What is a message queue?

A message queue acts as an intermediary between systems that exchange event data. The sender - for example, a store - places a message in the queue and does not wait for a response. The recipient - for example, an ERP or CRM system - retrieves messages one by one when it is ready. The queue is managed by a separate program, known as a broker, such as RabbitMQ.

The queue protects against data loss, but not against duplicates. A message is delivered at least once, so if the connection is lost, it may be delivered a second time, and it is up to the recipient to recognize that it has already been processed. Magento has a built-in queuing mechanism, but for production environments, Adobe recommends an external broker because a simple database-based adapter does not scale.