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Service Bus and Scalability

One of the beauty of cloud is scalability. You can scale as much you want, without having problem. Of course this is in theory. A cloud provider offer this feature to you, but a developer/architect needs to know how to use it in a way to design a high scalable system. Each cloud provider offer different points for scalability. We need to know very well what this points are and how we can use them in our benefit.
As a messaging system, Windows Azure offer us Windows Azure Service Bus. This a powerful service when an application needs a messaging system. But of course, Service Bus, as well as other services has its own limitations.
In this moment, the maximum number of subscription for a specific topic is 2.000. If we would have one client for each subscription that the maximum number of clients per topic is 2.000.
What should we do if we would have 10.000 clients?
It is clear that we cannot have 10.000 subscriptions per topic. We could send an email to Microsoft to ask them why they set this limit and if they could make an exception for us. But what we will do when we will hit 50.000 clients?
In this moment we need to identify exactly what is the scalability point. It is pretty clear the subscription number is not what we need. If we go one level up to the topics, than we will observe that this is a perfect scalability point for our application.
We can have an ‘unlimited’ number of topics over the same namespace or using multiple namespace. I thing that for all real scenario a namespace is sufficient. We should think to use more namespaces, if the load of the topics is pretty high, but this is not the subject for this blog post.
Based on the clients ID or name we can develop an algorithm that resolve the topic name for a specific client. For example if we have 10.000 clients we can have 10 topics. First 1000 clients will be redirected to topic ‘myTopic1’, from 1001 to 2000 will be redirect to ‘myTopic2’ and so on.

This could be a good solution, but we can have some problems using this approach. What is happening when the distribution of active clients will change? For example from 1 to 1000 interval, in 6 month we will have only 100 clients active. We can end up with hundreds of topics that are not used at their full potential. From the security perspective if we are using Shared Access Signature (SAS) we will need a service that will provide the access token for each client. Also, if we want to change the distribution algorithm over the topics and subscription we will need to deploy in the same time the new algorithm to all the backend services and clients devices – we know that this is not possible).
My approach would be to create a services that can provide to each client the full configuration that is used to access a subscription (namespace, topic, subscription and credentials). In this way it will be very easy to manage, update and track the subscription distribution over the system. All the subscription management will be done on the server side. When a client would have problem accessing his own subscription, he can interrogate the server and check the current configuration for it.

In this post we saw that Service Bus Topics and Subscription are very scalable, but the solution that we provide need to be also scalable. It is not enough to run our solution over a scalable infrastructure, we also need to design a scalable components.


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