CloudFront Functions released for general availability

Posted on 04 May 21 by Gareth Evans - Principal Consultant

AWS have just released CloudFront Functions - what do we know so far and how do they compare to Lambda@Edge?


Today Amazon Web Services have announced the general availability of CloudFront Functions and have a corresponding blog post describing their usage. CloudFront Functions is a serverless platform to run your code on the CloudFront edge locations with low latency cheaply but with some important caveats. This blog post takes a dive into the hairy details, consolidating information about CloudFormation support, the runtime, billing and monitoring.


For the impatient here are the bullet points that the remainder of this post will elaborate on:


The runtime for Functions is pure JavaScript to the ECMAScript 5.1 specification with some features of later specifications included. It does not include some of the JS niceties that we’ve come to love over the years including:

The following major features have been made available from later specifications:

In addition some features have been deliberately restricted:

All code must be specified in a single file or string and the only modules made available are crypto and querystring.

Your code gets a maximum of 1 millisecond to run in 2MB of memory. I have not yet seen how the available compute compares.

The Functions are deployed to every CloudFront Edge location, which is currently at 218+, providing the lowest possible latency.

No environment variables or secrets management is available, requiring any secrets (such as JWT secret keys) to be baked into the code at build/deploy time.

CloudFormation support

Good news! CloudFormation support is available on release! 🎉 Whilst at the time of writing the documentation has yet to be published, if you examine the specification you can expect it to look something like this:

  "ExampleFunction": {
    "Type": "AWS::CloudFront::Function",
    "Properties": {
      "Name": "",
      "Stage": "",
      "AutoPublish": true,
      "FunctionCode": "",
      "FunctionConfig": {
        "Comment": "",
        "Runtime": ""

Interestingly it includes a Runtime parameter hinting that additional runtimes may be made available in the future, although this may never eventuate (array properties that must include exactly one element anyone?… 😞).

Additionaly it looks like the CDK team has a PR in the works to include L2 construct support so that we can enjoy it’s higher level abstractions in addition to L1 constructs reflecting pure CloudFormation resources.


The monitoring available on release is as you’d expect with several CloudWatch metrics being published:

All output from console.log(...) entries will be sent to CloudWatch Logs in the us-east-1 region regardless of where the Function executed.


In exchange for the greatly constrained runtime environment CloudFront Functions offers greatly reduced pricing that also only has a single per-invocation dimension. There are no considerations in which region the invocations occur in and pricing is, at this time, US$0.10 per 1 million requests. In addition there is also a free tier available for new customers with 2,000,000 invocations per month for the first year.


In short CloudFront Functions are intended for much simpler use cases such as token validation and request/response manipulation. In exchange for a restricted runtime you gain lower pricing and lower latency.

Compared to Lambda@Edge, CloudFront Functions are:


In all, the runtime, whilst bespoke, seems capable of fulfilling it’s intended purpose of request/response manipulation. It is understandable that the runtime is somewhat limited given the execution environment although it will be interesting to see if it becomes a target for transpilers given the available features are quite selective. This would also make local testing more viable, reducing the need to deploy each iteration. Perhaps the runtime will be made available?

I know I’m going to be cutting tickets to update this very site to use CloudFront Functions.

Thanks for reading! 🙏