Azure Functions Cold Start Workaround


I love Azure Functions. So much power for so little effort or cost. The only downside is that the consumption model that keeps the cost so dirt-cheap means that unless you are using your Function constantly (in which case, you might be better off with the non-consumption options anyway), you will often be hit with a long delay as your Function wakes up from hibernation.

So very cold…

This isn’t a big deal if you are dealing with a fire and forget queue trigger scenario, but if you have web app that is calling the HTTP trigger and you need to wait for the Function to do it’s job before responding with a 200 OK… that’s a long wait (well over 15 seconds in my experience with a PowerShell function that loads a bunch of modules).

Now, the blunt way to mitigate this (as suggested by some in github issues on the subject) is to set up a timer function in the same Function App to run every 5 minutes to keep things warm. This to me seems like a wasteful and potentially expensive approach. For my use-case, there was a better way that would work.

The Classic CRUD Use-case

Here’s my use case: I’m building some custom SharePoint forms for a customer and using my preferred JS framework, good old Angular 1.x. Don’t believe the hype around the newer frameworks, ng1 still gets the job done with no performance problems at the scale I’m dealing with in SharePoint. It also comes with a very strong ecosystem of libraries to support doing amazing things in the browser. But that’s a topic for another blog.

Anyway, the only thing I couldn’t do effectively on the client-side was break permissions on the list item created using the form and secure it to the creator and some other users (eg. their manager, etc). You need elevated permissions for that. I called on the awesome power of the PnP PowerShell library (specifically the Set-PnPListItemPermission cmdlet) to do this and wrapped it in a PowerShell Azure Function:

Pretty simple. Nice and clean – gets the job done. My Angular service calls this right after saving the item based on the form input by the user. If the Azure Function is starting from cold, then that adds an extra 20 seconds to save operation. Unacceptable and avoidable.

A more nuanced warmup for those who can…

This seemed like such an obvious solution once I hit on it – but I hadn’t thought of it before. When the form is first opened, it’s pretty reasonable to assume that (unless the form is a monster), the user should be hitting that submit/save button in under 5 mins. So that’s when we hit our function with a modified HTTP payload of ‘WARMUP’.

Just a simple ‘If’ statement to bypass the function if the ‘WARMUP’ payload is detected. The Function immediately responds with a 200. We ignore that – this is effectively fire-and-forget. Yes, this would be even simpler if we had a separate warmup Function that did absolutely nothing except warm up the Functions App, but I like that this ensures that my dependent PnP dlls (in the ‘modules’ folder of my Function) have been fired up on the current instance before I hit the function for real. Maybe it makes no difference. Don’t really care – this is simple enough anyway.

Here’s the Angular code that calls the Function (both as a warmup and for real):

Anyway, nothing revolutionary here, I know. But I hadn’t come across this approach before, so I thought it was worth writing up as it suits this standard CRUD forms over data scenario so nicely.

Till next time!