fix(pipelines): make CdkPipeline build stage optional

[tomas-mazak]

In PR #10148, @rix0rrr made it possible to provide a custom CodePipeline pipeline instance to CdkPipeline. This also made the sourceAction (Source stage) and synthAction (Build stage) props optional.

However, validation was added to ensure that if synthAction is not provided, the pipeline already contains at least two stages (assuming that would be Source and Build).

Logically though, CdkPipeline works perfectly fine without Build stage, if an already-built cloud assembly is provided in the source stage (e.g. S3 source action). A use case for this is, for example, separating CI and CD logic, where CDK synthesis happens within the CI build and the assembly is stored as an artefact to be deployed by a pipeline.

This PR makes the Build stage optional, to allow this use case without a need for a dummy build stage.

Example pipeline code:

export class PipelineStack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string, props: cdk.StackProps) {
    super(scope, id, props);

    const versionsBucket = new s3.Bucket(this, 'VersionsBucket', {
      bucketName: 's3pipeline-app-versions',
      versioned: true,
    });

    // The CodePipeline
    const cloudAssemblyArtifact = new codepipeline.Artifact()
    const codePipeline = new codepipeline.Pipeline(this, 'CodePipeline', {
      pipelineName: 'S3Pipeline',
      restartExecutionOnUpdate: true,
      stages: [{
        stageName: 'Source',
        actions: [new actions.S3SourceAction({
            actionName: 'S3',
            bucket: versionsBucket,
            bucketKey: 'cloudassembly.zip',
            output: cloudAssemblyArtifact
          })]
      }]
    });

    // CDK Pipeline
    const cdkPipeline = new pipelines.CdkPipeline(this, 'CdkPipeline', {
      codePipeline,
      cloudAssemblyArtifact,
    });

    // Add application stage
    cdkPipeline.addApplicationStage(new MyAppStage(this, "PreProd"));
  }
}

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[rix0rrr]

You are technically correct that this is not necessary, and I see you have a use case where they aren't necessary.

Your use case is outside the 99% though, so I'm hesitant to give up a guard rail that would help all people out there who aren't in your position.

How about a compromise: providing your own pipeline is an "I know what I'm doing" switch, so we don't require 2 stages in the existing pipeline (but if no pipeline is given, we still require the synthAction?).

[tomas-mazak]

Hi @rix0rrr, thanks for your reply.

How about a compromise: providing your own pipeline is an "I know what I'm doing" switch, so we don't require 2 stages in the existing pipeline (but if no pipeline is given, we still require the synthAction?).

I actually believe that's the behaviour this PR introduces: You must provide either sourceAction or codePipeline, otherwise the validation fails on line 117. If you provide sourceAction, then you also need to provide synthAction, otherwise the validation fails on line 110. That means the only way of not providing the synthAction is to provide a codePipeline that already has the Source stage. Does that make sense?

[tomas-mazak]

If you like, I am happy to slightly reorder the validation steps to make the dependencies more obvious.

[mergify]

Thank you for contributing! Your pull request will be updated from master and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[aws-cdk-automation]

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• CodeBuild project: AutoBuildProject6AEA49D1-qxepHUsryhcu
• Commit ID: b7b2fbb
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[mergify]

Thank you for contributing! Your pull request will be updated from master and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).