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Split/Sync

Extract crates from monorepos to standalone repositories with full git history, then keep them in sync bidirectionally.

What This Replaces

Tool What It Does Limitation
Google Copybara Bidirectional sync between repos Java dependency, complex config, heavy runtime
git subtree One-way extraction Manual, no bidirectional sync
git filter-repo History rewriting One-time operation, no ongoing sync
git subrepo Subtree management Limited conflict resolution

cargo-rail split/sync provides:

  • Bidirectional sync with 3-way merge conflict resolution
  • Full git history preservation (deterministic SHAs via system git)
  • Zero Java/Python dependencies (pure Rust)
  • PR-based workflow (never merges to main without review)

Split Modes

Mode Use Case Example
single One crate → one repo tokio-macros → standalone repo
combined Multiple crates → one repo Shared utilities bundle
workspace Multiple crates → workspace structure Mirrors monorepo layout

Quick Start

# 1. Initialize split config for a crate
cargo rail split init my-crate --check    # preview config
cargo rail split init my-crate            # write config to rail.toml

# 2. Extract crate to standalone repo
cargo rail split run my-crate --check     # preview extraction
cargo rail split run my-crate             # execute extraction

# 3. Ongoing sync (after initial split)
cargo rail sync my-crate --to-remote      # push monorepo changes to split repo
cargo rail sync my-crate --from-remote    # pull split repo changes (creates PR branch)

Configuration

# rail.toml
[crates.my-crate.split]
remote = "git@github.com:org/my-crate.git"
branch = "main"
mode = "single"
paths = [{ crate = "crates/my-crate" }]

[crates.my-crate.release]
publish = true

Safety

  • Check mode first: Always run with --check before applying
  • Sandbox testing: Test on throwaway repos before production
  • Clean worktree: Keep source repo clean before split/sync
  • PR workflow: --from-remote creates a PR branch, never merges directly

See Also