Mirroring Repositories

Mirroring Repositories

Maintain synchronized copies of repositories with optional transformations.

Simple Mirror

Basic mirroring without transformations:

core.workflow(
    name = "mirror",
    origin = git.origin(
        url = "https://github.com/upstream/repo",
        ref = "main",
    ),
    destination = git.destination(
        url = "https://github.com/myorg/mirror",
        push = "main",
    ),
    origin_files = glob(["**"]),
    authoring = authoring.pass_thru("Mirror Bot <mirror@myorg.com>"),
    mode = "ITERATIVE",  # Preserve commit history
)

Mirror with Transformations

Apply changes during mirroring:

core.workflow(
    name = "mirror",
    origin = git.origin(
        url = "https://github.com/upstream/repo",
        ref = "main",
    ),
    destination = git.destination(
        url = "https://github.com/myorg/fork",
        push = "main",
    ),
    origin_files = glob(["**"]),
    authoring = authoring.pass_thru("Fork Bot <fork@myorg.com>"),
    transformations = [
        # Apply custom patches
        core.replace(
            before = "upstream.example.com",
            after = "myorg.example.com",
        ),
    ],
    mode = "SQUASH",
)

Partial Mirror

Mirror only specific parts:

core.workflow(
    name = "partial-mirror",
    origin = git.origin(
        url = "https://github.com/upstream/monorepo",
        ref = "main",
    ),
    destination = git.destination(
        url = "https://github.com/myorg/specific-package",
        push = "main",
    ),
    origin_files = glob(["packages/specific/**"]),
    transformations = [
        core.move("packages/specific/", ""),
    ],
    authoring = authoring.pass_thru("Mirror <mirror@myorg.com>"),
    mode = "SQUASH",
)

Fork Maintenance

Keep a fork in sync with upstream while maintaining patches:

core.workflow(
    name = "sync-fork",
    origin = git.origin(
        url = "https://github.com/upstream/repo",
        ref = "main",
    ),
    destination = git.destination(
        url = "https://github.com/myorg/fork",
        fetch = "upstream-sync",
        push = "upstream-sync",
    ),
    destination_files = glob(
        include = ["**"],
        exclude = ["myorg-patches/**"],  # Keep our patches
    ),
    authoring = authoring.pass_thru("Sync <sync@myorg.com>"),
    mode = "SQUASH",
)

Read-Only Mirror

Add a notice that the mirror is read-only:

transformations = [
    metadata.add_header(
        text = """\
This is a read-only mirror. Do not submit changes here.

Source: https://github.com/upstream/repo
""",
    ),
]

Scheduled Mirroring

Run in GitHub Actions on a schedule:

.github/workflows/mirror.yml

name: Mirror Upstream

on:
  schedule:
    - cron: "0 */6 * * *" # Every 6 hours
  workflow_dispatch:

jobs:
  mirror:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0

      - name: Run Copybara
        run: |
          java -jar copybara.jar migrate copy.bara.sky mirror

Multi-Branch Mirror

Mirror multiple branches:

def mirror_branch(branch):
    return core.workflow(
        name = "mirror-" + branch,
        origin = git.origin(
            url = "https://github.com/upstream/repo",
            ref = branch,
        ),
        destination = git.destination(
            url = "https://github.com/myorg/mirror",
            push = branch,
        ),
        authoring = authoring.pass_thru("Mirror <mirror@myorg.com>"),
        mode = "ITERATIVE",
    )

# Create workflows for each branch
mirror_branch("main")
mirror_branch("release/v1")
mirror_branch("release/v2")

Next Steps

• Monorepo extraction
• GitHub Actions