Glossary

Glossary

Quick reference for terminology used throughout this documentation. If you’re new to code synchronization tools or coming from a different ecosystem, this page will help you understand the concepts.

Code Review Systems

Gerrit

Gerrit is an open-source, web-based code review tool developed by Google. It’s widely used in enterprise environments and open-source projects (including Android, Chromium, and Go).

Key differences from GitHub:

• Uses “Change Lists” (CLs) instead of Pull Requests
• Built-in support for multi-stage review workflows
• Tighter integration with Git (changes are pushed to special refs)
• Often used with Bazel build system

Tip

If you’ve only used GitHub, think of Gerrit as a more enterprise-focused alternative with stricter review workflows.

CL (Change List)

A Change List (CL) is Gerrit’s equivalent of a Pull Request. It represents a set of code changes submitted for review.

When Copybara creates a CL, it pushes to Gerrit’s special refs/for/<branch> reference, which triggers the review process.

Related: Gerrit Change Documentation

PR (Pull Request)

A Pull Request (PR) is GitHub’s mechanism for proposing code changes. GitLab calls these “Merge Requests” (MRs).

When Copybara uses git.github_pr_destination, it creates a branch and opens a PR for review.

Related: GitHub Pull Request Documentation

Merge Request (MR)

GitLab’s term for a Pull Request. Functionally equivalent to GitHub PRs.

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Copybara Concepts

Origin

The origin is the source repository that Copybara reads code from. This is where your “source of truth” code lives.

origin = git.github_origin(
    url = "https://github.com/org/repo",
    ref = "main",
)

Related: Origins & Destinations Overview

Destination

The destination is the target repository that Copybara writes code to. This is where transformed code ends up.

destination = git.destination(
    url = "https://github.com/org/public-repo",
    push = "main",
)

Related: Origins & Destinations Overview

Workflow

A workflow is a named Copybara configuration that defines a complete sync operation: where to read from, where to write to, and what transformations to apply.

core.workflow(
    name = "export",
    origin = ...,
    destination = ...,
    transformations = [...],
)

You can define multiple workflows in a single config file (e.g., export and import).

Related: Workflow Modes

SOT (Source of Truth)

The Source of Truth (SOT) is the authoritative repository where canonical code lives. Changes always flow FROM the SOT to mirrors, never the reverse (unless explicitly importing contributions).

• For export workflows: Internal repo is SOT → External repo is mirror
• For import workflows: External repo is SOT → Internal repo receives contributions

Related: Core Concepts

Transformation

A transformation is a modification applied to code during the sync process. Transformations run in sequence and can move files, replace text, filter content, and more.

transformations = [
    core.move("internal/", "src/"),
    core.replace("internal.corp.com", "example.com"),
]

Related: Transformations Overview

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Git Concepts

Squash

Squashing combines multiple commits into a single commit. In Copybara’s SQUASH mode, all origin changes since the last sync become one destination commit.

Commits A, B, C → Single commit "A + B + C"

Related: SQUASH vs ITERATIVE

Rebase

Rebasing means reapplying commits on top of a different base commit. This creates a linear history without merge commits.

Copybara’s ITERATIVE mode works best with rebased (linear) histories.

Related: Git Rebase Documentation

SHA / Commit Hash

A SHA (Secure Hash Algorithm) or commit hash is a unique 40-character identifier for a Git commit (e.g., abc123def456...). Often abbreviated to 7-8 characters.

Copybara tracks sync state using these hashes in the GitOrigin-RevId label.

Ref

A ref (reference) is a human-readable name pointing to a commit. Common refs include:

• Branch names: main, develop
• Tags: v1.0.0
• Special refs: HEAD, refs/for/main (Gerrit)

Merge Commit

A merge commit has two or more parent commits, created when merging branches. Copybara’s ITERATIVE mode may fail on merge commits; SQUASH handles them gracefully.

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Repository Patterns

Monorepo

A monorepo (monolithic repository) is a single repository containing multiple projects, services, or components. Google, Meta, and Microsoft use monorepos internally.

Copybara is often used to extract portions of a monorepo for open-source release.

Example: Extracting //third_party/project/ from an internal monorepo to a public GitHub repo.

Related: Monorepo Extraction Use Case

Mirroring

Mirroring keeps two repositories synchronized. Unlike simple git push --mirror, Copybara can apply transformations during mirroring.

Related: Mirroring Repos Use Case

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Configuration

Starlark

Starlark is a Python-like configuration language created by Google. It’s used by:

• Bazel - Google’s build system
• Buck2 - Meta’s build system
• Copybara - For .bara.sky config files

Starlark is intentionally limited (no I/O, no infinite loops) to ensure configs are deterministic and fast to evaluate.

Related: Starlark Language Spec

Glob

A glob is a pattern for matching file paths. Copybara uses globs to specify which files to include or exclude.

Pattern	Matches
*	Any characters except /
**	Any characters including / (recursive)
?	Single character

glob(["src/**/*.js"], exclude = ["**/*_test.js"])

Related: Glob Patterns, Glob Reference

Bazel

Bazel is Google’s open-source build system. It uses Starlark for configuration (BUILD files). Many Copybara users come from Bazel-based environments.

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See Also

• Core Concepts - Foundational Copybara concepts
• Introduction - What Copybara is and why use it
• GitHub Copybara Repository - Official source code