What exactly are Azure website instance

Question

Azure website clarity regarding the instances, are these instances running on multiple machine like a web server farm environment where one client request (session) can be server by different server in the farm.

Issue with server farm asp.net application is that we can loose session variable if that is the case, for this issue to get around we add machine key in web.config that assure client session will always be server by same server so there wont be any lose of session variable.

How does Azure instance work. doesnt it mean we can have 10 websites under same plan let ex s1 Standard.

I am hosting asp.net webform based website on Azure I am confused how exactly instance work. there are lot of article on internet to create confusion.

Answer 1

Adding to Martin Brandl's answer.

An App Service Plan can have multiple instances. These instances run all of the apps in that plan. So it is pretty much like a classic web farm. Except it is way easier to manage.

There is a better way of keeping session information in a web farm rather than relying on sticky sessions with ARR Affinity. Put the session data in a central place. For that you can use Azure Redis Cache or SQL Database. There are ready-made session state providers for these that you can just plug in to an ASP.NET application. That way your session state is not stored on the instances, so you don't need to care if clients hit the same server.

The problem I have with ARR Affinity, is that Azure does not guarantee your instances will remain running. A hardware failure could bring one down and you lose a part of your session data. If you use auto-scale, same thing.

Answer 2

The thing with PaaS is that you don't have to care about that. If you have a statefull site, you can use the ARR Affinity setting to ensure a client gets always to the same instance: