auto update AWS Lambda Application with lambda referencing layer

Question

I have a the following template.yml configs referencing a layer.

```
Resources:
  helloFromLambdaFunction:
    Type: AWS::Serverless::Function
    Properties:
      Layers:
        - arn:aws:lambda:us-east-2:xxxxxxxxx:layer:matchmaking-lobby:63
```
I have many lambda apps referencing the same layer. Whenever I make changes to this layer I need to manually update each lambda app. I've tried replacing the version number with `$LATEST`, but that doesn't work. Is there anyway to have the lambdas pull the latest version of the layer?

Accepted Answer

This behavior isn't possible with CloudFormation. The reason is that referring to `$LATEST` cannot guarantee a reproducible state for your stack.

Suppose you performed a successful deployment. Then, after the deployment, the latest version of your Lambda Layer changed. Later, you attempt a new deployment that fails. If this happens, CloudFormation will then attempt to revert to the previous known good state. To revert to that known state, CloudFormation needs the specific version number of the Lambda Layer. That's why the specific version number needs to be in the stack template.

Many customers overcome this limitation by including the layer itself in the stack template (via `AWS::Lambda::LayerVersion`) and making a reference to it. Otherwise, you can do things like put placeholders like `$LAYER_VERSION` into your CloudFormation YAML template, and substitute the value of the Lambda version into it after you upload it using an external program such as `envsubst`, before updating the stack with the rendered template.

auto update AWS Lambda Application with lambda referencing layer

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