Introducing… CloudFormation extrinsic functions ;-)

Ok, ok, AWS hasn’t announced a whole new class of intrinsic functions for you to use in CloudFormation. But if you’re using the Serverless framework then I have a treat for you :-) I created a plugin for the Serverless framework that lets you use a number of custom Fn:: functions in your serverless.yml.

The problem

The motivation for this plugin came from a challenge we had encountered at DAZN. We had a use case where we wanted to specify a CloudFormation condition that behaves like Javascript’s startsWith function.

The available set of Condition functions is fairly constrained. Even simple logic such as the one described above proved tricky and the code was not easily comprehensible.

“Extrinsic” functions

This plugin lets you use a number of custom functions anywhere in your serverless.yml as if they’re CloudFormation’s intrinsic functions. For example, you can use them as part of CloudFormation Conditions. To implement the startsWith logic, we can simply use the Fn::StartsWith function:

All and all, 8 custom functions are supported out-of-the-box. And it’s pretty trivial to add support for additional functions in the future. Let me know in the comments below if you have use cases that are difficult to implement with CloudFormation’s intrinsic functions. Maybe I can make your life easier by supporting them as extrinsic functions ;-)


Hi, my name is Yan Cui. I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

You can contact me via Email, Twitter and LinkedIn.


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Originally published at https://theburningmonk.com on April 30, 2019.