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hyperhive/swarm-builder/README.md
iris 6739bb903d docs: sort the top-level directories into five sections by what the reader wants
Every docs directory moves under 1-getting-started, 2-handbook,
4-implementation or 5-project; the options reference (section 3) comes from
the website and has no directory here. trust-boundary stays whole under the
handbook for now. web-ui, swarm and tools split between handbook and
implementation, and the command-line references move to
2-handbook/reference. Links, path references in code and nix, and the docs
index follow the moves; no page content changes.
2026-11-06 18:46:57 +01:00

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# swarm-builder
The swarm builder's build service, in the `swarm-builder` container on the
host that sets `deploy.builder.enable`. The design is
[`docs/4-implementation/swarm/builder.md`](../docs/4-implementation/swarm/builder.md).
swarm-controller sends one request per config PR head: an agent, a commit and
the PR. This service builds that agent config on the agent base this host was
deployed with, through the host's nix daemon, and answers with the system
closure's store path or the tail of what `nix` printed. The controller turns
the answer into a `swarm-builder` commit status on the PR's head.
## What a build is
- `nix build` of `nix/builder/check-flake`'s `check` output, with its three
inputs overridden: `hyperhive` and `nixpkgs` to this host's own sources,
and `agent` to `git+https://<forge>/agent-configs/<agent>?ref=refs/pull/<pr>/head&rev=<rev>`.
- The evaluation runs here, pure; the builds run in the host daemon's sandbox
as an untrusted client, so a config can neither set daemon options nor read
this service's environment.
- Nothing is pushed and nothing is recorded: the closure stays in the host's
store until it is collected. (A meta build can push, see below.)
## Meta builds
The service also builds one agent from `internal/meta` on request: an agent and
a commit of the repo. It runs `nix build` of
`nixosConfigurations.<agent>.config.system.build.toplevel` from that commit,
with every input at the flake's own lock, and answers with the store path and
the platform it builds for. The controller records the answer; the service
writes nothing. It takes the same one-build slot as the PR checks. See
[`docs/4-implementation/swarm/builder.md`](../docs/4-implementation/swarm/builder.md#deploy-gate).
Where `services.hyperhive.swarm.attic.{domain,cache}` are not null (they default
to the swarm's own cache), it first pushes a green build's closure to that cache with `attic push`, and the answer says
whether that worked. A failed push leaves the build green. See
[`docs/4-implementation/swarm/builder.md`](../docs/4-implementation/swarm/builder.md#binary-cache).
## Metrics
The service exports build counts, durations, queue depth and wait, and reply
publish failures through its container's forwarder to the swarm collector. The
names and attributes are in
[`docs/4-implementation/scheduler/observability.md`](../docs/4-implementation/scheduler/observability.md#swarm-builder-metrics).
## Credentials
The container holds one credential: its own store certificate, which nspawn
hands in at start without a copy on disk. With it the service reads its
queue client secret at start and again when authelia refuses it, and its
read-only forge token at every build, and the cache push token at every push.
`git` gets the token through a credential helper reading the child's
environment, never in a URL or an argument, and the helper answers only an
`https` request for the forge's host. Its store role reads exactly
those three paths.
## Archetypes
The service also answers archetype evaluations: `nix eval` of the `meta` of
every `hyperhiveArchetypes` entry in this host's hyperhive source, with
nixpkgs pointed at this host's. It fetches nothing and uses no token, and it
doesn't wait for a running build. See
[`docs/2-handbook/swarm/archetypes.md`](../docs/2-handbook/swarm/archetypes.md).
## Meta-flake lock
The service also locks the swarm meta flake on request: `nix flake lock` of
the `flake.nix` it was sent, in a scratch directory, with the build's forge
credential. It answers with the lock and commits nothing. See
[`docs/4-implementation/swarm/builder.md`](../docs/4-implementation/swarm/builder.md#meta-flake-lock).