"Always On" in Azure Function Apps running on an App Service Plan has any additional cost?

Question

Does enabling "Always On" on a Linux/ Windows Function App have additional cost when running on an App Service Plan?

I went through this links - https://learn.microsoft.com/en-us/azure/azure-functions/dedicated-plan#always-on

1. I want to understand why upon receiving a HTTP request, the HTTP Trigger function (after idling out) still responds with a 404, and Azure does not "wake up" the function?
2. If I turn on "Always On", would it add to the monthly charges of running the function app on the app service plan?

Answer 1

@Priyanka Chaudhuri Thanks for reaching out. Enabling “Always On” for a Function App running on an App Service Plan does not have an additional cost. The “Always On” feature keeps the app loaded even when there’s no traffic.

It’s particularly useful for scenarios where you want to avoid the latency that comes with loading the app after idle periods.

If your Function App is not up and running, it might respond with a 404 error. Turning on “Always On” could help in avoiding such issues as it ensures that your Function App is always loaded and ready to respond to traffic.

please refer: https://github.com/Azure/azure-functions-host/issues/8021#issuecomment-1004303750

As per the above reference if it is a http triggered app the http request itself works as the trigger to wake up the app if it had idled out. this will lead to delayed start.

https://techcommunity.microsoft.com/t5/apps-on-azure-blog/how-to-troubleshoot-azure-functions-http-trigger-404-error/ba-p/3766817#:~:text=One%20of%20the%20common%20causes%20of%20the%20Azure,be%20due%20to%20a%20typo%20in%20the%20URL.

let me know incase of further queries, I would be happy to assist you.

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