How Scale-out work in Azure App Services

Question

I am trying to wrap my head around the concept of Azure App Service plan and Azure App Services, with no luck.

My understanding is that an App Service Plan defines the capacity and the pricing, all apps assigned to a specific App Service plan will share the same resources, is that right?

If that is right, then what is the benefit of the scaling-out? If the scale out will create more instances of the same app which at the end will be hosted on the same App Service Plan (sharing the same resources)?

I read almost all the official and non-official documentation about Azure App Service plan and App Services but couldn't find an answer to this question, they are all saying that scaling is working on the app level (not the app service plan) and at the same time saying that the apps assigned to the same app service plan are actually sharing the same resources, so what is the benefit of the scale-out feature?

Regards,

Answer 1

As I know, the scale out would create multiple copies of your web app and add a Load Balance to distribute the requests between them automatically. And you don't need to configure the load balance separately by yourself.

Assuming that you create a website (a windows server with IIS), then your website would has the App Pool which defines the available resources for your website. Each instance could handle a limited number of requests, in order to reduce the response time, you could scale out your website into multiple instances, then each web-server could split the work load. For more details, you could refer to Scaling Up and Scaling Out in Windows Azure Web Sites and this tutorial for a better understanding of Azure Web App auto scale.

Answer 2

As @4c74356b41 said when you scale out you are going to get more physical resources (i.e VM's with more compute, memory and storage). Also one correction as per Azure documentation, scale out is going to effect all apps in app service plan. see below link and the point to note is

"The scale settings take only seconds to apply and affect all apps in your App Service plan. They do not require you to change your code or redeploy your application" - https://azure.microsoft.com/en-us/documentation/articles/web-sites-scale/

Answer 3

My understanding is that an App Service Plan defines the capacity and the pricing, all apps assigned to a specific App Service plan will share the same resources, is that right?

Yes.

If that is right, then what is the benefit of the scaling-out? If the scale out will create more instances of the same app which at the end will be hosted on the same App Service Plan (sharing the same resources)?

No one forces you to put all your apps on the same App Service Plan. When you create an App you put it into some App Service Plan. All the Apps on that (and only that) App Service Plan would share resources, but you could create a lot of App Service Plans.
Also, when you scale out you create more PaaS instances of the VM's hosting your App, so when you scale out you are not getting another App Pool in the same IIS, you are getting another App Pool on the other IIS on the other VM.

edit: to clarify the comment, the App Service Plan is a collection of Windows VM's with IIS installed on them. All the Apps assigned to that App Service Plan are hosted on ALL the instances of those VM's, when you scale out or scale up you change the number or capacity of those VM's.
There's no temporary App Service Plan. You pay for the Service Plan, not for the App. Apps cost nothing, they are simply consuming resources on the Service Plan, its the Service Plan that "eats" money. You are getting billed according to the Service Plan tier and scale.
Pricing is based on the size and number of VM instances you run.