How to Use Service Bus Topics/Subscriptions
This guide will show you how to use Service Bus topics and subscriptions. The samples are written in Java and use the Windows Azure SDK for Java. 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.
Table of Contents
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, they instead exchange messages via a topic, which acts as an intermediary.
In contrast to Service Bus queues, where 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 topic subscription 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 allows you to filter/restrict which messages to a topic are received by which topic subscriptions.
Service Bus topics and subscriptions enable you to scale to process a very large number of messages across a very large number of users and applications.
Create a Service Namespace
To begin using Service Bus topics and subscriptions 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:
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Log on to the Windows Azure Management Portal.
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In the left navigation pane of the Management Portal, click Service Bus.
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In the lower pane of the Management Portal, click Create.
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In the Add a new namespace dialog, enter a namespace name. The system immediately checks to see if the name is available.
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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.
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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.
Obtain the Default Management Credentials for the Namespace
In order to perform management operations, such as creating a topic or subscription, on the new namespace, you need to obtain the management credentials for the namespace.
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In the left navigation pane, click the Service Bus node to display the list of available namespaces:
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Select the namespace you just created from the list shown:
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Click Access Key.
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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.
Configure Your Application to Use Service Bus
Add the following import statements to the top of the Java file:
// Include the following imports to use service bus APIs
import com.microsoft.windowsazure.services.serviceBus.*;
import com.microsoft.windowsazure.services.serviceBus.models.*;
import com.microsoft.windowsazure.services.core.*;
import javax.xml.datatype.*;
How to Create a Topic
Management operations for Service Bus topics can be performed via the ServiceBusContract class. A ServiceBusContract object is constructed with an appropriate configuration that encapsulates the token permissions to manage it, and the ServiceBusContract class is the sole point of communication with Azure.
The ServiceBusService class provides methods to create, enumerate, and delete topics. The example below shows how a ServiceBusService can be used to create a topic named "TestTopic" within a "HowToSample" service namespace:
String issuer = "<obtained from portal>";
String key = "<obtained from portal>";
Configuration config =
ServiceBusConfiguration.configureWithWrapAuthentication(
“HowToSample”,
issuer,
key);
ServiceBusContract service = ServiceBusService.create(config);
TopicInfo topicInfo = new TopicInfo("TestTopic");
try
{
CreateTopicResult result = service.createTopic(topicInfo);
}
catch (ServiceException e) {
System.out.print("ServiceException encountered: ");
System.out.println(e.getMessage());
System.exit(-1);
} There are methods on TopicInfo that allow properties of the topic to be tuned (for example: to set the default "time-to-live" value to be applied to messages sent to the topic). The following example shows how to create a topic named "TestTopic" with a maximum size of 5GB:
long maxSizeInMegabytes = 5120;
TopicInfo topicInfo = new TopicInfo("TestTopic"); TopicInfo.setMaxSizeInMegabytes(maxSizeInMegabytes);
CreateTopicResult result = service.createTopic(topicInfo); Note: You can use the listTopics method on ServiceBusContract objects to check if a topic with a specified name already exists within a service namespace..
How to Create Subscriptions
Topic subscriptions are also created with the ServiceBusService class. Subscriptions are named and can have an optional filter that restricts the set of messages passed to the subscription's virtual queue.
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.
String issuer = "<obtained from portal>";
String key = "<obtained from portal>";
Configuration config = ServiceBusConfiguration.configureWithWrapAuthentication(
“HowToSample”,
issuer,
key);
ServiceBusContract service = ServiceBusService.create(config);
try
{
TopicInfo topicInfo = new TopicInfo("TestTopic");
CreateTopicResult result = service.createTopic(topicInfo);
SubscriptionInfo subInfo = new SubscriptionInfo("AllMessages");
CreateSubscriptionResult result =
service.createSubscription("TestTopic", subInfo);
}
catch (ServiceException e) {
System.out.print("ServiceException encountered: ");
System.out.println(e.getMessage());
System.exit(-1);
} 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.
The example below creates a subscription named "HighMessages" with a SqlFilter that only selects messages that have a custom MessageNumber property greater than 3:
// Create a "HighMessages" filtered subscription
SubscriptionInfo subInfo = new SubscriptionInfo("HighMessages");
CreateSubscriptionResult result =
service.createSubscription("TestTopic", subInfo);
RuleInfo ruleInfo = new RuleInfo();
ruleInfo = ruleInfo.withSqlExpressionFilter("MessageNumber > 3");
CreateRuleResult ruleResult =
service.createRule("TestTopic", "HighMessages", ruleInfo); 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:
// Create a "LowMessages" filtered subscription
SubscriptionInfo subInfo = new SubscriptionInfo("HighMessages");
CreateSubscriptionResult result =
service.createSubscription("TestTopic", subInfo);
RuleInfo ruleInfo = new RuleInfo();
ruleInfo = ruleInfo.withSqlExpressionFilter("MessageNumber <= 3");
CreateRuleResult ruleResult =
service.createRule("TestTopic", "HighMessages", ruleInfo); When a message is now sent to the "TestTopic", 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 will obtain a ServiceBusContract object. The below code demonstrates how to send a message for the "TestTopic" topic we created above within our "HowToSample" service namespace:
String issuer = "<obtained from portal>";
String key = "<obtained from portal>";
Configuration config = ServiceBusConfiguration.configureWithWrapAuthentication(
“HowToSample”,
issuer,
key);
ServiceBusContract service = ServiceBusService.create(config);
TopicInfo topicInfo = new TopicInfo("TestTopic");
try
{
CreateTopicResult result = service.createTopic(topicInfo);
BrokeredMessage message = new BrokeredMessage("sendMessageWorks");
service.sendTopicMessage("TestTopic", message);
}
catch (ServiceException e) {
System.out.print("ServiceException encountered: ");
System.out.println(e.getMessage());
System.exit(-1);
} Messages sent to Service Bus Topics are instances of the BrokeredMessage class. BrokeredMessage objects have a set of standard methods (such as setLabel and TimeToLive), a dictionary that is used to hold custom application specific properties, and a body of arbitrary application data. An application can set the body of the message by passing any serializable object into the constructor of the BrokeredMessage, and the appropriate DataContractSerializer will then be used to serialize the object. Alternatively, a java.io.InputStream can be provided.
The following example demonstrates how to send five test messages to the "TestTopic" MessageSender we obtained in the code snippet above. Note how the MessageNumber property value of each message varies on the iteration of the loop (this will determine which subscriptions receive it):
for (int i=0; i<5; i++) {
//Create message, passing a string message for the body
BrokeredMessage message = new BrokeredMessage("Test message " + i);
// Set some additional custom app-specific property
message.setProperty("TestProperty", "TestValue" + i);
// Send message to the topic
service.sendTopicMessage("TestTopic", message);
} 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
The primary way to receive messages from a subscription is to use a ServiceBusContract object. Received messages can work in two different modes: ReceiveAndDelete and PeekLock.
When using the ReceiveAndDelete mode, receive is a single-shot operation - that is, when Service Bus receives a read request for a message, it marks the message as being consumed and returns it to the application. ReceiveAndDelete mode 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.
In PeekLock mode, 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 on the received message. When Service Bus sees the Delete call, it will mark the message as being consumed and remove it from the topic.
The example below demonstrates how messages can be received and processed using PeekLock mode (not the default mode). The example below does an infinite loop and processes messages as they arrive to our "HighMessages" subscription. Note that the path to our "HighMessages" subscription is supplied in the form "<topic path>/subscriptions/<subscription name>".
ReceiveMessageOptions opts = ReceiveMessageOptions.DEFAULT; opts.setReceiveMode(ReceiveMode.PEEK_LOCK);
while(true) {
ReceiveSubscriptionMessageResult resultQM =
service.receiveSubscriptionMessage("TestTopic", opts);
BrokeredMessage message = resultQM.getValue();
if (message != null && message.getMessageId() != null)
{
try
{
System.out.println("Body: " + message.toString());
System.out.println("MessageID: " + message.getMessageId());
System.out.println("Custom Property: " +
message.getProperty("TestProperty"));
// Remove message from topic
System.out.println("Deleting this message.");
service.deleteMessage(message);
}
catch (Exception ex)
{
// Indicate a problem, unlock message in topic
System.out.println("Inner exception encountered!");
service.unlockMessage(message);
}
}
else
{
System.out.println("Finishing up - no more messages.");
break;
// Added to handle no more messages in the topic.
// Could instead wait for more messages to be added.
}
} 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 unlockMessage method on the received message (instead of the deleteMessage method). This will cause Service Bus to unlock the message within the topic 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 topic, 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 deleteMessage request is issued, 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 getMessageId method of the message, which will remain constant across delivery attempts.
How to Delete Topics and Subscriptions
The primary way to delete topics and subscriptions is to use a ServiceBusContract object. Received messages can work in two different modes: ReceiveAndDelete and PeekLock
// Delete Topic
service.deleteTopic("TestTopic");
// Delete subscription service.deleteSubscription("TestTopic", "HighMessages"); Deleting a topic will also delete any subscriptions that are registered with the topic. Subscriptions can also be deleted independently.
Next Steps
Now that you've learned the basics of Service Bus queues, see the MSDN topic Queues, Topics, and Subscriptions for more information.