Message queues overview
The Message Queue Framework (MQF) is a system that allows a module to publish messages to queues. It also defines the consumers that will receive the messages asynchronously. The MQF uses 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.
The following diagram illustrates the Message Queue Framework:
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A publisher is a component that sends messages to an exchange. It knows which exchange to publish to and the format of the messages it sends.
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An exchange receives messages from publishers and sends them to queues. Although RabbitMQ supports multiple types of exchanges, Commerce uses topic exchanges only. A topic includes a routing key, which contains text strings separated by dots. The format for a topic name is
string1.string2: for example,customer.createdorcustomer.sent.email.The broker allows you to use wildcards when setting rules for forwarding messages. You can use an asterisk (
*) to replace one string or a pound sign (#) to replace 0 or more strings. For example,customer.*would filter oncustomer.createandcustomer.delete, but notcustomer.sent.email. Howevercustomer.#would filter oncustomer.create,customer.delete, andcustomer.sent.email. -
A queue is a buffer that stores messages.
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A consumer receives messages. It knows which queue to consume. It can map processors of the message to a specific queue.
A basic message queue system can also be set up without using RabbitMQ. In this system, a MySQL adapter 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.