(/* ... */));
+
+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...
+       }
+    }
+
+.. code-block:: xml
+
+    <service id="App\Handler\MyMessageHandler">
+       <tag name="message_handler" />
+    </service>
+
+.. note::
+
+    If the message cannot be guessed from the handler's type-hint, use the
+    ``handles`` attribute on the tag.
+
+Asynchronous messages
+~~~~~~~~~~~~~~~~~~~~~
+
+Using the Message Component is useful to decouple your application but it also
+very useful when you want to do some asynchronous processing. This means that
+your application will produce a message to a queuing system and consume this
+message later in the background, using a _worker_.
+
+Adapters
+~~~~~~~~
+
+The communication with queuing system or third parties is delegated to
+libraries for now. You can use one of the following adapters:
+
+#. `PHP Enqueue bridge`_ to use one of their 10+ compatible queues such as
+   RabbitMq, Amazon SQS or Google Pub/Sub.
+
+Routing
+-------
+
+When doing asynchronous processing, the key is to route the message to the right
+sender. As the routing is application-specific and not message-specific, the
+configuration can be made within the ``framework.yaml`` configuration file as
+well:
+
+.. code-block:: yaml
+
+    framework:
+       message:
+           routing:
+               'My\Message\MessageAboutDoingOperationalWork': my_operations_queue_sender
+
+Such configuration would only route the ``MessageAboutDoingOperationalWork``
+message to be asynchronous, the rest of the messages would still be directly
+handled.
+
+If you want to do route all the messages to a queue by default, you can use such
+configuration:
+
+.. code-block:: yaml
+
+    framework:
+       message:
+           routing:
+               'My\Message\MessageAboutDoingOperationalWork': my_operations_queue_sender
+               '*': my_default_sender
+
+Note that you can also route a message to multiple senders at the same time:
+
+.. code-block:: yaml
+
+    framework:
+       message:
+           routing:
+               'My\Message\AnImportantMessage': [my_default_sender, my_audit_sender]
+
+Same bus received and sender
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To allow us to receive and send messages on the same bus and prevent a loop, the
+message bus is equipped with the ``WrapIntoReceivedMessage`` received. It will
+wrap the received messages into ``ReceivedMessage`` objects and the
+``SendMessageMiddleware`` middleware will know it should not send these messages.
+
+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 Symfony\Component\Message\SenderInterface;
+    use App\Message\ImportantAction;
+
+    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'
+                   )
+           );
+       }
+    }
+
+Then, register your sender service:
+
+.. code-block:: yaml
+
+    services:
+         App\MessageSender\ImportantActionToEmailSender:
+             arguments:
+                 - "@mailer"
+                 - "%to_email%"
+
+             tags:
+                 - message.sender
+
+Finally, route your important message to the sender:
+
+.. code-block:: yaml
+
+    framework:
+       message:
+           routing:
+               'App\Message\ImportantAction': [App\MessageSender\ImportantActionToEmailSender, ~]
+
+.. note::
+
+    This example shows you how you can at the same time send your message and
+    directly handle it using a ``null`` (``~``) sender.
+
+Your own receiver
+-----------------
+
+A consumer is responsible for receiving messages from a source and dispatching
+them to the application.
+
+Let's say you already proceed some "orders" on 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 your custom CSV consumer and Symfony will do the rest.
+
+First, create your receiver::
+
+    namespace App\MessageReceiver;
+
+    use Symfony\Component\Message\ReceiverInterface;
+    use Symfony\Component\Serializer\SerializerInterface;
+
+    use App\Message\NewOrder;
+
+    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']);
+           }
+       }
+    }
+
+Then, register your receiver service:
+
+.. code-block:: yaml
+
+    services:
+       App\MessageReceiver\NewOrdersFromCsvFile:
+           arguments:
+               - "@serializer"
+               - "%new_orders_csv_file_path%"
+
+           tags:
+               - message.receiver
+
+Finally, use your consumer:
+
+.. code-block:: terminal
+
+    $ bin/console message:consume App\MessageReceived\NewOrdersFromCsvFile
+
+.. _`PHP Enqueue bridge`: https://github.com/sroze/enqueue-bridge