Cloudform Lambda functions from file without Zip packages

Do you like the convenience of defining a lambda function inline in your cloudformation to avoid the hassle of uploading and referencing a zip file but miss syntax highlighting, being able to update and test the function in a sane way? I ran into the same problem and realised there is a nice middle ground that is less documented.

TLDR: define your function in a separate file and use a cloudformation parameter to pass it in!

Step 1: Define lambda function in a separate file

This should be the obvious first step. We want our function in a separate file, so our editor can give us nice syntax highlighting, autocomplete and we are able to unit test the function if we choose to do so.

We’ll define a minimal NodeJS lambda function in a file called index.js

exports.handler = function (event, context) {
  console.log("Hello aws!");
};

Step 2: Read file contents into bash variable

We want to pass the contents of the file as a variable into the cloudformation template. So we need to get the contents of a file as a string variable and pass it in as a parameter override.

If using bash, you can do so like below (cat works as well, < is however a built-in function so it’s more portable).

lambda_src=$(< index.js)

Bash variables have a basically unlimited size capacity, so the length of the file shouldn’t pose an issue for this step.

Step 3: Pass source in variable as stack parameter

Next we’ll need to pass our file-as-string variable into the cloudformation template. This is where we do run into a stricter limit - cloudformation template parameters have a max size of 4KB

This is roughly equivalent to 150 lines of NodeJS code - so it’s definitely not the best solution for longer functions but perfect for medium sized ones that aren’t trivial enough to just be inline. You can use a site like Byte Size Matters to check the byte size of the file.

We can pass in our variable defined in step 3 into parameter-overrides - make sure to wrap it in quotes so bash doesn’t complain.

aws cloudformation deploy \
    --stack-name lambda-inline-file \
    --template-file ./cloudformation.yml \
    --parameter-overrides LambdaSrc="$lambda_src" \
    --capabilities=CAPABILITY_IAM 

Step 4: Pass parameter into ZipFile config

Next we’ll want to create our cloudformation file and ensure we pass the parameter defined above into the ZipFile config value.

AWSTemplateFormatVersion: '2010-09-09'

Parameters:
  LambdaSrc:
    Type: String

Resources:
  LambdaFn:
    Type: AWS::Lambda::Function
    Properties:
      Handler: index.handler
      Role: !GetAtt LambdaExecutionRole.Arn
      Code:
        ZipFile: !Ref LambdaSrc
      Runtime: nodejs20.x

  # lambda execution role
  # ...

And we’re done!

Deploy the cloudformation file and we’ll see our lambda code has updated correctly.

AWS Console

You can see the full repo here

Happy coding!

Likes