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Server-first source control for monorepos, coding agents and CI.
Scion is a Rust implementation of the design in Anthropic's Redesigning source control: history, merges and rebases are computed on the server, clients stay online and fetch immutable objects on demand, and a checkout holds only the paths a job needs. It ships a server, scion, and a client, scn, over a language-neutral protocol, and is built by OpenCSG for starhub-server and CSGLite.
Four agents change one repository at the same time (left), each committing and landing on main; the right pane watches main on the server. When main has moved, the server rebases a commit as it lands (rebased onto the latest main, Your commit X is now Y). When two agents edit the same lines of CHANGELOG.md, the land is refused, and the agent runs scn sync, keeps both sides, scn resolve, and lands again: every agent's line ends up on main. The agents are scripts running real scn commands against a server; docs/demo has them and how to record the video (MP4).
CSG Scion vs. Git and CarterT27/scion on the OpenScion workloads, one Apple M5. Median ms, lower is better.
| Scenario | Git, 1k files | CarterT27, 1k | CSG Scion, 1k | Git, 10k files | CarterT27, 10k | CSG Scion, 10k |
|---|---|---|---|---|---|---|
| Import directory | 365.0 | 32.8 | 30.6 | 3,364.4 | 413.5 | 354.9 |
| Commit one changed file | 28.4 | 29.2 | 13.9 | 250.8 | 142.5 | 35.5 |
| Full clone / checkout | 243.0 | 115.8 | 52.2 | 2,361.9 | 1,047.3 | 589.4 |
| One-directory clone / checkout | 60.3 | 25.0 | 23.1 | 172.9 | 123.1 | 49.5 |
Fastest in every cell: 2–12× faster than Git, 1.1–4× faster than CarterT27/scion on checkouts and commits.
Real repository: pingdotgg/t3code, 5,573 commits and 28,068 files; CSG Scion on PostgreSQL and MinIO. The M4 Pro columns are from CarterT27/scion's README.
| Scenario | Git, M5 | CSG Scion, M5 | Git, M4 Pro | CarterT27, M4 Pro |
|---|---|---|---|---|
| Full checkout (28,068 files) | 2.13 s | 2.20 s | 2.90 s | 14.96 s |
Checkout of apps/web (1,521 files) |
0.173 s | 0.178 s | 0.21 s | 0.97 s |
| Clean status | 98 ms | 75 ms | — | — |
| Full history: Git's clone, Scion's import | 3.96 s | 103 s + 42 s repack | 3.56 s | 23 min 18 s |
| Storage of the history | 557 MB pack | 247 MB in MinIO + 90 MB in PostgreSQL | — | — |
Method and more: OpenScion workloads · starhub-server · component benchmarks
flowchart LR
scn["scn<br/>client"] --> daemon["scn daemon<br/>local cache"] --> cache["scion cache<br/>regional cache"] --> server["scion server<br/>commit · land · history"]
server --> meta[("SQLite / PostgreSQL")]
server --> blobs[("Local dir / S3")]
Commits are created and rebased on the server; clients fetch only the files they check out; history needs no clone. Also: scn import git with git SHAs kept, ACLs by repository, ref and path, and redaction.
# Install the client and run the server as a service on 127.0.0.1:7080 (Linux or macOS)
curl -fsSL https://raw.githubusercontent.com/OpenCSGs/scion/main/install.sh | sh -s -- --server
scn repo create http://127.0.0.1:7080/mono
scn clone http://127.0.0.1:7080/mono && cd mono
echo hello > README.md
scn commit -m "add readme"
scn land # move main to the commit; the server rebases if main moved
scn logWithout --server, the script installs only the client, scn. From source instead: cargo run -p scion -- server --lite --data-dir ./.scion-data and cargo install --path client/cli.
Know git? Most git commands work in scn as you type them, starting with scn clone http://host/REPO: see git commands in scn for the full list.
A tour of scion and scn goes through the everyday commands, and coming from git explains how the model differs. Tools that only speak git can clone, fetch and push the same URL: see git clients.
# Client only
curl -fsSL https://raw.githubusercontent.com/OpenCSGs/scion/main/install.sh | sh
# Client and server, as a service on 127.0.0.1:7080
curl -fsSL https://raw.githubusercontent.com/OpenCSGs/scion/main/install.sh | sh -s -- --server
# For a team: listen on all interfaces with authentication on; prints the first admin token
curl -fsSL https://raw.githubusercontent.com/OpenCSGs/scion/main/install.sh | sh -s -- --server --listen 0.0.0.0:7080Runs under systemd on Linux (data in /var/lib/scion) or launchd on macOS (data in ~/Library/Application Support/scion). sh install.sh --help lists the options; release binaries covers installing by hand.
docker run -d --name scion -p 7080:7080 -v scion-data:/data opencsghq/scion:latest
docker exec scion scion admin token create --lite --subject admin --group admins # shown only once
scn login http://localhost:7080 --token scn_pat_…
scn repo create http://localhost:7080/monoThe image is built for linux/amd64 and linux/arm64 and requires authentication. Members of admins manage the server and anyone signed in can read, write and land; to change that, see access control.
docker run -d --name scion -p 7080:7080 \
-e SCION_META=postgres://scion:PASSWORD@db:5432/scion \
-e SCION_BLOBS=s3://scion-objects -e SCION_S3_ENDPOINT=https://s3.example.com \
-e SCION_S3_ACCESS_KEY_ID=… -e SCION_S3_SECRET_ACCESS_KEY=… \
opencsghq/scion:latest --log-format json server --listen 0.0.0.0:7080 --auth-required
docker exec scion scion admin token create --subject admin --group adminsSeveral nodes behind a load balancer share one database and bucket (clusters). To try it all on one machine, deploy/compose starts PostgreSQL, SeaweedFS and the server.
Each user or CI job signs in once with scn login SERVER scn_pat_… (or sets SCION_TOKEN). An administrator creates tokens with scion admin token create --subject alice, where alice is the user's name as commands and policies see it, adding --group admins for another administrator or --ttl 90d for expiry:
| Deployment | Command |
|---|---|
| install script, Linux | sudo -u scion scion admin token create --lite --data-dir /var/lib/scion --subject alice |
| install script, macOS | scion admin token create --lite --data-dir ~/Library/Application\ Support/scion --subject alice |
| Docker, one node | docker exec scion scion admin token create --lite --subject alice |
| Docker Compose or production | docker compose exec scion scion admin token create --subject alice |
With an OIDC provider or a GitHub OAuth app configured, people sign in with scn login SERVER in a browser instead. Organizations (scn org), personal namespaces and several authorization sources can be combined: the policy, GitHub teams, and webhooks such as starhub or OPA. Deployment and authentication have the details.
| Use | Deploy · CLI tour · Git commands · Coming from git · Git import · Sparse checkout · Repos · Archives · Browse · Git SHAs · git clients |
| Operate | Storage · Clusters · Caching · Auth · Redaction · Events · Observability |
| Design | Server · Client · starhub · Object format · Protocol |
- M1–M4: write path, working copy, history, server-side landing, clusters, caches
- M5, production hardening: auth, redaction, observability, events, packs and release builds done; YugabyteDB and Helm remain
Details: milestones · roadmap
cargo test --workspace
cargo clippy --workspace --all-targets -- -D warningsDevelopment describes the repository layout, the dependency rules between crates and the testing requirements (at least 90% coverage of new code).
Apache-2.0