Multi-repo support🔗

This page explains how to use argocd-diff-preview when your Argo CD Applications and their managed Kubernetes resources live in separate repositories.

Terminology🔗

Let's define the two repositories we're working with:

Repository	Contains
Application Repository	Argo CD `Application` and `ApplicationSet` manifests
Resource Repository	Kubernetes resources (Helm charts, Kustomize overlays, plain YAML) referenced by the applications

If you want diffs triggered by changes in either repository, you'll need a pipeline in both.

Two key rules🔗

Rule 1: `--repo` must match the repository with the PR🔗

The --repo flag tells argocd-diff-preview which repository has the change. The tool uses this to redirect the correct targetRevision to the PR branch.

Flag	Purpose
`--repo`	Must match the repository where the PR was opened
`--target-branch`	The PR branch that applications should be redirected to

docker run \
  --network=host \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v $(pwd)/main:/base-branch \
  -v $(pwd)/pull-request:/target-branch \
  -v $(pwd)/output:/output \
  -e REPO=<org>/<repo-with-pr> \            ⬅️ Must match the repo with the PR
  -e TARGET_BRANCH=<pr-branch> \            ⬅️ The branch to redirect applications to
  dagandersen/argocd-diff-preview:v0.1.20

Why does this matter?

The tool only redirects applications whose spec.source.repoURL matches the --repo flag. If you specify the wrong repo, the applications won't point to your PR branch.

Rule 2: Always clone the Application Repository🔗

The /base-branch and /target-branch folders must contain the Application Repository - this is where argocd-diff-preview looks for Application and ApplicationSet manifests.

This means even when running a pipeline in the Resource Repository, you still need to clone the Application Repository into these folders.

GitHub Workflows Examples🔗

Pipeline in the Application Repository🔗

This is the standard setup - nothing unusual here:

Clone the Application Repository (this repo)
Provide secrets for both repositories
Set --repo to the Application Repository

.github/workflows/diff.yml (Application Repository)

name: Argo CD Diff Preview

on:
  pull_request:
    branches: [main]

jobs:
  build:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pull-requests: write

    steps:
      - uses: actions/checkout@v4       ⬅️ We pull the Application Repository
        with:
          path: pull-request

      - uses: actions/checkout@v4       ⬅️ We pull the Application Repository
        with:
          ref: main
          path: main

      - name: Prepare secrets
        run: |
          mkdir secrets

          # Secret for Application Repository
          cat > secrets/app-repo-creds.yaml << "EOF"
          apiVersion: v1
          kind: Secret
          metadata:
            name: app-repo-creds
            namespace: argocd
            labels:
              argocd.argoproj.io/secret-type: repo-creds
          stringData:
            url: https://github.com/<org>/<application-repository>
            password: ${{ secrets.GITHUB_TOKEN }}
            username: not-used
          EOF

          # Secret for Resource Repository
          cat > secrets/resource-repo-creds.yaml << "EOF"
          apiVersion: v1
          kind: Secret
          metadata:
            name: resource-repo-creds
            namespace: argocd
            labels:
              argocd.argoproj.io/secret-type: repo-creds
          stringData:
            url: https://github.com/<org>/<resource-repository>
            password: ${{ secrets.RESOURCE_REPO_TOKEN }}
            username: not-used
          EOF

      - name: Generate Diff
        run: |
          docker run \
            --network=host \
            -v /var/run/docker.sock:/var/run/docker.sock \
            -v $(pwd)/main:/base-branch \
            -v $(pwd)/pull-request:/target-branch \
            -v $(pwd)/output:/output \
            -v $(pwd)/secrets:/secrets \               ⬅️ Mount the secrets folder
            -e TARGET_BRANCH=<pr-branch> \             ⬅️ The PR branch on the Application Repository
            -e REPO=<org>/<application-repository> \   ⬅️ Application Repository
            dagandersen/argocd-diff-preview:v0.1.20

      - name: Post diff as comment
        run: |
          gh pr comment ${{ github.event.number }} --repo ${{ github.repository }} --body-file output/diff.md --edit-last || \
          gh pr comment ${{ github.event.number }} --repo ${{ github.repository }} --body-file output/diff.md
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

Local equivalent🔗

git clone https://github.com/<org>/<application-repository> base-branch --depth 1 -q -b main
git clone https://github.com/<org>/<application-repository> target-branch --depth 1 -q -b pr-branch

mkdir secrets
# store some secrets in the ./secrets folder

docker run \
    --network host \
    -v /var/run/docker.sock:/var/run/docker.sock \
    -v $(pwd)/output:/output \
    -v $(pwd)/base-branch:/base-branch \
    -v $(pwd)/target-branch:/target-branch \
    -v $(pwd)/secrets:/secrets \
    -e TARGET_BRANCH=<pr-branch> \
    -e REPO=<org>/<application-repository> \
    dagandersen/argocd-diff-preview:v0.1.20

Pipeline in the Resource Repository🔗

This is where it gets interesting:

Clone the Application Repository (not the current repo!)
Provide secrets for both repositories
Set --repo to the Resource Repository (where the PR is)

.github/workflows/diff.yml (Resource Repository)

name: Argo CD Diff Preview

on:
  pull_request:
    branches: [main]

jobs:
  build:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pull-requests: write

    steps:
      # ⚠️ Clone the APPLICATION Repository, not this Resource Repository!
      # Both clone `main` because the PR is not on the Application Repository
      - uses: actions/checkout@v4
        with:
          repository: <org>/<application-repository>
          token: ${{ secrets.APP_REPO_TOKEN }}
          ref: main
          path: pull-request

      - uses: actions/checkout@v4
        with:
          repository: <org>/<application-repository>
          token: ${{ secrets.APP_REPO_TOKEN }}
          ref: main
          path: main

      - name: Prepare secrets
        run: |
          mkdir secrets

          # Secret for Application Repository
          cat > secrets/app-repo-creds.yaml << "EOF"
          apiVersion: v1
          kind: Secret
          metadata:
            name: app-repo-creds
            namespace: argocd
            labels:
              argocd.argoproj.io/secret-type: repo-creds
          stringData:
            url: https://github.com/<org>/<application-repository>
            password: ${{ secrets.APP_REPO_TOKEN }}
            username: not-used
          EOF

          # Secret for Resource Repository
          cat > secrets/resource-repo-creds.yaml << "EOF"
          apiVersion: v1
          kind: Secret
          metadata:
            name: resource-repo-creds
            namespace: argocd
            labels:
              argocd.argoproj.io/secret-type: repo-creds
          stringData:
            url: https://github.com/<org>/<resource-repository>
            password: ${{ secrets.GITHUB_TOKEN }}
            username: not-used
          EOF

      - name: Generate Diff
        run: |
          docker run \
            --network=host \
            -v /var/run/docker.sock:/var/run/docker.sock \
            -v $(pwd)/main:/base-branch \
            -v $(pwd)/pull-request:/target-branch \
            -v $(pwd)/output:/output \
            -v $(pwd)/secrets:/secrets \               ⬅️ Mount the secrets folder
            -e TARGET_BRANCH=<pr-branch> \             ⬅️ The PR branch on the Resource Repository
            -e REPO=<org>/<resource-repository> \      ⬅️ Resource Repository (where the PR is!)
            dagandersen/argocd-diff-preview:v0.1.20

      - name: Post diff as comment
        run: |
          gh pr comment ${{ github.event.number }} --repo ${{ github.repository }} --body-file output/diff.md --edit-last || \
          gh pr comment ${{ github.event.number }} --repo ${{ github.repository }} --body-file output/diff.md
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

Cross-repository access

The Resource Repository pipeline needs a Personal Access Token (PAT) or GitHub App token with access to the Application Repository. The default GITHUB_TOKEN only has access to the current repository.

Local equivalent🔗

git clone https://github.com/<org>/<application-repository> base-branch --depth 1 -q -b main
git clone https://github.com/<org>/<application-repository> target-branch --depth 1 -q -b main

mkdir secrets
# store some secrets in the ./secrets folder

docker run \
    --network host \
    -v /var/run/docker.sock:/var/run/docker.sock \
    -v $(pwd)/output:/output \
    -v $(pwd)/base-branch:/base-branch \
    -v $(pwd)/target-branch:/target-branch \
    -v $(pwd)/secrets:/secrets \
    -e TARGET_BRANCH=<pr-branch> \
    -e REPO=<org>/<resource-repository> \
    dagandersen/argocd-diff-preview:v0.1.20

Summary🔗

Pipeline location	Clone which repo?	`--repo` flag
Application Repository	Application Repository	Application Repository
Resource Repository	Application Repository	Resource Repository

With pipelines in both repositories, you'll get diffs for changes to either your Applications or the resources they manage.

Questions?

If you have questions or suggestions, please open an issue!

Multi-repo support🔗

Terminology🔗

Two key rules🔗

Rule 1: --repo must match the repository with the PR🔗

Rule 2: Always clone the Application Repository🔗

GitHub Workflows Examples🔗

Pipeline in the Application Repository🔗

Local equivalent🔗

Pipeline in the Resource Repository🔗

Local equivalent🔗

Summary🔗

Rule 1: `--repo` must match the repository with the PR🔗