hq/03-DESIGN/01-to-be/05-the-node-host.md

---
layer: to-be
status: in-progress
code: [mesh-host]
updated: 2026-08-30
decisions:
  - 02-DECISIONS/0019-how-this-repository-works.md
  - 02-DECISIONS/0004-a-node-and-how-it-joins.md
  - 02-DECISIONS/0005-the-node-host.md
  - 02-DECISIONS/0004-a-node-and-how-it-joins.md
  - 02-DECISIONS/0004-a-node-and-how-it-joins.md
  - 02-DECISIONS/0010-delivery.md
  - 02-DECISIONS/0005-the-node-host.md
  - 02-DECISIONS/0005-the-node-host.md
  - 02-DECISIONS/0005-the-node-host.md
  - 02-DECISIONS/0005-the-node-host.md
  - 02-DECISIONS/0005-the-node-host.md
---

# The node host

Tier 0. The one thing ever installed by hand, and the only thing that changes a machine.

## What it is

A **statically linked binary that requires nothing to be present** — copy it onto a machine and
run it, and that is the whole installation
([ADR 0005](../../02-DECISIONS/0005-the-node-host.md)). Written in Go, because the
job is system-level and because the host shares no code with any other tier.

A single binary with one job: **apply declared state on this machine**
([ADR 0005](../../02-DECISIONS/0005-the-node-host.md)). Overlay
membership, packet filtering, packages, services, containers and filesystems are not six
concerns it carries; they are six instances of the one.

It replaces three things that exist today
([`00-as-is/05`](../00-as-is/05-runtime-and-installation.md)): the three hand-run bootstrap
scripts — first node, joining, rescue — and the synchronisers that rewrite managed files
([`00-as-is/06`](../00-as-is/06-configuration-and-secrets.md)).

**What it is not:** it does not decide anything that needs another node, it never queries the
mesh database, and it has no listening surface.

## The parts

| Part | Owns |
|---|---|
| `apply` | reconciling declared state on this machine |
| `store` | local state, authoritative while disconnected |
| `link` | the single outbound connection to the control plane |
| `profile` | what this machine can be asked to do |
| `inventory` | what this machine is and has |
| `substrate.lock` | the pinned tier-1 descriptor, appliable with no mesh present |

### apply

Takes a **declaration** and makes the machine match it. Idempotent: applying the same
declaration twice changes nothing the second time, and applying it to a drifted machine
returns it.

Three properties, each following a recorded decision:

**A failed step fails the apply.** Not "logs and continues"
([ADR 0010](../../02-DECISIONS/0010-delivery.md)). A partial apply that
reports success is the mesh's most expensive shape.

**Each applier reads back.** Setting a value is not evidence the value took. The firewall is
asked whether the rule loaded; conntrack is asked what timeout it holds. This is `how-we-build`
§5 as a component requirement rather than a review habit.

**What was applied is recorded after it works, never before**
([ADR 0018](../../02-DECISIONS/0018-a-picture-is-read-from-what-runs.md)). A failed apply leaves
the machine in whatever state it reached, and nothing must claim otherwise.

### store

Local, and **authoritative while disconnected**. Not a cache of the control plane — the record
of what this node has applied and what it currently holds.

This is structural rather than convenient: if disconnection is an ordinary situation rather
than an exception ([ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md)), the
store is what makes it ordinary. A laptop shut for a week comes back and reconciles; it does
not come back and ask what it is.

### link

The node's one connection to the control plane, and its security boundary
([ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md)).

It is the broker connection that already exists
([ADR 0002](../../02-DECISIONS/0002-nodes-communicate-over-a-broker.md)) — outbound,
node-initiated, per-node addressed — carrying **per-node identity instead of a shared
credential**. The node owns no password. It owns an identity, and that identity is what it
presents.

What arrives is bounded by form: **declarations of known shape, never a command to run.**

**It must fail legibly.** A node that cannot link says which side refused and on what grounds.
A boundary that refuses without saying why is worse than the password it replaced.

### profile

What this machine can be asked to do — a graphical session, a container runtime, an
architecture, a network position.

**Detected, never assumed.** A package being installed does not mean a capability is present
([04-ISSUES/007](../../04-ISSUES/007-an-installed-package-is-not-a-capability/00-report.md)): a
capability is real when it is present, running and working, and the difference is the whole
point of detecting it.

The profile is what makes [ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md)
work: a node is a node, and what varies between them is here rather than in the definition.

### inventory

What this machine *is* — its identity, what it holds, what it has applied. Reported upward over
the link; never asked downward.

## What it is not

- It does not decide anything that needs another node.
- It never queries the mesh database.
- It has no listening surface.
- **It does not manage its own unit.** It manages `service` resources and its own unit is one —
  the temptation is obvious and it ends with a host stopping itself half way through an apply,
  leaving a machine with nothing running to fix it. The installation owns the host; the host owns
  everything else.

**How it is installed, enrolled, run, upgraded and retired is
[`09-the-node-lifecycle.md`](09-the-node-lifecycle.md)**, in full and in one place. This document
is the component; that one is what happens to it.

## Where a declaration comes from

One behaviour, two sources
([ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md)):

| Situation | Source |
|---|---|
| no mesh reachable | `substrate.lock` — the pinned bundle the host carries |
| mesh reachable | the control plane, over the link |

**The first node is not a different kind of node.** It is a node whose mesh is not up yet. It
applies the bundle it carries, the control plane comes up on top of it, and from that moment it
takes declarations like every other node. Its specialness is temporary and self-erasing.

**A joining node does the minimum to be reachable and nothing else** — an identity, an address,
one peer — and then stops deciding. It does not compute the overlay; it needs one peer to reach
the mesh, and the full peer set arrives derived.

## What a declaration is

Settled by [ADR 0005](../../02-DECISIONS/0005-the-node-host.md).

**JSON**, because the host has no dependencies to spend and the standard library carries no
YAML. **An ordered list of typed resources**, each with a stable identity — the order is stated
rather than derived, because deriving it would be the host deciding the thing most likely to
differ from what the control plane intended.

**Unknown is refused, never skipped.** An unknown version, type or field refuses the whole
declaration. A host that skipped what it did not understand would apply most of it and report
success.

**Complete for what the host owns, and only that.** It removes what it previously applied and
is no longer declared — a fact it holds, from the store, rather than an inference — and never
removes anything it did not create.

**Addressed.** A host with an identity refuses a declaration addressed elsewhere; a host
without one, applying the bundle it carries, has nothing to check against.

## Build order

Staged so each stage is verifiable in the lab before the next exists.

**1 — profile and inventory.** The host runs on a machine, detects what it can do, and reports
what it is. No control plane, no declarations, no network. Verifiable immediately: the lab's
`place:` gains its first implementation, and a raised scenario finally contains something.

**2 — apply, from the bundle.** The host applies `substrate.lock` with no mesh present. This is
the first node's path, and it is the claim the skeleton's Move 1 rests on and has never proved:
that one host can raise the substrate alone.

Raising the substrate needs six shapes in the host's vocabulary, and **all six are built**:

| | | |
|---|---|---|
| `directory`, `file` | **built** | no machine dependency at all |
| `service` | **built** | running/stopped **and** enabled/disabled at boot — a unit started but not enabled stops being true at the next reboot |
| `package` | **built** | present, never upgraded, and **never uninstalled** — the host cannot know what else needs it, so dropping one is *forgotten*, not *removed* |
| `container` | **built** | pinned by digest ([ADR 0006](../../02-DECISIONS/0006-the-substrate-and-the-control-plane.md)); identified by a label carrying a digest of the declaration that made it, because a runtime normalises what it is given and that is indistinguishable from drift |
| `action` | **built** | bundle-only ([ADR 0005](../../02-DECISIONS/0005-the-node-host.md)); verify is mandatory and is the idempotency check as well as the read-back |

**The parser enforces the boundary rather than the caller remembering it.** `Parse` refuses an
action and is what the link uses; `ParseTrusted` permits one and is what the bundle uses. The
safe path is the default and the permissive one has to be named.

**The lab still cannot exercise the last three**, and that is now the only thing in the way: a
scenario is a closed address space, so nothing can be fetched there, and its machines carry no
container runtime. All three were instead verified against a real machine — a container created,
labelled, replaced when its declaration changed, exec'd into and removed; an action that exits
zero and satisfies nothing failing the apply. That is lab-installation work
([`04-lab-installation.md`](04-lab-installation.md)) rather than a constraint on the design, but
until it is done the substrate bootstrap has no end-to-end test.

**3 — link and store.** The node connects, receives declarations, and holds what it applied.

**4 — enrolment.** The one genuinely new mechanism in
[ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md); everything else there
is configuration of what already runs.

Stage 1 is deliberately the smallest useful thing. The lab currently raises **empty machines**
([`00-as-is/11`](../00-as-is/11-the-lab.md)) because `place:` has nothing to place; stage 1 ends
that, and every later stage is tested by a lab that already works.

## How it is verified

**The lab is the harness.** A scenario places a host on a machine and asserts what it did —
against a real hypervisor, with the boundary never mocked
([ADR 0017](../../02-DECISIONS/0017-a-test-defends-a-decision.md)).

Each decision above owes a test:

| Decision | What asserts it |
|---|---|
| 0037 — the host never queries the mesh database | no database client in the dependency tree; a dependency-direction lint failing on an upward import |
| 0038 — one behaviour, two sources | the same code path raises a first node and joins a second |
| 0039 — a node holds no shared credential | a raised node's store contains no credential to any service |
| 0036 — disconnection is a situation | a node cut off and returned reconciles without being re-adopted |
| 0008 — a failed step fails the apply | an apply with a failing step reports failure |

## Open

- **Whether one host can raise the substrate alone.** Move 1 assumes it. Stage 2 tests it, and
  if it is false the tier boundary moves.
- **What the host carries versus what it finds.** It manages `wg`, `nft`, `pacman`, `docker`;
  it does not contain them, and how it obtains one it lacks is undecided —
  [04-ISSUES/007](../../04-ISSUES/007-an-installed-package-is-not-a-capability/00-report.md).
- **Six vocabularies.** Zero dependencies, but the host must still know what a peer, a rule, a
  package, a unit, a container and a dataset *are*. Nothing has measured that surface, and it is
  the residue of the question [`host-size.md`](../../01-RESEARCH/006-mesh-from-scratch/host-size.md)
  answered.
- **Rescue.** [ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md) suggests it is
  a node whose local state is discarded so the mesh re-derives it, and does not decide it.
- **What may expire.** An identity needing refresh to stay valid would make a laptop fail for
  being a laptop ([ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md)).

## What was added to the vocabulary, and why each cost was worth paying

*Written 2026-08-30. Every addition widens what a compromised control plane can express, so the
count is asserted by a test and a change to it is a decision rather than a convenience.*

Six shapes raised the substrate. Two more exist because most of what a person installs is not a
service:

| | why |
|---|---|
| **user** | a shell, a terminal, a chat client, a desktop are a package plus configuration **in somebody's home**. A mesh with no user owns `/etc` and nothing anybody looks at |
| **archive** | a theme is hundreds of files. Inlining them makes every declaration enormous and rewrites all of them when one changes |

And `file` gained two fields: `bytes`, because a wallpaper is not a string, and `owner`, because a
file in a home belongs to somebody.

**`user` also makes a login shell declared state.** `chsh` is a command, the link may not carry
one, and a shell that could only be set by hand is a shell the mesh cannot manage — which is most
of the reason to manage a machine.

### The refusals that came with them

- **A file says what is in it exactly once.** `content`, `bytes` and `sealed` are exclusive, so
  *what is in this file* is answerable by looking rather than by knowing which field wins.
- **Groups are added, never pruned.** The tool that sets them replaces the set unless told
  otherwise, which would silently remove every group that makes a login able to use the machine.
  A machine's own groups are not the mesh's to know about.
- **An archive is pinned by digest, checked before a single file is written.** This is the one
  place the host reaches out on its own — everywhere else it holds one outbound connection and
  fetches nothing — so the only thing making those bytes safe to unpack is that they hash to what
  was declared.
- **An entry naming a path outside the archive is refused, not sanitised.** Rewriting it to land
  inside would put a file somewhere nobody asked for and report success. The first implementation
  quietly relocated it, and a test caught that.
- **Symlinks and device nodes are refused rather than skipped**, or an archive needing one arrives
  silently incomplete.

**A partial host does archives and refuses users**: an archive needs a filesystem and a way to
fetch; a user needs a user database the host is allowed to write.