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

layer, status, code, updated, decisions

layer	status	code	updated	decisions
to-be	in-progress	mesh-host	2026-08-27	02-DECISIONS/0011-how-this-repository-works.md 02-DECISIONS/0015-a-node-and-how-it-joins.md 02-DECISIONS/0016-the-node-host.md 02-DECISIONS/0015-a-node-and-how-it-joins.md 02-DECISIONS/0015-a-node-and-how-it-joins.md 02-DECISIONS/0023-delivery.md 02-DECISIONS/0016-the-node-host.md 02-DECISIONS/0016-the-node-host.md 02-DECISIONS/0016-the-node-host.md 02-DECISIONS/0016-the-node-host.md 02-DECISIONS/0016-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 0016). 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 0016). 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): the three hand-run bootstrap scripts — first node, joining, rescue — and the synchronisers that rewrite managed files (00-as-is/06).

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 0023). 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 0014). 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 0015), 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 0015).

It is the broker connection that already exists (ADR 0001) — 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): 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 0015 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, 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 0015):

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 0016.

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 0021); 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 0016); 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) 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 0015; 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) 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 0013).

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.
• 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 answered.
• Rescue. ADR 0015 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 0015).