aws_lambda: Only update Docker image based Lambdas when changes occur

[DFEvans]

Describe the feature

cdk deploy is pretty insistent on pushing new Docker images and updating Lambdas every time it is run - even if the contents of the Docker images haven't changed.

CDK could instead only push new images and Lambda updates when changes have occurred, reducing the amount of deployment work that is done.

Use Case

We have a CDK stack with a few dozen Lambdas, defined via aws_lambda.DockerImageCode.from_image_asset, all pointing to a local Dockerfile.

Our CI/CD system currently takes 10-15 minutes to run cdk deploy, mostly spent on updating all defined Lambdas with every deployment, regardless of whether a new image needed to be built or not. This is despite caching the cdk.out directory and pushing/pulling the Docker images for the Lambdas to an external repository as part of the build process.

While Docker realises that there are no changes (i.e. it outputs CACHED for all steps in the Dockerfile, both when running docker build and cdk deploy), it has proven challenging to stop CDK from pushing the images as if they were new each time. It appears that CDK is unable to determine that no changes need to be pushed.

If CDK could selectively update Lambdas, this would significantly improve our deploy time - often, only a single Lambda is affected and needs updating.

Proposed Solution

CDK caches, stores, or references Docker images in some way which mean that identical images (at least, as seen by Docker) do not get re-submitted as a new version of the Lambda each time.

Other Information

A potential alternative is that we build the Docker images outside of CDK, pushing them to ECR, and passing the tags through to CDK. However, this is a very heavyweight solution that requires us to re-implement much of the Docker image management that CDK undertakes.

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

CDK version used

2.44.0

Environment details (OS name and version, etc.)

CI/CD workers running Ubuntu-20.04

[peterwoodworth]

Thanks for the request, we're already tracking this feature request here 🙂

[github-actions]

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