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Duplicate Detection in Windows Azure Service Bus

One great feature of Windows Azure Service Bus is the ability to identify the messages that are duplicate. Using this feature we can automatically remove messages from Service Bus Queue or Topic when we have more than one the same message. By default this mechanism is deactivated, but we can activate it very easily.
Let’s see how this mechanism works. When we activate this mechanism Windows Azure Service Bus will start to store a history with our messages. This period of time can be configures from only a few minutes to days. If a duplicate message is send to Service Bus, the service will automatically ignore the message.
Because the content of each message need to be serializable, the duplicate detection mechanism will look to the serialized items that were added to the message. If the content is the same, than the message will be consider as duplicate. For example we have the following class that we add to each message:
public Foo
    public string Id { get; set;}
    public string Name { get; set;}
If we will add two messages with objects of type Foo with the same id and name, than the second message will be consider duplicate. We will not receive any kind of notification when we will try to send the second message.
MessagingFactory factory = MessagingFactory.Create([uri], [tokenProvider]);
MessageSender sender = factory.CreateMessageSender("FooQueue")

Foo obj1 = new Foo() { Id = “1”, Name = “One” };
BrokeredMessage message1 = new BrokeredMessage(obj1);

Foo obj2 = new Foo() { Id = “1”, Name = “One” };
BrokeredMessage message2 = new BrokeredMessage(obj2);

It is very important to remember that the duplicate detection mechanism will use the space from our Service Bus to persist the old messages. Because of this it is very important to be careful when we set the history time period.
This configuration can be made from two places. One is from portal, when we can activate this feature and set the history time period. This feature will be available only if you select “Custom create”.

The second place from where this can be configuration can done is from code. In the moment when we create a new Service Bus Topic or Service Bus Queue we can specify in the create command. To be able to do this we will need to create a TopicDescription or a QueueDescription object. In this object we can activate this functionality and set the history time period.
TopicDescription fooTopicDescription = new TopicDescription("FooTopic")
        RequiresDuplicateDetection = true,
        DuplicateDetectionHistoryTimeWindow = new TimeSpan(10, 0, 0),
QueueDescription fooQueueDescription = new QueueDescription("FooQueue")
        RequiresDuplicateDetection = true,
        DuplicateDetectionHistoryTimeWindow = new TimeSpan(10, 0, 0),

These policy settings of Service Bus Queue or Service Bus Topic can be changed only on creating time. After this, if we want to change the history time period or to deactivate it, it will not be possible. To be able to do this, you will need to delete the resource and recreate it.
This is a great feature that can be used when we need to avoid duplicate messages to be added to our Service Bus.


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