Patterns in Windows Azure Service Bus - Recipient List Pattern

I will continue the blog series of post about patterns that can be user with Service Bus of Windows Azure with Recipient List Pattern. Did you ever try to send an email to a list of users? Using Exchange Server is quite simple. We can create a group list and send an email to the specific group list. For example we can have groups of email for different products categories. One can be for TVs, another one for Notebooks and so on. If we create a complex system for a company that have different products that want to send we will need to be able to send notifications about new products that are available. To be able to do something like this we will need a system that permits us to send notifications to a “group” – to a list of subscribers. In a system that is based on messages we will not know the list of subscribers for each group. Because of this we can decorate each message with some meta-information about the groups should receive the message. In Windows Azure, we can implement this pattern using Service Bus Topics. For each message we can add a property to the message that specifies the groups of subscribers that will receive the message. This list of messages can be separate with a comma or any kind of character.

BrokeredMessage message = new BrokeredMessage();
message.Properties.Add(“Groups”,”Review, Test”);
…

Each subscriber will need to create a custom filter. For this purpose we can use the ‘LIKE’ operator of SqlFilter.

TopicClient topicClient = TopicClient.CreateFromConnectionString(
CloudConfigurationManager.GetSetting(
    "ServiceBusConnectionString"),
    "myFooTopic");
SqlFilter sqlFilterReviewGroup = new SqlFilter(“Groups LIKE ‘%Review%’”);
topicClient.AddSubscription(“ReviewSubscription”, sqlFilterReviewGroup);
SqlFilter sqlFilterTestGroup = new SqlFilter(“Groups LIKE ‘%Test%’”);
topicClient.AddSubscription(“ReviewSubscription”, sqlFilterTestGroup);

Even if this solution will work without any kind of problem, we should be aware that we use the “LIKE” operator. From performance perspective we know that this is not the fastest solution. If we don’t have a lot of messages that are send on the wire that this is not a reals issue. For better performance we can find different solutions, from complicated one that use bits to simpler one like adding different property for each group. In this way the subscriber of a filter will only need to check if the property is set or not. For this purpose we can use the “EXISTS” operator or SqlFilter.

SqlFilter sqlFilterReviewGroup = new SqlFilter(“EXISTS Review”);
topicClient.AddSubscription(“ReviewSubscription”, sqlFilterReviewGroup);

If you don’t want to have a lot of properties you can a prime numbers. The only problem with this solution is related to how easy the code can be read and maintain. Each prime number can represent a different group. And a message should be received by a subscription if the group property can be divided to the prime number that represents our group. We could imagine another solutions also.
To be able to use this pattern we need to define the list of recipients. This list can be a static list of can be created dynamically and can be change on runtime. Using Service Bus Topics from Windows Azure the list can be dynamically created and changing the list will not require changing the code that send or receive the messages. All this can be done in the configuration files.
The important thing that we need to remember about Recipient List Pattern is when we need to use it and Windows Azure Service Bus supports an implementation of this pattern.
Last edit: A list of all patterns that can be used with Windows Azure Service Bus, that were described by me LINK.