Hi Champs,

Today I am going to help you in different aspect of scaled environment in Azure where I will explain what is ARR in scaled Sitecore Azure instances. So without delaying we will start this with below pointer which will explain all the things related to this.

What is ARR Affinity?

Application Request Routing (ARR) is a feature where when a client (or browser) request to any Azure based website, a cookie will be created and stick to the first time request received web site instance.

The same cookie will be used for subsequent requests from this client or browser and these requests will be guided to the same web site instance the one which was served for the first time.

Advantages:
With this feature, we can get an advantage if in case the web site instance is maintaining lots of data in it’s memory and moving the subsequent requests to other instance leads to copy entire data to other instance and this is a more performance and pain to the system.

Dis-Advantages:
We can see many disadvantages when compared with advantages. If a client request and unfortunately the sticky instance is not available, this request cannot be guided to any other available instance instead it will send an unavailable message to the client.

Also, In case if an instance is a too much load with other request and this request will be in request queue instead can guide to other instance to balance the load.

How ARR Affinity works?

  1. Client connects to an Azure Web Sites website
  2. ARR runs on the front-end Azure server and receives the request
  3. ARR decides to which of the available instances the request should go
  4. ARR forwards the request to the selected server, crafts and attaches an ARRAffinity cookie to the request
  5. The response comes back to the client, holding the ARRAffinity cookie.
  6. When the client receives the request, it stores the cookie for later use (browsers are designed to do this for cookies they receive from servers)
  7. When the client submits a subsequent request, it includes the cookie in it
  8. When ARR receives the request, it sees the cookie, and decodes it.
  9. The decoded cookie holds the name of the instance that was used earlier, and so ARR forwards the request to the same instance, rather than choosing one from the pool
  10. The same thing (steps 7-9) repeat upon every subsequent request for the same site, until the user closes the browser, at which point the cookie is cleared
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This is how the affinity cookie looks:
 
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Disabling the affinity can be done in different ways: (Every one prefer that last way explained)

  1. In your application
    To control this behavior in an application, you need to write code to send out a special HTTP header, which will tell the Application Request Router to remove the affinity cookie. This header is Arr-Disable-Session-Affinity, and if you set it to true, ARR will strip out the cookie. For example, you could add a line similar to this to your applications’ code: 
     
    headers.Add(“Arr-Disable-Session-Affinity”, “True”);
     
  2. In a site configuration
    If you prefer to have it completely disabled, you could have ARR remove the cookie always by having IIS itself inject that header directly. This is done with a customHeaders configuration section in web.config. Simply add the following into your web.config, and upload it to the root of the site: 

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There is one more way yo disable this is which require us to follow below steps.
  1. Go to Azure App service.
  2.  Then go to Configuration.
  3. In configurations you will find General settings tab click on it.
  4. Here in this tab you will find the Sections called Platform settings.
  5. In this sections you will find the ARR affinity radio button.
  6. As per requirement you can disable it.(by default this is on)

ARR

To test this you need to go into HTTP header and check the value is coming or not. Another simple way is once you disabled it you can clear your browser cookies and load the website again and check in cookies sections and you will find that ARR cookie is not created.

Happy Learning!!!!