I am trying to understand better how Azure App Service Plan (ASP) load-balances the traffic when multiple/different App Services are deployed in it. Let's assume my ASP is made of 2 nodes (VMs or instances) and I deploy 2 apps (total 4 app instances running) and with following URL:
https://app1.azurewebsites.net
https://app2.azurewebsites.net
I know that there are ASP front-ends acting as load balancers. So here if I understand correctly it is like when I have a web-server hosting different web-sites and address distinction is based on virtual hostnames (which are the URL above). Right?
App Service is a multitenant service, so it uses the host header in the request to route the request to the correct endpoint. The default domain name of App Services, *.azurewebsites.net (say, contoso.azurewebsites.net), is different from the application gateway's domain name (say, contoso.com). ref.1
When using App Service, you can scale your apps by scaling the App Service plan they run on. When multiple apps are run in the same App Service plan, each scaled-out instance runs all the apps in the plan.
Apps are allocated to available App Service plan using a best effort approach for an even distribution across instances. While an even distribution is not guaranteed, the platform will make sure that two instances of the same app will not be hosted on the same App Service plan instance.
The platform does not rely on metrics to decide on worker allocation. Applications are rebalanced only when instances are added or removed from the App Service plan.
You can also now do Per-app scaling, which can be enabled at the App Service plan level to allow for scaling an app independently from the App Service plan that hosts it. This way, an App Service plan can be scaled to 10 instances, but an app can be set to use only five. ref.2