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This guide shows you how to perform common scenarios using the Azure Queue storage service. The samples are written in Python and use the Microsoft Azure Storage SDK for Python. The scenarios covered include inserting, peeking, getting, and deleting queue messages, as well as creating and deleting queues. For more information on queues, refer to the [Next Steps] section.
What is Queue Storage?
Azure Queue storage is a service for storing large numbers of messages that can be accessed from anywhere in the world via authenticated calls using HTTP or HTTPS. A single queue message can be up to 64 KB in size, and a queue can contain millions of messages, up to the total capacity limit of a storage account.
Common uses of Queue storage include:
- Creating a backlog of work to process asynchronously
- Passing messages from an Azure web role to an Azure worker role
Queue Service Concepts
The Queue service contains the following components:
URL format: Queues are addressable using the following URL format:
The following URL addresses a queue in the diagram:
Storage Account: All access to Azure Storage is done through a storage account. See Azure Storage Scalability and Performance Targets for details about storage account capacity.
- Queue: A queue contains a set of messages. All messages must be in a queue. Note that the queue name must be all lowercase. For information on naming queues, see Naming Queues and Metadata.
- Message: A message, in any format, of up to 64 KB. The maximum time that a message can remain in the queue is 7 days.
Create an Azure storage account
If you prefer not to create a storage account at this time, you can also use the Azure storage emulator to run and test your code in a local environment. For more information, see Use the Azure Storage Emulator for Development and Testing.
How To: Create a Queue
The QueueService object lets you work with queues. The following code creates a QueueService object. Add the following near the top of any Python file in which you wish to programmatically access Azure Storage:
from azure.storage.queue import QueueService
The following code creates a QueueService object using the storage account name and account key. Replace 'myaccount' and 'mykey' with your account name and key.
queue_service = QueueService(account_name='myaccount', account_key='mykey') queue_service.create_queue('taskqueue')
How To: Insert a Message into a Queue
To insert a message into a queue, use the put_message method to create a new message and add it to the queue.
queue_service.put_message('taskqueue', u'Hello World')
How To: Peek at the Next Message
You can peek at the message in the front of a queue without removing it from the queue by calling the peek_messages method. By default, peek_messages peeks at a single message.
messages = queue_service.peek_messages('taskqueue') for message in messages: print(message.content)
How To: Dequeue Messages
Your code removes a message from a queue in two steps. When you call get_messages, you get the next message in a queue by default. A message returned from get_messages becomes invisible to any other code reading messages from this queue. By default, this message stays invisible for 30 seconds. To finish removing the message from the queue, you must also call delete_message. This two-step process of removing a message assures that when your code fails to process a message due to hardware or software failure, another instance of your code can get the same message and try again. Your code calls delete_message right after the message has been processed.
messages = queue_service.get_messages('taskqueue') for message in messages: print(message.content) queue_service.delete_message('taskqueue', message.id, message.pop_receipt)
There are two ways you can customize message retrieval from a queue. First, you can get a batch of messages (up to 32). Second, you can set a longer or shorter invisibility timeout, allowing your code more or less time to fully process each message. The following code example uses the get_messages method to get 16 messages in one call. Then it processes each message using a for loop. It also sets the invisibility timeout to five minutes for each message.
messages = queue_service.get_messages('taskqueue', num_messages=16, visibility_timeout=5*60) for message in messages: print(message.content) queue_service.delete_message('taskqueue', message.id, message.pop_receipt)
How To: Change the Contents of a Queued Message
You can change the contents of a message in-place in the queue. If the message represents a work task, you could use this feature to update the status of the work task. The code below uses the update_message method to update a message. The visibility timeout is set to 0, meaning the message appears immediately and the content is updated.
messages = queue_service.get_messages('taskqueue') for message in messages: queue_service.update_message('taskqueue', message.id, message.pop_receipt, 0, u'Hello World Again')
How To: Get the Queue Length
You can get an estimate of the number of messages in a queue. The get_queue_metadata method asks the queue service to return metadata about the queue, and the approximate_message_count. The result is only approximate because messages can be added or removed after the queue service responds to your request.
metadata = queue_service.get_queue_metadata('taskqueue') count = metadata.approximate_message_count
How To: Delete a Queue
To delete a queue and all the messages contained in it, call the delete_queue method.
Now that you've learned the basics of Queue storage, follow these links to learn more.