How to Use Service Bus Queues
This guide will show you how to use Service Bus queues. The samples are written in Python and use the Python Azure module. The scenarios covered include creating queues, sending and receiving messages, and deleting queues. For more information on queues, see the Next Steps section.
Table of Contents
What are Service Bus Queues?
Service Bus Queues support a brokered messaging communication model. When using queues, components of a distributed application do not communicate directly with each other, they instead exchange messages via a queue, which acts as an intermediary. A message producer (sender) hands off a message to the queue and then continues its processing. Asynchronously, a message consumer (receiver) pulls the message from the queue and processes it. The producer does not have to wait for a reply from the consumer in order to continue to process and send further messages. Queues offer First In, First Out (FIFO) message delivery to one or more competing consumers. That is, messages are typically received and processed by the receivers in the order in which they were added to the queue, and each message is received and processed by only one message consumer.
Service Bus queues are a general-purpose technology that can be used for a wide variety of scenarios:
- Communication between web and worker roles in a multi-tier Windows Azure application
- Communication between on-premises apps and Windows Azure hosted apps in a hybrid solution
- Communication between components of a distributed application running on-premises in different organizations or departments of an organization
Using queues can enable you to scale out your applications better, and enable more resiliency to your architecture.
Create a Service Namespace
To begin using Service Bus queues in Windows Azure, you must first create a service namespace. A service namespace provides a scoping container for addressing Service Bus resources within your application.
To create a service namespace:
Log on to the Windows Azure Management Portal.
In the left navigation pane of the Management Portal, click Service Bus.
In the lower pane of the Management Portal, click Create.
In the Add a new namespace dialog, enter a namespace name. The system immediately checks to see if the name is available.
After making sure the namespace name is available, choose the country or region in which your namespace should be hosted (make sure you use the same country/region in which you are deploying your compute resources).
IMPORTANT: Pick the same region that you intend to choose for deploying your application. This will give you the best performance.
Click the check mark. The system now creates your service namespace and enables it. You might have to wait several minutes as the system provisions resources for your account.
The namespace you created will then appear in the Management Portal and takes a moment to activate. Wait until the status is Active before continuing.
Obtain the Default Management Credentials for the Namespace
In order to perform management operations, such as creating a queue, on the new namespace, you must obtain the management credentials for the namespace.
In the left navigation pane, click the Service Bus node, to display the list of available namespaces:
Select the namespace you just created from the list shown:
Click Access Key.
In the Connect to your namespace dialog, find the Default Issuer and Default Key entries. Make a note of these values, as you will use this information below to perform operations with the namespace.
Note: If you need to install Python or the Client Libraries, 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 Windows Azure Service Bus:
from azure.servicebus import *
The following code creates a ServiceBusService object. Replace 'mynamespace', 'mykey' and 'myissuer' with the real namespace, key and issuer.
bus_service = ServiceBusService(service_namespace='mynamespace', account_key='mykey', issuer='myissuer')
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'
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('Test Message')
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')
Messages are deleted from the queue as they are read; however, you can read (peek) and lock the message without deleting it from the queue by setting the optional parameter peek_lock to True.
The default 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)
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.
Now that you’ve learned the basics of Service Bus queues, follow these links to learn more.