shep-deploy

A deploy dog for shep.

Watches a git branch, builds a release in an isolated directory, swaps to it, reloads the sheep, and rolls back on its own if the new release does not come up.

It is an external dog, the same shape as shep-log-rotate: an ordinary binary you adopt, talking to the daemon over the socket the CLI already uses.

What one deploy does

1. Fetch into a bare clone, and compare the branch head to the last deployed sha.
2. git worktree add the new sha, sharing the object store.
3. Symlink the shared files in: whatever git ignores and .shepignore does not.
4. Run the build, as the app's user if it sets one.
5. rename(2) current onto the new release.
6. Reload the sheep.
7. Verify. On failure, put current back and reload again.

Steps 1 to 4 never touch the running app. A build that fails costs a directory, not an outage.

Verification waits for a new process to reach Online, not for any process to be Online. shep answers a reload before it has finished one, and keeps the old instance when the replacement never becomes ready, so "something is online" is true throughout a deploy that failed.

Layout

Everything lives under $SHEP_HOME/deploy/<sheep>/:

git/                 one bare clone, shared by every release
releases/<sha>/      a worktree per release
current -> releases/<sha>
deploy.toml          remote, branch, deployed sha, held sha, verify mode, watch mode

The sheep's cwd is current, permanently. Set it explicitly when you register the app: a Flockfile cwd left to default is resolved at registration, which pins the sheep to one release and makes every later swap invisible to it.

Usage

shep-deploy deploy <sheep>
shep-deploy deploy <sheep> --watch auto|manual
shep-deploy setup <sheep>
shep-deploy survey
shep-deploy on-remove

Adopted as a dog, shep-deploy takes no arguments and polls instead. Every 30 seconds by default, it deploys any watch = "auto" target whose branch has moved. Configure it in shep.toml:

[dog.deploy]
interval = "30s"
retention = 5

Both are read once, when the dog starts, so changing either takes a shep restart deploy. retention is how many releases each target keeps. It cannot be below 2: the release a failed deploy rolls back to is the second newest, so anything lower would silently disable rollback, and it is refused rather than clamped.

One target's failure never stops the others, and never stops the dog. Each target's outcome is reported on its own and the loop carries on. up to date prints nothing at all, which is the answer to almost every tick of almost every target, and a line that repeats is said once rather than every interval.

Every tick also paints each target's smit, so shep flock shows which branch and sha it is on without a second command:

▲ main@a1b2c3      watched
⏸ main@f6e5d4      manual

Republished every tick rather than on change: shep holds a smit in memory only for as long as the connection that painted it stays open, so a dog that only published on change would show nothing at all after a daemon restart until its next deploy. A refused smit is logged and otherwise ignored - it is cosmetic, never worth failing a deploy over.

A commit that does not land is left alone until the branch moves. It is written into deploy.toml as failed, because otherwise the branch head and the deployed sha stay different and every tick runs the whole sequence again: fetch, full rebuild, swap, reload, wait out the verification budget, roll back, reload again. Two reloads of a live app and a build every thirty seconds, from one bad commit, until somebody notices. Pushing a fix clears it, which is what CI does with a red commit; so does shep deploy <sheep>, which retries the same commit deliberately. The tradeoff is deliberate too: a deploy that failed on a network blip rather than on the commit waits for one of those two, rather than being retried on the next tick. shep-deploy survey shows such a target as held, naming the commit it is holding, so a target stuck since yesterday does not read like one with nothing to do. Survey reads the record, never the remote, so a hold that a push already cleared still shows until the next tick.

A tick never begins while the previous one is still running, so a push landing during a build is deployed on the next tick rather than aborting the build in flight.

--watch changes the setting and returns without deploying. survey reports where every registered sheep stands and starts, registers and writes nothing.

shep-deploy on-remove is the lifecycle hook: shep runs it before forgetting the dog, and it puts every sheep back where it ran before the dog took over. A sheep the dog bootstrapped has nowhere to go back to, so it is left running from current and the report says exactly that, with the path. The deploy tree is never deleted. It is not the dog's to delete, and in the bootstrap case a running app is still pointing into it.

setup takes a sheep over: it builds the tree, fetches the repository, links the shared files in, builds the first release, and re-registers the sheep with its cwd set to current. The first cutover is the one deploy that may have downtime. It runs two instances at once, so an app that does not bind with SO_REUSEPORT cannot take its own port while the original still holds it, and the new instance is then removed and the original left serving. Every deploy after the first replaces the instance rather than joining it, and does not meet this.

A cutover that was abandoned leaves the tree behind, and a sheep is not a deploy target until a cutover lands. Its record names no deployed release, so shep deploy against it does not stop: it builds, swaps, reloads the sheep at its own checkout, sees a real turnover, and reports success for a release nothing served. Remove its tree and run setup again once the cause is fixed. Both setup and the failure message say this, and both print the resolved path to remove rather than a $SHEP_HOME you would have to expand yourself. shep deploy refuses such a target outright, setup leaves it watch = manual until the cutover lands, and --watch auto on one is refused for the same reason: it would ask for that deploy once every interval, unattended.

It is also the one deploy that is not verified against the readiness probe. shep reports a freshly started process Online once its listen_timeout elapses, whatever the probe said, and only aborts a reload whose replacement was not ready. So setup checks what it can: a new process started and was still the same process, not errored and not restarted, ten seconds later. A release that starts, stays up and serves nothing passes that. Every deploy after the first is verified properly, against the probe, with automatic rollback.

Exit codes follow shep's own taxonomy: 0 deployed or already up to date, 2 bad arguments, 4 bad configuration, 5 no daemon answered, 1 anything else. Two are this dog's own. 12 means the deploy was rejected and the previous release was put back. 13 means a first cutover landed and then could not tidy up: the sheep is live on the new release and something after the swap failed. A script that treats any nonzero code as "the deploy broke" will be wrong about both, because in each case the flock is healthy and serving, on the old release for 12 and on the new one for 13.

verify = "probed" (the default) needs the app to have a readiness_probe or wait_ready; without one, shep reports a process Online for not having died yet, so there is nothing to verify against and the deploy is refused. verify = "alive" is the deliberate downgrade: a new process, still running ten seconds later.

Security

A deploy runs the build command from the repository being deployed. That is the point of it, and it is also the whole of the risk: bun install's postinstall scripts and make build are arbitrary code, chosen by whoever can land a commit on the branch you track.

That build runs as this process's uid unless the app sets user. Supervised as a dog, this process is the shepherd's child and shares its uid, and shep's own docs recommend running the shepherd as root so it can drop privileges per app. So the default arrangement is a repository's build script running as root, once per deploy.

Set user on the app and the build drops to that user's uid and primary group, with the shepherd's supplementary groups cleared, before it runs anything. A compromised build then gets that app's privileges and nothing more.

shep-deploy warns when it is about to run a build as root with no user set. It does not refuse. Whether an app runs without a user is shep's call and the operator's, not a deploy dog's.

Nothing else here handles credentials. Git auth is inherited from the user the build runs as, so a private repository works exactly as it does in that user's own shell, and no token passes through any URL or argument this crate builds.

Platform

Unix only. This is deliberate, not a gap waiting to be filled by accident: the deploy model is rename(2) over a symlink, the build's privilege drop is a uid and a gid, and both are Unix concepts the code uses directly rather than through a portability layer. Building on Windows fails with one sentence saying so. Windows support is planned and will be scoped on its own.

Status

Working: the deploy sequence, the operator commands, opt-in, the poll loop, retention, and restore on removal. Tested against a real shepherd.

Not built: Windows.

See docs/writing-plans/plans/2026-08-26-deploy-engine.md for what was built, and the design spec for what it is for.

License

MIT OR Apache-2.0, at your option.