Queues and Consumer Flow

Objective

Choose a queue's lifecycle and delivery flow, including durable topology, queue type, competing consumers, and bounded prefetch.

Why it matters

Queues buffer work between producers and consumers; queue choice and consumer credit control reliability, ordering, and resource use.

Mental model

A queue is a broker-owned work backlog. Consumers borrow deliveries from it; prefetch is the maximum number each consumer may hold before returning credit with acknowledgements.

Explanation

Queues are generally FIFO, but multiple consumers, requeues, and priorities change observed order. Durable queue metadata plus persistent messages are both needed to retain messages across a broker restart. Competing consumers share work from one queue; a second queue bound to the same exchange gets its own copy. Use manual acknowledgements and bounded prefetch to cap in-flight work. Queue declarations must remain property-equivalent; changing immutable declaration arguments requires a migration strategy.

Worked example

orders.billing has three workers with prefetch 10. At most 30 deliveries can be unacknowledged across them. A fourth analytics queue bound to the same event exchange receives an independent copy, not a share of the billing queue's work.

Common misconceptions

• Two consumers of one queue compete; they do not each receive every message.
• Durable queues alone do not preserve transient messages across a restart.
• FIFO does not imply end-to-end processing order with concurrent consumers or redelivery.

Misconception log

Retrieval questions

• Predict distribution with two consumers and two queues.
• Choose prefetch for a handler that calls a slow external service.

Connections

• Prerequisites: N01-connections-channels-topology.
• Enables: N04-acknowledgements-confirms-idempotency, N07-priority-lazy-queues, N08-javascript-typescript-python-clients.
• Knowledge base: knowledge-base.

Sources

• RabbitMQ Queues
• Consumer Acknowledgements and Publisher Confirms