How you can deactivate session affinity on Azure Web App and ARR

In today's post we will talk about REST API and stick sessions.

Nowadays, REST API and Web Services are very common. We can find a lot of system and devices worldwide that are using HTTP(s) protocol and REST Services to communicate.
It not very common to send a cookie or session information when you calling a REST endpoint. All information are send in the heather, but not like cookies or session data.

In this situation you may want to disable sending cookies and session information to the clients. In this way you could use a little less resources on backend. Especially when you have a system that will need to manage hundreds of thousands of messages per second.
This two things can be manage and disabled pretty easily.

Going further, let's analyze what is happening when you host an endpoint like this on Azure like a Web App. It is important to know that in front of a Web App on Microsoft Azure we have out of the box a Load Balancer when multiple instances are deployed for the same Web App.
Behind this Load Balancer you will have of course an ARR (Application Request Routing). This feature is inherited from ISS - is one of the coolest extension of ISS.
ARR will keep track of different request. Once a request ends on a specific instance, the ARR will make sure that all other requests from the same client will hit the same instance of Web App.

This feature is important for application where there are session sensitive data or you have stateless connections. In this scenario you will always want to end up on the same instance.

But of course, not all the time you will need something like this. This feature is great when you need it. In most of the cases you will not need it. For this scenario, we can disable ARR to manage the session affinity.
The most easy solution is to set this configuration from web.config file, like below.

<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeather>
        <add name="Arr-Disable-Session-Affinity" value="true" />
      </customHeather>
    </httpProtocol>
  </system.webServer>
</configuration>

There is another option, to set it from code, but it will require a deployment. Personally, I prefer to keep this kind this configuration in configuration files, where you can easily spot and change.

headers.Add(
     "Arr-Disable-Session-Affinity", 
     "True");

In this way we can reduce a little bit the stress that is on ARR system.