[doc] Add docs about message processors.

Author: makasim

Diff for: docs/bundle/message_processor.md

@@ -0,0 +1,81 @@
+# Message processor
+
+Message processors and usage examples described in [consumption/message_processor](../consumption/message_processor.md)
+Here we just show how to register a message processor service to enqueue. Let's say we have app bundle and a message processor there
+
+* [Container tag](#container-tag)
+* [Topic subscriber](#topic-subscriber)
+
+# Container tag
+
+```yaml
+# src/AppBundle/Resources/services.yml
+
+services:
+  app.async.say_hello_processor:
+    class: 'AppBundle\Async\SayHelloProcessor'
+    tags:
+        - { name: 'enqueue.client.message_processor', topicName: 'aTopic' }
+        
+```
+
+The tag has some additional options:
+
+* topicName [Req]: Tells what topic to consume messages from.
+* queueName: By default message processor does not require an extra queue on broker side. It reuse a default one. Setting the option you can define a custom queue to be used.
+* processorName: By default the service id is used as message processor name. Using the option you can define a custom name.
+
+# Topic subscriber
+
+There is a `TopicSubscriber` interface (like [EventSubscriberInterface](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/EventDispatcher/EventSubscriberInterface.php)). 
+It allows to keep subscription login and process logic closer to each other. 
+
+```php
+<?php
+namespace AppBundle\Async;
+
+use Enqueue\Client\TopicSubscriberInterface;
+use Enqueue\Psr\Processor;
+
+class SayHelloProcessor implements Processor, TopicSubscriberInterface
+{
+    public static function getSubscribedTopics()
+    {
+        return ['aTopic', 'anotherTopic'];
+    }
+}
+```
+
+On the topic subscriber you can also define queue and processor name:
+
+```php
+<?php
+use Enqueue\Client\TopicSubscriberInterface;
+use Enqueue\Psr\Processor;
+
+class SayHelloProcessor implements Processor, TopicSubscriberInterface
+{
+    public static function getSubscribedTopics()
+    {
+        return [
+            'aTopic' => ['queueName' => 'fooQueue', 'processorName' => 'foo'], 
+            'anotherTopic' => ['queueName' => 'barQueue', 'processorName' => 'bar'], 
+        ];
+    }
+}
+```
+
+In the container you can just add the tag `enqueue.client.message_processor` and omit any other options:
+
+```yaml
+# src/AppBundle/Resources/services.yml
+
+services:
+  app.async.say_hello_processor:
+    class: 'AppBundle\Async\SayHelloProcessor'
+    tags:
+        - { name: 'enqueue.client.message_processor'}
+
+```
+
+[back to index](../index.md)

Diff for: docs/consumption/message_processor.md

@@ -0,0 +1,133 @@
+# Message processor
+
+The message processor is an object that actually process the message and must return a result status.
+Here's example:
+
+```php
+<?php
+use Enqueue\Psr\Processor;
+use Enqueue\Psr\Message;
+use Enqueue\Psr\Context;
+
+class SendMailProcessor implements Processor
+{
+    public function process(Message $message, Context $context) 
+    {
+        $this->mailer->send('[email protected]', $message->getBody());
+        
+        return self::ACK;
+    }
+}
+```
+
+Usually there is no need to catch exceptions. 
+The message broker can detect consumer has failed and redeliver the message.
+Sometimes you have to reject messages explicitly. 
+
+```php
+<?php
+use Enqueue\Psr\Processor;
+use Enqueue\Psr\Message;
+use Enqueue\Psr\Context;
+use Enqueue\Util\JSON;
+
+class SendMailProcessor implements Processor
+{
+    public function process(Message $message, Context $context) 
+    {
+        $data = JSON::decode($message->getBody());
+        if ($user  = $this->userRepository->find($data['userId'])) {
+            return self::REJECT;
+        }
+        
+        $this->mailer->send($user->getEmail(), $data['text']);
+        
+        return self::ACK;
+    }
+}
+```
+
+It is possible to find out whether the message failed previously or not. 
+There is `isRedelivered` method for that. 
+If it returns true than there was attempt to process message. 
+   
+```php
+<?php
+use Enqueue\Psr\Processor;
+use Enqueue\Psr\Message;
+use Enqueue\Psr\Context;
+
+class SendMailProcessor implements Processor
+{
+    public function process(Message $message, Context $context) 
+    {
+        if ($message->isRedelivered()) {
+            return self::REQUEUE;
+        }
+        
+        $this->mailer->send('[email protected]', $message->getBody());
+        
+        return self::ACK;
+    }
+}
+```
+
+The second argument is your context. You can use it to send messages to other queues\topics.
+ 
+```php
+<?php
+use Enqueue\Psr\Processor;
+use Enqueue\Psr\Message;
+use Enqueue\Psr\Context;
+
+class SendMailProcessor implements Processor
+{
+    public function process(Message $message, Context $context) 
+    {
+        $this->mailer->send('[email protected]', $message->getBody());
+        
+        $queue = $context->createQueue('anotherQueue');
+        $message = $context->createMessage('Message has been sent');
+        $context->createProducer()->send($queue, $message);
+        
+        return self::ACK;
+    }
+}
+```
+
+The consumption component provide some useful extensions, for example there is an extension that makes RPC processing simplier.
+ 
+```php
+<?php
+use Enqueue\Psr\Processor;
+use Enqueue\Psr\Message;
+use Enqueue\Psr\Context;
+use Enqueue\Consumption\ChainExtension;
+use Enqueue\Consumption\QueueConsumer;
+use Enqueue\Consumption\Extension\ReplyExtension;
+use Enqueue\Consumption\Result;
+
+class SendMailProcessor implements Processor
+{
+    public function process(Message $message, Context $context) 
+    {
+        $this->mailer->send('[email protected]', $message->getBody());
+        
+        $replyMessage = $context->createMessage('Message has been sent');
+        
+        return Result::reply($replyMessage);
+    }
+}
+
+/** @var \Enqueue\Psr\Context $psrContext */
+
+$queueConsumer = new QueueConsumer($psrContext, new ChainExtension([
+    new ReplyExtension()
+]));
+
+$queueConsumer->bind('foo', new SendMailProcessor());
+
+$queueConsumer->consume();
+```
+
+[back to index](../index.md)

Diff for: docs/index.md

@@ -8,6 +8,7 @@
     - [Null](null_transport.md)
 * Consumption
     - [Extensions](consumption/extensions.md)
+    - [Message processor](consumption/message_processor.md)
 * Client
     - [Message examples](client/message_examples.md)
     - [Supported brokers](client/supported_brokers.md)