AMQP 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
redeliveredflag is set totrueon 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 consumersif_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.
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