How to use Service Bus topics and subscriptions with Python

This article describes how to use Service Bus topics and subscriptions. The samples are written in Python and use the Azure Python SDK package. 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 about topics and subscriptions, see the Next Steps section.

What are Service Bus topics and subscriptions?

Service Bus topics and subscriptions support a publish/subscribe messaging communication model. When using topics and subscriptions, components of a distributed application do not communicate directly with each other; instead they exchange messages via a topic, which acts as an intermediary.

TopicConcepts

In contrast with Service Bus queues, in which each message is processed by a single consumer, topics and subscriptions provide a "one-to-many" form of communication, using a publish/subscribe pattern. It is possible to register multiple subscriptions to a topic. When a message is sent to a topic, it is then made available to each subscription to handle/process independently.

A subscription to a topic resembles a virtual queue that receives copies of the messages that were sent to the topic. You can optionally register filter rules for a topic on a per-subscription basis, which enables you to filter or restrict which messages to a topic are received by which topic subscriptions.

Service Bus topics and subscriptions enable you to scale and process a very large number of messages across many users and applications.

Create a namespace

To begin using Service Bus topics and subscriptions in Azure, you must first create a service namespace. A namespace provides a scoping container for addressing Service Bus resources within your application.

To create a namespace:

  1. Log on to the Azure portal.
  2. In the left navigation pane of the portal, click New, then click Enterprise Integration, and then click Service Bus.
  3. In the Create namespace dialog, enter a namespace name. The system immediately checks to see if the name is available.
  4. After making sure the namespace name is available, choose the pricing tier (Basic, Standard, or Premium).
  5. In the Subscription field, choose an Azure subscription in which to create the namespace.
  6. In the Resource group field, choose an existing resource group in which the namespace will live, or create a new one.
  7. In Location, choose the country or region in which your namespace should be hosted.

    Create namespace

  8. Click the Create button. The system now creates your namespace and enables it. You might have to wait several minutes as the system provisions resources for your account.

Obtain the credentials

  1. In the list of namespaces, click the newly created namespace name.
  2. In the Service Bus namespace blade, click Shared access policies.
  3. In the Shared access policies blade, click RootManageSharedAccessKey.

    connection-info

  4. In the Policy: RootManageSharedAccessKey blade, click the copy button next to Connection string–primary key, to copy the connection string to your clipboard for later use.

    connection-string

Note

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

Create a topic

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

from azure.servicebus import ServiceBusService, Message, Topic, Rule, DEFAULT_RULE_NAME

The following code creates a ServiceBusService object. Replace mynamespace, sharedaccesskeyname, and sharedaccesskey with your actual namespace, Shared Access Signature (SAS) key name, and key value.

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

You can obtain the values for the SAS key name and value from the Azure portal.

bus_service.create_topic('mytopic')

The create_topic method also supports additional options, which enable you to override default topic settings such as message time to live or maximum topic size. The following example sets the maximum topic size to 5 GB, and a time to live (TTL) value 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)

Create subscriptions

Subscriptions to topics 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 to which they are subscribed, 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 define filters that enable you to specify which messages sent to a topic should show up within a specific topic subscription.

The most flexible type of filter supported by subscriptions is a SqlFilter, which implements a subset of SQL92. SQL filters operate on the properties of the messages that are published to the topic. For more information about the expressions that can be used with a SQL filter, see the SqlFilter.SqlExpression syntax.

You can add filters 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

Because 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 following example 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)

Now, when a message is sent to mytopic it is always delivered to receivers subscribed to the AllMessages topic subscription, and selectively delivered to receivers subscribed to the HighMessages and LowMessages topic subscriptions (depending on the message content).

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 determines which subscriptions receive it):

for i in range(5):
    msg = Message('Msg {0}'.format(i).encode('utf-8'), custom_properties={'messagenumber':i})
    bus_service.send_topic_message('mytopic', msg)

Service Bus topics support a maximum message size of 256 KB in the Standard tier and 1 MB in the Premium tier. 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 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. For more information about quotas, see Service Bus quotas.

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', peek_lock=False)
print(msg.body)

Messages are deleted from the subscription 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 delete method on the Message object. The delete method marks the message as being consumed and removes 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 (for example, if the application crashes), then Service Bus unlocks the message automatically and makes 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.

Delete topics and subscriptions

Topics and subscriptions are persistent, and must be explicitly deleted either through the Azure portal or programmatically. The following example shows how to delete the topic named mytopic:

bus_service.delete_topic('mytopic')

Deleting a topic also deletes any subscriptions that are registered with the topic. Subscriptions can also be deleted independently. The following code shows 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.