[lambda] deployment failure on updates to cross-stack layers

Here is the application structure:

• Stack A:
  • lambda layer
• Stack B: Depends on A:
  • Lambda which is using the layer

new lambda.Function(this, 'some-lambda'
   {
     ...
        layers: [stackA.lambdaLayer]
   }
)

where stackA.lambdaLayer is an instance of lambda.LayerVersion.

The initial deployment works well. However, when the layer code is changed and the second deployment takes place, Stack A deployment fails with error message:

lambdas-layer (…) Requested update requires the creation of a new physical resource; hence creating one. Resource creation Initiated dev-StackA Export dev-CoreStack:corelambdaslayerLayerVersionArn261FB3DB cannot be updated as it is in use by dev-StackB

[alexdilley]

@eladb I’ve worked around this by using a SSM parameter store to reference the latest layerVersionArn and then accessing the layer instance via Layer.fromLayerVersionArn(). (Nod to @rhboyd for this idea.)

I wrote a custom construct to act as such a proxy:

const cdk = require('@aws-cdk/core');
const lambda = require('@aws-cdk/aws-lambda');
const ssm = require('@aws-cdk/aws-ssm');

// Proxy to the LATEST version of the Lambda Layer shared between stacks via a
// SSM paramter store.
module.exports = class SharedLayer extends cdk.Construct {
  static parameterName(construct) {
    return `/${cdk.Stack.of(construct).stage}/shared/layerVersionArn`;
  }

  static of(construct) {
    const layerVersionArn = ssm.StringParameter.valueForStringParameter(
      construct,
      this.parameterName(construct)
    );

    return lambda.LayerVersion.fromLayerVersionArn(
      construct,
      'SharedLayer',
      layerVersionArn
    );
  }

  constructor(scope, id, props) {
    super(scope, id, props);

    new ssm.StringParameter(this, 'VersionArn', {
      parameterName: SharedLayer.parameterName(this),
      stringValue: props.layerVersionArn,
    });
  }
};

[ranguard]

I split the layers into a stack of their own, and share the ARN via SSM.Params, to then be used by a lambda in another stack. So no hard dependency…

You can see both the layer creation and then usage (which would usually be in a separate stack)

https://github.com/ranguard/cdk-talk-examples/tree/master/code/lambda_layer_example/lib

We have a ‘layers’ stack

AWS keeps layer versions around, whilst any lambda is pointing at them - so new layer deploys won’t break existing functions - but you do need to re-deploy any stack that is using the layers