How are multiple websites load balanced across multiple instances on App Service Plans?

Question

We're hosting 3 separate websites on a single app service plan. This plan is configured to run 3 small instances (S1). I understand this to mean 3 separate servers each with 1 CPU.

If only one of the three websites experiences very high traffic from a single source (another offsite server) how is the load balanced across the 3 instances?

Are all of the requests going to round-robin or all try to go to a single instance? How do I control or test this?

Is the single website restricted to a fraction/portion of the resources available on a single instance when there are other sites?

What are the advantages of running multiple single CPU instances vs a single instance with multiple CPUs with Azure Websites? (Besides availability)

We've attempted to load test one of the sites and it seems to never use more than 10-15% of the CPU when we are slamming only 1 site and the other 2 have no traffic. This is what made me suspicious of the load balance.

Answer 1

Your first paragraph is correct. You will have 3 S1 servers allocated to your sites.

Each server will host all 3 sites. The requests to any site will be routed at random to any of the servers.

However, by default sticky sessions are enabled, which means that a single client will always hit the same server for the same site. So if one client is really hammering a site all the load will go to a single server.

You can disable this behavior by disabling ARR's Instancy Affinity for the site. You do that by setting the custom header Arr-Disable-Session-Affinity = false in your web.config file.

More detailed instructions here: https://azure.microsoft.com/blog/2013/11/18/disabling-arrs-instance-affinity-in-windows-azure-web-sites/

• Vyrotek: Perfect thanks! How can I learn about how the CPU itself is shared? Is a free-for-all between the sites or are they limited and given equal portions?
• Zain Rizvi: It's free for all between the sites. It's a good idea to turn on Autoscale if you're running your sites in a Standard or Premium App Service Plan. Turning on autoscale doesn't cost you anything until your sites start eating more resources and the service provisions extra servers for you to handle the load