How to Use Service Bus Topics/Subscriptions
This guide will show you how to use Service Bus topics and subscriptions from Python applications. The scenarios covered include creating topics and subscriptions, creating subscription filters, sending messages to a topic, receiving messages from a subscription, and deleting topics and subscriptions. For more information on topics and subscriptions, see the Next Steps section.
Table of Contents
Note: If you need to install Python or the Client Libraries, please see the Python Installation Guide.
How to Create a Topic
The ServiceBusService object lets you work with topics. 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')
bus_service.create_topic('mytopic') create_topic also supports additional options, which allow you to override default topic settings such as message time to live or maximum topic size. The following example shows demonstrates setting the maximum topic size to 5GB a time to live of 1 minute:
topic_options = Topic()
topic_options.max_size_in_megabytes = '5120'
topic_options.default_message_time_to_live = 'PT1M'
bus_service.create_topic('mytopic', topic_options) How to Create Subscriptions
Topic subscriptions are also created with the ServiceBusService object. Subscriptions are named and can have an optional filter that restricts the set of messages delivered to the subscription's virtual queue.
Note: Subscriptions are persistent and will continue to exist until either they, or the topic they are associated with, are deleted.
Create a Subscription with the default (MatchAll) Filter
The MatchAll filter is the default filter that is used if no filter is specified when a new subscription is created. When the MatchAll filter is used, all messages published to the topic are placed in the subscription's virtual queue. The following example creates a subscription named 'AllMessages' and uses the default MatchAll filter.
bus_service.create_subscription('mytopic', 'AllMessages') Create Subscriptions with Filters
You can also setup filters that allow you to scope which messages sent to a topic should show up within a specific topic subscription.
The most flexible type of filter supported by subscriptions is the SqlFilter, which implements a subset of SQL92. SQL filters operate on the properties of the messages that are published to the topic. For more details about the expressions that can be used with a SQL filter, review the SqlFilter.SqlExpression syntax.
Filters can be added to a subscription by using the create_rule method of the ServiceBusService object. This method allows you to add new filters to an existing subscription.
Note: Since the default filter is applied automatically to all new subscriptions, you must first remove the default filter or the MatchAll will override any other filters you may specify. You can remove the default rule by using the delete_rule method of the ServiceBusService object.
The example below creates a subscription named 'HighMessages' with a SqlFilter that only selects messages that have a custom messagenumber property greater than 3:
bus_service.create_subscription('mytopic', 'HighMessages')
rule = Rule()
rule.filter_type = 'SqlFilter'
rule.filter_expression = 'messagenumber > 3'
bus_service.create_rule('mytopic', 'HighMessages', 'HighMessageFilter', rule)
bus_service.delete_rule('mytopic', 'HighMessages', DEFAULT_RULE_NAME) Similarly, the following example creates a subscription named 'LowMessages' with a SqlFilter that only selects messages that have a messagenumber property less than or equal to 3:
bus_service.create_subscription('mytopic', 'LowMessages')
rule = Rule()
rule.filter_type = 'SqlFilter'
rule.filter_expression = 'messagenumber <= 3'
bus_service.create_rule('mytopic', 'LowMessages', 'LowMessageFilter', rule)
bus_service.delete_rule('mytopic', 'LowMessages', DEFAULT_RULE_NAME) When a message is now sent to 'mytopic', it will always be delivered to receivers subscribed to the 'AllMessages' topic subscription, and selectively delivered to receivers subscribed to the 'HighMessages' and 'LowMessages' topic subscriptions (depending upon the message content).
How to Send Messages to a Topic
To send a message to a Service Bus topic, your application must use the send_topic_message method of the ServiceBusService object.
The following example demonstrates how to send five test messages to 'mytopic'. Note that the messagenumber property value of each message varies on the iteration of the loop (this will determine which subscriptions receive it):
for i in range(5):
msg = Message('Msg ' + str(i), custom_properties={'messagenumber':i})
bus_service.send_topic_message('mytopic', msg) Service Bus topics support a maximum message size of 256 MB (the header, which includes the standard and custom application properties, can have a maximum size of 64 MB). There is no limit on the number of messages held in a topic but there is a cap on the total size of the messages held by a topic. This topic size is defined at creation time, with an upper limit of 5 GB.
How to Receive Messages from a Subscription
Messages are received from a subscription using the receive_subscription_message method on the ServiceBusService object:
msg = bus_service.receive_subscription_message('mytopic', 'LowMessages')
print(msg.body) Messages are deleted from the subscription as they are read; however, you can read (peek) and lock the message without deleting it from the subscription 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 delete method on the Message object. The delete method will mark the message as being consumed and remove it from the subscription.
msg = bus_service.receive_subscription_message('mytopic', 'LowMessages', 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 subscription 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 subscription, 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.
How to Delete Topics and Subscriptions
Topics and subscriptions are persistent, and must be explicitly deleted either through the Windows Azure Management portal or programmatically. The example below demonstrates how to delete the topic named 'mytopic':
bus_service.delete_topic('mytopic') Deleting a topic will also delete any subscriptions that are registered with the topic. Subscriptions can also be deleted independently. The following code demonstrates how to delete a subscription named 'HighMessages' from the 'mytopic' topic:
bus_service.delete_subscription('mytopic', 'HighMessages') Next Steps
Now that you've learned the basics of Service Bus topics, follow these links to learn more.