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.

The queue list endpoints (GET /api/queues and GET /api/queues/:vhost) accept a state query parameter to only return queues in the given state(s), comma separated, e.g. ?state=closed or ?state=paused,closed.

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.