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Types of Subscriptions Filter on Windows Azure Service Bus

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Service Bus from Windows Azure is a great services to distribute messages to multiple subscribers. A topic can have one or more subscriptions. A message added to the topic will be send to all subscriptions that are register in that moment.
To be able to control what kind of messages are send to each subscription we can define filters. A filter is added on each subscription and specify what messages will be accepted by subscription.
The types of filters that can used are:

 Correlation Filter
This is used when you want to accept messages that has a specific correlation value. This value can be set directly to the BrokenMessage and is great if you have messages with different priority or rank.
CorrelationFilter filter = new CorrelationFilter(“low”);
namespaceManager.CreateSubscription(topicPath, subscriptionName, filter);

BrokenMessage message = new BrokenMessage();
message.CorrelationId = “low”;
…

SqlFilter
This filter give us the ability to specify what kind of messages will be accepted in a subsection based on a custom rules. This rule can be related to the properties list of a BrokenMessage that are specific by the producer or system properties (SessionId for example is a system property).
The syntax is very simple and is based on SQL92 standard and T/SQL. If you know to write a query in SQL than you will not have problem to use this filter. The syntax give us the possibility to use = != < > <> IS IN LIKE EXISTS ESCAPE NO + - * / AND OR …
For a normal use this is enough to define rules over subscriptions.
SqlFilter filter = new SqlFilter(“CustomProperty == 1”);
namespaceManager.CreateSubscription(topicPath, subscriptionName, filter);
…
BrokenMessage message = new BrokenMessage();
message.Properties[“CustomProperty”] = 1;
The “user” or “sys” prefix need to be used when you refers to properties that are in different scope. The “sys” refers to properties that are on BrokenMessage (like Correlationid or SessionId).

 FalseFilter
This is a filter that can be used to block all the messages to the given subscription. This can be used if you create a subscription and you don’t want to accept message to it (temporarily). Also, this can be used when you have a subscription and you don’t want to receive message anymore to it, but you don’t want to delete it.
FalseFilter filter = new FalseFilter()
namespaceManager.CreateSubscription(topicPath, subscriptionName, filter);
…

TrueFilter
This filter is similar with SqlFilter. It will accept the message that respect the specific rule.

We looked over what kind of filter can be defined over a subscription. I think that FalseFilter can be very useful when you are in production and you need to control what messages are receive by one of subscription. For example in a scenario when a client didn’t pays the monthly subscription and you want to stop temporary his subscription. Is more simple that deleting it or set a SqlFilter – “ 1 <> 1”.
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