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How to Use Service Bus Queues

This guide describes how to use Service Bus queues. The samples are written in Python and use the Python Azure package. The scenarios covered include creating queues, sending and receiving messages, and deleting queues. For more information on queues, see the [Next Steps][] section.

[AZURE.INCLUDE howto-service-bus-queues]

Note: If you need to install Python or the Python Azure package, please see the Python Installation Guide.

How to create a queue

The ServiceBusService object lets you work with queues. Add the following near the top of any Python file in which you wish to programmatically access Azure Service Bus:

from azure.servicebus import ServiceBusService, Message, Queue

The following code creates a ServiceBusService object. Replace 'mynamespace', 'sharedaccesskeyname' and 'sharedaccesskey' with the real namespace, shared access signature (SAS) key name and value.

bus_service = ServiceBusService(
    service_namespace='mynamespace',
    shared_access_key_name='sharedaccesskeyname',
    shared_access_key_value='sharedaccesskey')

The values for the SAS key name and value can be found in the Azure Portal connection information, or in Visual Studio Properties window when selecting the service bus namespace in Server Explorer (as shown in the previous section).

bus_service.create_queue('taskqueue')

create_queue also supports additional options, which allow you to override default queue settings such as message time to live or maximum queue size. The following example shows setting the maximum queue size to 5GB a time to live of 1 minute:

queue_options = Queue()
queue_options.max_size_in_megabytes = '5120'
queue_options.default_message_time_to_live = 'PT1M'

bus_service.create_queue('taskqueue', queue_options)

How to send messages to a queue

To send a message to a Service Bus queue, your application will call the send_queue_message method on the ServiceBusService object.

The following example demonstrates how to send a test message to the queue named taskqueue using send_queue_message:

msg = Message(b'Test Message')
bus_service.send_queue_message('taskqueue', msg)

Service Bus queues support a maximum message size of 256 KB (the header, which includes the standard and custom application properties, can have a maximum size of 64 KB). There is no limit on the number of messages held in a queue but there is a cap on the total size of the messages held by a queue. This queue size is defined at creation time, with an upper limit of 5 GB.

How to receive messages from a queue

Messages are received from a queue using the receive_queue_message method on the ServiceBusService object:

msg = bus_service.receive_queue_message('taskqueue', peek_lock=False)
print(msg.body)

Messages are deleted from the queue as they are read when the parameter peek_lock is set to False. You can read (peek) and lock the message without deleting it from the queue by setting the parameter peek_lock to True.

The behavior of reading and deleting the message as part of the receive operation is the simplest model, and works best for scenarios in which an application can tolerate not processing a message in the event of a failure. To understand this, consider a scenario in which the consumer issues the receive request and then crashes before processing it. Because Service Bus will have marked the message as being consumed, then when the application restarts and begins consuming messages again, it will have missed the message that was consumed prior to the crash.

If the peek_lock parameter is set to True, the receive becomes a two stage operation, which makes it possible to support applications that cannot tolerate missing messages. When Service Bus receives a request, it finds the next message to be consumed, locks it to prevent other consumers receiving it, and then returns it to the application. After the application finishes processing the message (or stores it reliably for future processing), it completes the second stage of the receive process by calling the delete method on the Message object. The delete method will mark the message as being consumed and remove it from the queue.

msg = bus_service.receive_queue_message('taskqueue', peek_lock=True)
print(msg.body)

msg.delete()

How to handle application crashes and unreadable messages

Service Bus provides functionality to help you gracefully recover from errors in your application or difficulties processing a message. If a receiver application is unable to process the message for some reason, then it can call the unlock method on the Message object. This will cause Service Bus to unlock the message within the queue and make it available to be received again, either by the same consuming application or by another consuming application.

There is also a timeout associated with a message locked within the queue, and if the application fails to process the message before the lock timeout expires (e.g., if the application crashes), then Service Bus will unlock the message automatically and make it available to be received again.

In the event that the application crashes after processing the message but before the delete method is called, then the message will be redelivered to the application when it restarts. This is often called At Least Once Processing, that is, each message will be processed at least once but in certain situations the same message may be redelivered. If the scenario cannot tolerate duplicate processing, then application developers should add additional logic to their application to handle duplicate message delivery. This is often achieved using the MessageId property of the message, which will remain constant across delivery attempts.

Next Steps

Now that you have learned the basics of Service Bus queues, follow these links to learn more.

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