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Queues

A queue is a named, ordered store of messages, owned by a vhost, that delivers those messages to one or more consumers. It is the destination that exchanges route messages to via bindings.

Queue Types

The queue type is selected at declaration time and cannot be changed afterwards.

Type x-queue-type Description
Standard (default) FIFO queue with optional durability
Priority (use x-max-priority) Delivers messages by priority
Stream stream Append-only log for multiple consumers
MQTT Session mqtt Internal, used by MQTT sessions

See also: Priority Queues, Streams, Delayed Queues

Queue Properties

Property Description
durable Queue survives server restart. Messages in a durable queue are persisted to disk.
exclusive Queue is exclusive to the declaring connection. Deleted when the connection closes. Cannot be accessed by other connections.
auto_delete Queue is deleted when the last consumer unsubscribes.

A non-durable, non-exclusive queue is called a transient queue. Its messages are still written to disk but the queue is removed on server restart.

Queue Arguments

Argument Type Description
x-message-ttl Int (>= 0) Default message TTL in milliseconds. See TTL.
x-expires Int (>= 1) Queue expiration after inactivity in milliseconds. See TTL.
x-max-length Int (>= 0) Maximum number of messages in the queue
x-max-length-bytes Int (>= 0) Maximum total size of messages in bytes
x-overflow String Overflow behavior (see below)
x-dead-letter-exchange String Dead letter exchange. See Dead Lettering.
x-dead-letter-routing-key String Routing key for dead-lettered messages
x-delivery-limit Int (>= 0) Max redelivery attempts before dead-lettering
x-consumer-timeout Int (>= 0) Consumer idle timeout in milliseconds
x-single-active-consumer Bool Only one consumer receives messages at a time
x-max-priority Int (0-255) Enable priority queue with this many priority levels
x-message-deduplication Bool Enable message deduplication. See Deduplication.
x-cache-size Int (>= 0) Deduplication cache size
x-cache-ttl Int (>= 0) Deduplication cache TTL in milliseconds
x-deduplication-header String Header to use for deduplication key

Overflow Behavior

When a queue reaches its x-max-length or x-max-length-bytes limit, the overflow policy determines what happens:

Policy Behavior
drop-head (default) The oldest message is removed from the head of the queue
reject-publish New messages are rejected (basic.nack sent to publisher)

Overflow behavior can be set via the x-overflow queue argument or the overflow policy.

Requeue Behavior

When a consumer rejects or nacks a message with requeue=true:

  • The message is placed back in the queue
  • It may be delivered to a different consumer
  • The redelivered flag is set to true on the next delivery

When requeue=false, the message is either dead-lettered (if a DLX is configured) or discarded.

Queue Declaration

Declaring a queue that already exists succeeds only if the properties and arguments match the existing queue. A mismatch results in a PRECONDITION_FAILED channel error.

Queue Deletion

Deleting a queue removes it and all its messages. Options:

  • if_unused — only delete if no consumers
  • if_empty — only delete if no messages

Purge

queue.purge removes all messages from a queue without deleting the queue itself.

Queue States

A queue can be in one of the following states:

State Description
running Normal operation, delivering messages to consumers
paused Queue stops delivering messages but continues accepting publishes. Resume via the API.
closed Queue is closed due to an error. Durable, non-exclusive queues can be restarted via the API.
deleted Queue has been deleted

Pause and resume are available via the management API (PUT /api/queues/:vhost/:name/pause and /resume). A closed queue can be restarted with PUT /api/queues/:vhost/:name/restart.

Reserved Queue Name Prefixes

Queue names starting with amq. or mqtt. are reserved for server-internal use. Client queue declarations using these prefixes will be rejected, except for amq.direct.reply-to.* queues used for direct reply-to consumers.