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message-queues.md

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99_Module Development
Message Queues
Message Queues
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/guides/v2.0/extension-dev-guide/message-queues.html
Services

Message queues provide an asynchronous communications mechanism in which the sender and the receiver of a message do not contact each other. Nor do they need to communicate with the message queue at the same time. When a sender places a messages onto a queue, it is stored until the recipient receives them.

In {{site.data.var.ee}}, the Message Queue Framework (MQF) is a fully-functional system that allows a {% glossarytooltip c1e4242b-1f1a-44c3-9d72-1d5b1435e142 %}module{% endglossarytooltip %} to publish messages to queues. It also creates consumers to receive them asynchronously. The MQF primarily uses RabbitMQ as the messaging broker, which provides a scalable platform for sending and receiving messages. It also includes a mechanism for storing undelivered messages. RabbitMQ is based on the Advanced Message Queuing Protocol (AMQP) 0.9.1 specification.

A basic message queue system can also be set up without using RabbitMQ. In this system, a MySQL {% glossarytooltip edb42858-1ff8-41f9-80a6-edf0d86d7e10 %}adapter{% endglossarytooltip %} stores messages in the database. Three database tables (queue, queue_message, and queue_message_status) manage the message queue workload. Cron jobs ensure the consumers are able to receive messages. This solution is not very scalable. RabbitMQ should be used whenever possible.

See [Configure message queue topology]({{ page.baseurl }}/extension-dev-guide/message-queues/config-mq.html) for information about setting up the message queue system.

Send a message from the publisher to a queue

The following code sends a message to the queue. The publish method is defined in PublisherInterface

{% highlight php startinline=true %} $publisher->publish($topic, $message) {% endhighlight %}

In an MySQL adapter environment, when a message is published to multiple queues, create a single record in queue_message and multiple records in queue_message_status: one for each queue. (A join on the queue, queue_message, and queue_message_status tables is required).

Instantiate a consumer

The procedure for instantiating a consumer differs, depending on which message queue system is being used.

RabbitMQ

This instantiates a consumer that is defined in a queue.xml file. The consumer (customer_created_listener)listens to the queue and receives all new messages. For every message, it invokes Magento\Some\Class::processMessage($message)

{% highlight php startinline=true %} $this->consumerFactory->get('customer_created_listener') ->process(); {% endhighlight %}

MySQL adapter

Implement \Magento\Framework\MessageQueue\ConsumerInterface::process($maxNumberOfMessages) to instantiate a consumer.

Perform the following actions:

  1. Define the queue name associated with current consumer using \Magento\Framework\MessageQueue\ConsumerConfigurationInterface::getQueueName.
  2. Select $maxNumberOfMessages message records, filtering on the queue_name field. You must join on all 3 tables. To accomplish this, you may want to extract fewer records at a time to improve load distribution between multiple consumers.
  3. Decode the message using topic name taken from the \Magento\Framework\MessageQueue\ConsumerConfigurationInterface.
  4. Invoke callback Magento\Framework\MessageQueue\ConsumerConfigurationInterface::getCallback and pass the decoded data as an argument.

Override topic configuration

The following sample introduces a runtime configuration that allows you to redefine the adapter for a topic.

{% highlight php startinline=true %} 'queue' => array( 'topics' => array( 'customer.created' => [publisher="default-rabitmq"], 'order.created' => [publisher="default-rabitmq"], ), ), {% endhighlight %}

Related Topics

  • [Message Queues Overview]({{ page.baseurl }}/config-guide/mq/rabbitmq-overview.html)
  • [Configure message queue topology]({{ page.baseurl }}/extension-dev-guide/message-queues/config-mq.html)
  • [Install RabbitMQ]({{ page.baseurl }}/install-gde/prereq/install-rabbitmq.html)