Patterns in Windows Azure Service Bus - Message Filter Pattern

In the last post I tacked about Splitter Pattern.Today we will continue with Message Filter Pattern. This pattern can be used with success with Windows Azure Service Bus.

As the name says, all the messages are filtered based on specific rules. Any message that will reach the consumer will be filter based on this rules. All the producers will use the same entry point where they will add messages. They don’t have to know that messages are filtered based on same rules. In this way, the message system will create a decoupling between the producer and the consumer. This pattern is used to be able to control the messages that where not routed to any subscriber.

Windows Azure Service Bus Topics can be used for this purpose. It gives us the ability to define rules that can check the messages content based on meta-information. These rules will be added to each subscriber and will specify if the messages will be accepted or not.

SqlFilter myCustomFilter =
    new SqlFilter("grade < 5");
namespaceManager.CreateSubscription(
               "StudentsGradesTopic",
               "StudentsWithProblemsSubscription",      
               myCustomFilter);  

 In the following example I created a subscription that accept messages that have the grade under 5. For more information about defining custom rules: http://www.vunvulearadu.blogspot.hu/2012/08/service-bus-topics-how-to-use-it-part-2.html

When we are using this pattern, consumers will only receive and process messages that where filtered. Because of this, we can have messages in the system that will not pass any filter. This messages need to be tracked in one way or another. For these situations we can define a custom rule in Service Bus Topic that will receive messages that didn’t pass the rest of the rules of the subscribers. The name of the filter expression is “MatchNoneFilterExpression”. In the following example we setup a rule that accept messages that didn’t pass the rest of the rules.

RuleDescription notConsumedMessagesRule = new RuleDescription()
{
     FilterAction = new SqlFilterAction(“set isNotConsumed = true;”),
     FilterExpression = new MatchNoneFilterExpression()
};
subscription.Add(notConsumedMessagesRule);

For more information about this time type filters: http://www.vunvulearadu.blogspot.hu/2012/09/service-bus-topics-define-custom-rules.html

This pattern can be used with success when we need to control what kind of messages is received by each consumer. We can imagine that we need to manage grades from a university. For this purpose each department want to receive information related to them. The history department doesn’t want to receive grades from the mathematic department. For this case the Message Filter Pattern can help us a lot because it created only one entry point for the applications and services that add these messages.
Last edit: A list of all patterns that can be used with Windows Azure Service Bus, that were described by me LINK.