Why do Azure Functions take an excessive time to "wake up"?

Question

We have a simple Azure Function that makes a DocumentDB query. It seems like the first time we call it there is a long wait to finish, and then successive calls are very fast.

For example, I just opened our app and the first Function call took 10760ms, definitely noticeable by any end user. After this, all Function calls take approximately 100ms to process and are nearly imperceptible.

It seems as though there is some "wake up" cycle in Azure Functions. Is there some way to minimize this, or better yet is this documented somewhere so we can understand what's really going on here?

Answer 1

Function apps running on a consumption plan do indeed have an idle time after which they effectively go to sleep. The next invocation is required to "wake them up" as you've observed and people have mentioned in the comments.

As to why this happens, it's so that Microsoft can most optimally distribute compute workloads in a multi-tenant environment while ensuring that you're only billed to the second for the time where your function is actually doing work. This is the beauty of serverless.

For workloads where this is not acceptable behavior, you could consider moving off of the consumption plan and on to the actual App Service plan. Alternatively, you could implement a timer triggered function that goes off every minute for example and use that as a "keep alive" mechanism by pinging the function that you don't want to go to sleep.