When you create an app in App Service, it is put into an App Service plan. When the app runs, it runs on all the VM instances configured in the App Service plan. If multiple apps are in the same App Service plan, they all share the same VM instances. If you have multiple deployment slots for an app, all deployment slots also run on the same VM instances. If you enable diagnostic logs, perform backups, or run WebJobs, they also use CPU cycles and memory on these VM instances.
In this way, the App Service plan is the scale unit of the App Service apps. If the plan is configured to run five VM instances, then all apps in the plan run on all five instances. If the plan is configured for autoscaling, then all apps in the plan are scaled out together based on the autoscale settings.
Since you pay for the computing resources your App Service plan allocates, you can potentially save money by putting multiple apps into one App Service plan. You can continue to add apps to an existing plan as long as the plan has enough resources to handle the load. However, keep in mind that apps in the same App Service plan all share the same compute resources. To determine whether the new app has the necessary resources, you need to understand the capacity of the existing App Service plan, and the expected load for the new app. Overloading an App Service plan can potentially cause downtime for your new and existing apps.
Isolate your app into a new App Service plan when:
- The app is resource-intensive.
- You want to scale the app independently from the other apps in the existing plan.
- The app needs resource in a different geographical region.
This way you can allocate a new set of resources for your app and gain greater control of your apps.
In the example below the App service plan has a total of 7GB of memory available for use. That 7GB will be allocated on an “as needed” basis to the web apps and their respective instances.
Azure Advice 