On-Demand Build for Monorepo

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This article shows how to achieve on-demand builds in a Monorepo — triggering build tasks only for modified directories, avoiding the resource waste of full builds.

Prerequisites

Syntax guide: Pipeline.ifModify
System default environment variables: CNB_PIPELINE_KEY

Example Configuration

For multiple packages, use a unified template + variable differentiation approach to avoid copy-paste:

.cnb.yml
main:
  push:
    package-1:
      services:
        - docker
      ifModify:
        - packages/package-1/**
      env:
        PACKAGE: package-1
      stages:
        - name: set docker tag
          script: echo -n "${CNB_DOCKER_REGISTRY}/${CNB_REPO_SLUG_LOWERCASE}:$CNB_COMMIT_SHORT"
          exports:
            info: IMAGE_TAG
        - name: docker build
          script: cd packages/$PACKAGE && docker build -t $IMAGE_TAG .
        - name: push image
          script: docker push $IMAGE_TAG
    package-2:
      services:
        - docker
      ifModify:
        - packages/package-2/**
      env:
        PACKAGE: package-2
      stages:
        - name: set docker tag
          script: echo -n "${CNB_DOCKER_REGISTRY}/${CNB_REPO_SLUG_LOWERCASE}:$CNB_COMMIT_SHORT"
          exports:
            info: IMAGE_TAG
        - name: docker build
          script: cd packages/$PACKAGE && docker build -t $IMAGE_TAG .
        - name: push image
          script: docker push $IMAGE_TAG

When there are many packages, it is recommended to extract the common stages into a template file via include, where each package only needs to declare ifModify and env to reuse it.