---
status: proposed
date: 2026-08-27
deciders: jochen
reconstructed: false
extends: 0041-the-host-depends-on-nothing.md
---

# 57. The host is a root service, installed as a package, that never manages itself

## Context

[`05-the-node-host.md`](../03-DESIGN/01-to-be/05-the-node-host.md) describes what the host *does*
and never says what it *is* at runtime. Searched: the words *daemon*, *long-running*, *interval*,
*poll* and *heartbeat* appear in none of it, nor in
[ADR 0038](0038-a-node-joins-by-linking-first.md) or
[ADR 0039](0039-the-link-is-the-security-boundary.md).

What exists today is a command that runs and exits — `mesh-host apply FILE`. What the design
requires is a process holding an outbound link to the control plane. Nobody wrote down that
these are different things, so several questions have no answer: does it reconcile on a timer,
what happens when the link drops, and who installs the unit that starts it — given that the host
is the thing that installs units.

## Decision

### It runs on every node, and that is what a node is

[ADR 0036](0036-a-node-is-a-managed-machine.md) defines a node as a managed machine. **The host
is what makes it managed**, so a machine without one is not a node with a missing component; it
is not a node. There is no partial mode, no agentless node, and no second way in.

### It is a root service

**Root**, because there is no useful subset of its job that is not privileged: it writes under
`/etc`, installs packages, manages units, and runs containers. A host that dropped privilege
could apply almost nothing, and the almost is where the confusion would live.

**A service rather than a command**, because it holds the link, and something must survive a
reboot to hold it. The command-line entry points remain — they are how a person inspects and
rescues a machine — but the ordinary case is a unit that is always up.

### It never manages its own unit

**The host's own service file is not a resource the host applies.** The temptation is obvious —
it manages units, and its own unit is a unit — and it ends with a host stopping itself half way
through an apply, leaving a machine in a state nothing is running to fix.

So the boundary is: **the installation owns the host; the host owns everything else.** A
declaration that names the host's own unit is refused rather than obeyed.

### It is installed as a package, and a tarball is the floor

Two mechanisms, and the second is not a fallback for the first failing — it is what makes the
first possible.

| | |
|---|---|
| **package** — `pacman -S nox-mesh-host` | the ordinary path. Carries the binary, the unit file, the state directory, and an upgrade path |
| **tarball** — `curl … \| tar -xz` | the floor. One static binary, no repository, no distribution assumed |

**Why a package rather than only a binary.** [ADR 0041](0041-the-host-depends-on-nothing.md) says
copying the binary onto a machine is the whole installation, and that remains true of the
*binary*. But a unit file, a state directory and an upgrade path are real, and something has to
own them. A package that installs one statically linked binary plus a unit file adds no runtime
dependency — 0041 is about what must already be present for the host to work, not about how the
bytes arrived.

**Why the tarball must keep working.** The package lives in a repository, and the mesh's own
repository is hosted on the mesh. A first node cannot fetch from a mesh that does not exist yet,
and neither can a node whose mesh is down — which is exactly when somebody is trying to fix it.
**Any path that requires the mesh to install the thing that joins the mesh is a circle**, so the
tarball is the path that is never allowed to acquire a dependency.

### The host may replace its own binary; it may not stop its own unit

The first draft of this record said the mesh must not upgrade the host at all. That was too
broad, and it conflated two different acts.

**Replacing the binary is safe.** Unix keeps the running executable's inode open, so a package
upgrade writes a new file and the running process continues on the old one, undisturbed.
**Stopping the unit is what is unsafe** — that is the host killing itself part-way through an
apply, leaving a machine with nothing running to finish or fix it.

So the host may apply a `package` naming itself. What it must never do is ask the service
manager to restart it.

**The restart happens by exiting, not by asking.** When the host notices its own executable has
been replaced, it finishes the apply it is in, reports what it did, and **exits cleanly**. The
supervisor's `Restart=always` starts it again, on the new binary. Nothing stops the host; the
host stops, having finished.

Three conditions, and they are the whole safety argument:

- **after** the apply completes and its outcomes are recorded — never mid-way;
- **only** when the executable actually changed, which Linux reports plainly: a replaced
  `/proc/self/exe` reads as the old path marked deleted;
- **exit zero**, so a restart is what a supervisor does next rather than a failure it backs off
  from.

This makes a fleet-wide host upgrade an ordinary declaration, which the first draft gave up.

### Changes are pushed. The timer is for drift, and only for drift

**The host does not poll for work.** A new declaration arrives as a message on the link, and the
host applies it then ([ADR 0001](0001-nodes-communicate-over-a-broker.md)). Polling for updates
over a connection that already exists would be strictly worse in both directions: slower to
land, and constant traffic to learn nothing.

Four triggers, and only one of them is a clock:

| Trigger | Kind | Why |
|---|---|---|
| **a declaration arrives** | **pushed** | the ordinary path — this is how changes land |
| **start** | event | the machine may have changed while nothing was running |
| **reconnect** | event | declarations may have been missed while disconnected |
| **a timer** | periodic | **drift**, and nothing else |

**The timer cannot be replaced by an event, and the reason is definitional.** Drift is change the
*mesh did not make* — somebody edited a managed file, a distribution upgrade replaced a config,
a container was stopped by hand. **Nothing will ever send a message about it**, because the thing
that did it is not part of the mesh. A local periodic check is the only way to see it at all.

Without it, `owned` reports what the host *applied* rather than what is *there*, which is
[ADR 0035](0035-a-picture-is-read-from-what-runs.md) violated by omission.

**Ten minutes**, configurable. The check is cheap: it asks the package database, the service
manager and the container runtime about resources the host already knows it owns.

### It reports upward on a heartbeat

Separate from reconciling, and easy to conflate with it: the node tells the mesh what it is —
its running version, what it holds, what it last applied — on link, after every apply, and
periodically while idle.

**The heartbeat is what makes silence mean something.** Without it, the mesh cannot distinguish
a node that is fine and has had nothing to do from one that stopped. With it, *last heard from*
is a fact per node, and [ADR 0059](0059-a-host-that-cannot-start-rolls-itself-back.md) is what
handles the case where the node cannot report at all.

## Consequences

- **Adoption becomes two concrete steps**, which is the point of writing this down:
  install the package, then hand it a token. Nothing else.
- **The host gains a mode it does not have**, and it is the larger half of stage 3. Today every
  entry point runs and exits.
- **Refusing to manage its own unit needs enforcing, not just stating.** A declaration naming
  the host's unit must be refused by name, and that refusal is a test.
- **A host that cannot reach the mesh keeps reconciling from its store**, which is
  [ADR 0036](0036-a-node-is-a-managed-machine.md) made operational rather than aspirational: a
  disconnected node is not merely tolerated, it is actively holding its machine in the last
  state it was told to hold.
- **The timer makes drift visible and also makes it loud.** A resource the host cannot apply
  will now fail every ten minutes rather than once. That is correct and it needs somewhere to go
  other than a log nobody reads — which is `observability`'s, and it does not exist yet.
- **A host upgrade is an ordinary declaration**, which is worth the care it needs: the exit
  path is the only place the host deliberately stops, and a bug there is a node that restarts in
  a loop or never comes back. It wants a test that the host does **not** exit when its binary is
  unchanged, as much as one that it does when it changed.
- **A version-skewed fleet is now normal and needs saying.** Nodes restart onto the new binary
  at whatever moment their apply finishes, so "the fleet is upgraded" is a range rather than an
  instant. What a node reports as its version must be the **running** one, not the installed
  one, or the mesh will believe an upgrade landed before it took effect.

## References

- [ADR 0041](0041-the-host-depends-on-nothing.md) — the property the package must not break.
- [ADR 0036](0036-a-node-is-a-managed-machine.md) — what a node is, which this makes operational.
- [ADR 0039](0039-the-link-is-the-security-boundary.md) — the link the process exists to hold.
- [ADR 0051](0051-the-enrolment-token-carries-the-mesh.md) — the second of the two adoption steps.