I was wondering what would be the minimal recommended instance count for azure app service web app in production scenario ?

We currently have app services under Isolated app service plan with Autoscale rule in place (scaling up to 10 instances).

However the minimal instance count is currently set to 1.

I was wondering if this number shouldn't be higher (at least 2)? Can this impact SLA and apps availability somehow ?

CodePudding user response：

If availability is a concern you should use 2 or 3 instance to minimize impact. Having just one instance can cause downtime. If you have a web app running on 1 instance the Azure Portal warns you with the following advice:

Distributing your web app across multiple instances

The webapp is currently configured to run on only one instance. Since you have only one instance you can expect downtime because when the App Service platform is upgraded, the instance on which your web app is running will be upgraded. Therefore, your web app process will be restarted and will experience downtime.

Having 2 instances mitigates this, but can still cause downtime if one instance is not available due to a platform upgrade and one instance is not available due to a new deployment. 3 is best since in that regards.

Do note that if high availability is a must, then you need to also think multi-region. Think worst case scenario such as a natural disaster taking the one data center offline that you are using. It would be best to have something like Azure Traffic Manager (ATM) in front of two web apps (each with 3 instances) and if ATM detects one of your web app regions is offline, it can reroute traffic to minimize downtime. Customer's who are running storefronts and the availability of their app is tied directly to their app being online will often take this approach.

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