6.5. Automatically Deleted Queues
6.5.1. Automatically Deleted Queues
Queues can be configured to auto-delete. The broker will delete an auto-delete queue when it has no more subscribers, or if it is auto-delete and exclusive, when the declaring session ends.
Applications can delete queues themselves, but if an application fails or loses its connection it may not get the opportunity to clean up its queues. Specifying a queue as auto-delete delegates the responsibility to the broker to clean up the queue when it is no longer needed.
Auto-deleted queues are generally created by an application to receive messages, for example: a response queue to specify in the "reply-to" property of a message when requesting information from a service. In this scenario, an application creates a queue for its own use and subscribes it to an exchange. When the consuming application shuts down, the queue is deleted automatically. The queues created by the
qpid-config utility to receive information from the message broker are an example of this pattern.
A queue configured to
auto-delete is deleted by the broker after the last consumer has released its subscription to the queue. After the auto-delete queue is created, it becomes eligible for deletion as soon as a consumer subscribes to the queue. When the number of consumers subscribed to the queue reaches zero, the queue is deleted.
Here is an example using the Python API to create an auto-delete queue with the name "my-response-queue":
- Python
responsequeue = session.receiver('my-response-queue; {create:always, node:{x-declare:{auto-delete:True}}}')
Note
Because no bindings are specified in this queue creation, it is bound to the server's
default exchange: a pre-configured nameless direct exchange.
Custom Timeout
A custom timeout can be configured to provide a grace period before the deletion occurs.
Note
Starting from MRG-M 3.1.0, the C++ client adds a default value of 120 seconds to all durable subscriptions. The qpid python and Java clients do not have a default set, and must be configured manually.
If
qpid.auto_delete_timeout:0 is specified, the parameter has no effect: setting the parameter to 0 turns off the delayed auto-delete function.
If a timeout of 120 seconds is specified, the broker will wait for 120 seconds after the last consumer disconnects from the queue before deleting it. If a consumer subscribes to the queue within that grace period, the queue is not deleted. This is useful to allow for a consumer to drop its connection and reconnect without losing the information in its queue.
Here is an example using the Python API to create an auto-delete queue with the name "my-response-queue" and an auto-delete timeout of 120 seconds:
- Python
responsequeue = session.receiver("my-response-queue; {create:always, node:{x-declare:{auto-delete:True, arguments:{'qpid.auto_delete_timeout':120}}}}")
Be aware that a public auto-deleted queue can be deleted while your application is still sending to it, if your application is not holding it open with a receiver. You will not receive an error because you are sending to an exchange, which continues to exist; however your messages will not go to the now non-existent queue.
If you are publishing to a self-created auto-deleted queue, carefully consider whether using an auto-deleted queue is the correct approach. If the answer is "yes" (it can be useful for tests that clean up after themselves), then subscribe to the queue when you create it. Your subscription will then act as a handle, and the queue will not be deleted until you release it.
Using the Python API:
- Python
testqueue = session.sender("my-test-queue; {create:always, node:{x-declare:{auto-delete:True}}}") testqueuehandle = session.receiver("my-test-queue") ..... connection.close() # testqueuehandle is now released
An exception to the requirement that a consumer subscribe and then unsubscribe to invoke the auto-deletion is a queue configured to be
exclusive and auto-delete; these queues are deleted by the broker when the session that declared the queue ends, since the session that declared the queue is only possible subscriber.
