lambda: override cmd in DockerImageFunctionProps

[jstrunk]

Describe the feature

Add a command prop to DockerImageFunctionProps to override the value specified in https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_lambda.AssetImageCodeProps.html#cmd .

Use Case

I have a Docker image with multiple Lambda handlers. I would like to create the DockerImageCode once so it will build the image just once and then when creating each DockerImageFunction, I can specify the function to execute with command.

Proposed Solution

The synthesized CloudFormation for the AWS::Lambda::Function will include the value from the command prop as Command in ImageConfig. https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-lambda-function-imageconfig.html#cfn-lambda-function-imageconfig-command

Other Information

It is possible to override CMD from the dockerfile using https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_lambda.AssetImageCodeProps.html#cmd , but that will result in multiple Docker images.

Acknowledgements

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

CDK version used

2.143.0

Environment details (OS name and version, etc.)

MacOS 14.4.1 intel

[jstrunk]

Now that I understand the code involved, there are some design questions to resolve first.

https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-lambda/lib/function.ts#L1001-L1004

      imageConfig: undefinedIfNoKeys({
        command: code.image?.cmd,
        entryPoint: code.image?.entrypoint,
        workingDirectory: code.image?.workingDirectory,
      }),

DockerImageFunction extends Function and uses the same constructor with a code property. In order to enable reuse of the DockerImageCode object, I think we'd have to add the command prop to FunctionProps. We should probably allow overrides of entryPoint and workingDirectory, too.

I propose a new interface called ImageConfig from AssetImageCodeProps:

/**
 * Docker image options
 **/
export interface ImageConfig {
  /**
   * Specify or override the CMD on the specified Docker image or Dockerfile.
   * This needs to be in the 'exec form', viz., `[ 'executable', 'param1', 'param2' ]`.
   * @see https://docs.docker.com/engine/reference/builder/#cmd
   * @default - use the CMD specified in the docker image or Dockerfile.
   */
  readonly cmd?: string[];

  /**
   * Specify or override the ENTRYPOINT on the specified Docker image or Dockerfile.
   * An ENTRYPOINT allows you to configure a container that will run as an executable.
   * This needs to be in the 'exec form', viz., `[ 'executable', 'param1', 'param2' ]`.
   * @see https://docs.docker.com/engine/reference/builder/#entrypoint
   * @default - use the ENTRYPOINT in the docker image or Dockerfile.
   */
  readonly entrypoint?: string[];

  /**
   * Specify or override the WORKDIR on the specified Docker image or Dockerfile.
   * A WORKDIR allows you to configure the working directory the container will use.
   * @see https://docs.docker.com/engine/reference/builder/#workdir
   * @default - use the WORKDIR in the docker image or Dockerfile.
   */
  readonly workingDirectory?: string;
}

and update FunctionProps with:

  /**
   * Specify or override Docker image asset properties for this specific function.
   **/
  readonly imageConfig: ImageConfig;

Finally, we can update the CfnFunction call with:

      imageConfig: undefinedIfNoKeys({
        command: props.imageConfig?.cmd || code.image?.cmd,
        entryPoint: props.imageConfig?.entrypoint || code.image?.entrypoint,
        workingDirectory: props.imageConfig?.workingDirectory || code.image?.workingDirectory,
      }),

[jstrunk]

Or would it be better to use the generated interface: https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_lambda.CfnFunction.ImageConfigProperty.html ?

[pahud]

Are you asking for multiple DockerImageFunction sharing the same image asset with different command. Is it correct?

And if you check the assets.json from cdk.out there will be only one image asset but they are having their own custom command.

% cat cdk.out/dummy-stack.assets.json | jq '.dockerImages | keys' 
[
  "5151a43fb2ec70a430a9af0b3c3c6a5eda05076c464ab1a8657d01a6a7f5b97f"
]

I think it already works as you expect, doesn't it?

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[jstrunk]

That's neat! I think the documentation could be improved then. It's not clear to me that multiple calls to DockerImageCode.fromImageAsset with the same path will only build the image once.