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Patterns in Windows Azure Service Bus - Scatter-Gather Pattern

I will continue the series of posts related to patterns that can be used using Service Bus from Windows Azure. In the last post from this blog series I talked about Content-Based Router Pattern and we saw how easily can be implemented using Service Bus Topics.
Today we will look over another message pattern. I don’t know if you ever heard about Scatter-Gather message pattern. It is not a widely used pattern. Also there are a lot of cased when we use this pattern without realizing.
The first step is to try to define this pattern. As you can see, this pattern is formed from two different words – scatter and gather.
  • Scatter – refers how we can send a message to a list of receivers (consumers)
  • Gather – refers how we receive a collection of messages from more than one source
Before talking more about this pattern let’s see an example where this pattern can be used with success. Very easily we can imagine that we are a company that wants to by surfaces (Windows 8 Tables) for all your employ. To get the best price on the market you want to have a price offer from all your suppliers. To be able to do something like this you will need to send a message with the price request for each supplier. After this you will need to gather all the offers and process them.
I thing that you already notified the “scatter” and “gather” in our example:
Scatter - sending the message to our suppliers
Gather – gather the prices from all the suppliers
As you can see, this pattern doesn’t represent only a small picture, but it a little more complicated. From some perspective we could say that this pattern is a combination of two different patterns – splitter and aggregator patterns.
Windows Azure Service Bus supports this pattern without any kind of problems. The Scatter-Gather pattern can be implemented with success using Service Bus Topic.
From the Scatter perspective we will each of our supplier to register to our topic. Each subscriber will receive our message with our request and we will be able to send to us their offer.
Creating the TopicClient:
TopicClient topicClient = TopicClient.CreateFromConnectionString(
CloudConfigurationManager.GetSetting(
    "ServiceBusConnectionString"),
    "myFooTopic");
Create a subscription for each supplier:
NamespaceManager namespaceManager = NamespaceManager                             
                 .CreateFromConnectionString(“ServiceBusConnectionString”);
namespaceManager.CreateSubscription( “myFooTopic”, “subscriptionNameForSupplier1”);
>Each supplier needs to listen the subscription (this action don’t need to be done 24/24h) – Windows Azure will preserve the messages until the subscriber will consume them.
SubscriptionClient subscriptionClient = SubscriptionClient.CreateFromConnectionString(
    CloudConfigurationManager.GetSetting("ServiceBusConnectionString"),
    "myFooTopic",
    "Property1Equal10");
BrokeredMessage message = subscriptionClient.Receive();
Sending the message to our subscribers:
BrokeredMessage message = new BrokeredMessage();
…
topicClient.Send(message);
Create a queue where each supplier can send the price offer and listen the given queue by our company:
NamespaceManager nm = NamespaceManager.CreateFromConnectionString(
     CloudConfigurationManager.GetSetting(“ServiceBusConnectionString”));
if (!namespaceManager.QueueExists("FooQueue"))
{
    namespaceManager.CreateQueue(qd);
}
QueueClient queueClient = QueueClient.CreateFromConnectionString(
    myFooConnectionString,
    "FooQueue");
BrokeredMessage offerReceived = queueClient.Receive();

When we create the infrastructure used to receive the offers from each supplier, we can use Service Bus Topics or Service Bus Queues without any kind of problems. For this case I think that Service Bus Queues is better, because we don’t need to distribute the messages to more than one receiver.
Last edit: A list of all patterns that can be used with Windows Azure Service Bus, that were described by me LINK.  

Comments

  1. Hi Radu, very great post! I'm interested in seeing in more detail how you might incorporate the aggregator pattern. In many of the examples of the pattern that I've seen, the aggregator uses a canonical schema and a message envelope to aggregate responses. Furthermore, what would you use for correlation to the original message?

    ReplyDelete
    Replies
    1. Hi Chris. Next week I will prepare a post that will response to your questions.

      Delete

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