jschoubben
bea052753e
Stage 2 could not start without it. Three constraints already bound the shape and between them they decide most of it. JSON, because the standard library carries it and carries no YAML, and a YAML declaration would put a third-party parser inside the one binary whose whole argument is that it needs nothing — to gain authoring comfort in a document generated by a machine and read by a machine. An ordered list, because ordering is a DECISION. A host deriving order from declared dependencies would be deciding the thing most likely to differ between what the control plane intended and what the machine does. The control plane knows what depends on what; it says so by saying when. 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 a declaration and report success, which is 04-ISSUES/003 with the declaration on the other side of the wire. Complete for what the host OWNS, and only that. It removes what it previously applied and is no longer declared, which it knows from the store rather than by inference, and never removes what it did not create — a converger that treats 'not declared' as 'must not exist' deletes what the mesh never put there. Two consequences arriving earlier than the build order suggested: the store is load-bearing at stage 2, because nothing can be removed without knowing what was applied. And a closed address space bounds the first vocabulary to what needs no network, because a scenario has no route to a package repository.
213 lines
10 KiB
Markdown
213 lines
10 KiB
Markdown
---
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layer: to-be
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status: in-progress
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code: [mesh-host]
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updated: 2026-08-26
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decisions:
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- 02-DECISIONS/0030-the-repository-structure.md
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- 02-DECISIONS/0036-a-node-is-a-managed-machine.md
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- 02-DECISIONS/0037-the-host-applies-it-does-not-decide.md
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- 02-DECISIONS/0038-a-node-joins-by-linking-first.md
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- 02-DECISIONS/0039-the-link-is-the-security-boundary.md
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- 02-DECISIONS/0008-a-failed-step-fails-the-job.md
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- 02-DECISIONS/0041-the-host-depends-on-nothing.md
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- 02-DECISIONS/0043-a-declaration-is-an-ordered-list-of-owned-resources.md
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---
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# The node host
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Tier 0. The one thing ever installed by hand, and the only thing that changes a machine.
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## What it is
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A **statically linked binary that requires nothing to be present** — copy it onto a machine and
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run it, and that is the whole installation
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([ADR 0041](../../02-DECISIONS/0041-the-host-depends-on-nothing.md)). Written in Go, because the
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job is system-level and because the host shares no code with any other tier.
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A single binary with one job: **apply declared state on this machine**
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([ADR 0037](../../02-DECISIONS/0037-the-host-applies-it-does-not-decide.md)). Overlay
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membership, packet filtering, packages, services, containers and filesystems are not six
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concerns it carries; they are six instances of the one.
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It replaces three things that exist today
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([`00-as-is/05`](../00-as-is/05-runtime-and-installation.md)): the three hand-run bootstrap
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scripts — first node, joining, rescue — and the synchronisers that rewrite managed files
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([`00-as-is/06`](../00-as-is/06-configuration-and-secrets.md)).
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**What it is not:** it does not decide anything that needs another node, it never queries the
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mesh database, and it has no listening surface.
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## The parts
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| Part | Owns |
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| `apply` | reconciling declared state on this machine |
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| `store` | local state, authoritative while disconnected |
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| `link` | the single outbound connection to the control plane |
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| `profile` | what this machine can be asked to do |
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| `inventory` | what this machine is and has |
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| `substrate.lock` | the pinned tier-1 descriptor, appliable with no mesh present |
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### apply
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Takes a **declaration** and makes the machine match it. Idempotent: applying the same
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declaration twice changes nothing the second time, and applying it to a drifted machine
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returns it.
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Three properties, each following a recorded decision:
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**A failed step fails the apply.** Not "logs and continues"
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([ADR 0008](../../02-DECISIONS/0008-a-failed-step-fails-the-job.md)). A partial apply that
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reports success is the mesh's most expensive shape.
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**Each applier reads back.** Setting a value is not evidence the value took. The firewall is
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asked whether the rule loaded; conntrack is asked what timeout it holds. This is `how-we-build`
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§5 as a component requirement rather than a review habit.
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**What was applied is recorded after it works, never before**
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([ADR 0035](../../02-DECISIONS/0035-a-picture-is-read-from-what-runs.md)). A failed apply leaves
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the machine in whatever state it reached, and nothing must claim otherwise.
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### store
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Local, and **authoritative while disconnected**. Not a cache of the control plane — the record
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of what this node has applied and what it currently holds.
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This is structural rather than convenient: if disconnection is an ordinary situation rather
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than an exception ([ADR 0036](../../02-DECISIONS/0036-a-node-is-a-managed-machine.md)), the
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store is what makes it ordinary. A laptop shut for a week comes back and reconciles; it does
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not come back and ask what it is.
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### link
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The node's one connection to the control plane, and its security boundary
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([ADR 0039](../../02-DECISIONS/0039-the-link-is-the-security-boundary.md)).
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It is the broker connection that already exists
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([ADR 0001](../../02-DECISIONS/0001-nodes-communicate-over-a-broker.md)) — outbound,
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node-initiated, per-node addressed — carrying **per-node identity instead of a shared
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credential**. The node owns no password. It owns an identity, and that identity is what it
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presents.
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What arrives is bounded by form: **declarations of known shape, never a command to run.**
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**It must fail legibly.** A node that cannot link says which side refused and on what grounds.
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A boundary that refuses without saying why is worse than the password it replaced.
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### profile
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What this machine can be asked to do — a graphical session, a container runtime, an
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architecture, a network position.
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**Detected, never assumed.** A package being installed does not mean a capability is present
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([04-ISSUES/007](../../04-ISSUES/007-an-installed-package-is-not-a-capability/00-report.md)): a
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capability is real when it is present, running and working, and the difference is the whole
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point of detecting it.
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The profile is what makes [ADR 0036](../../02-DECISIONS/0036-a-node-is-a-managed-machine.md)
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work: a node is a node, and what varies between them is here rather than in the definition.
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### inventory
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What this machine *is* — its identity, what it holds, what it has applied. Reported upward over
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the link; never asked downward.
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## Where a declaration comes from
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One behaviour, two sources
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([ADR 0038](../../02-DECISIONS/0038-a-node-joins-by-linking-first.md)):
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| Situation | Source |
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| no mesh reachable | `substrate.lock` — the pinned bundle the host carries |
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| mesh reachable | the control plane, over the link |
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**The first node is not a different kind of node.** It is a node whose mesh is not up yet. It
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applies the bundle it carries, the control plane comes up on top of it, and from that moment it
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takes declarations like every other node. Its specialness is temporary and self-erasing.
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**A joining node does the minimum to be reachable and nothing else** — an identity, an address,
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one peer — and then stops deciding. It does not compute the overlay; it needs one peer to reach
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the mesh, and the full peer set arrives derived.
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## What a declaration is
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Settled by [ADR 0043](../../02-DECISIONS/0043-a-declaration-is-an-ordered-list-of-owned-resources.md).
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**JSON**, because the host has no dependencies to spend and the standard library carries no
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YAML. **An ordered list of typed resources**, each with a stable identity — the order is stated
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rather than derived, because deriving it would be the host deciding the thing most likely to
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differ from what the control plane intended.
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**Unknown is refused, never skipped.** An unknown version, type or field refuses the whole
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declaration. A host that skipped what it did not understand would apply most of it and report
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success.
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**Complete for what the host owns, and only that.** It removes what it previously applied and
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is no longer declared — a fact it holds, from the store, rather than an inference — and never
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removes anything it did not create.
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**Addressed.** A host with an identity refuses a declaration addressed elsewhere; a host
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without one, applying the bundle it carries, has nothing to check against.
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## Build order
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Staged so each stage is verifiable in the lab before the next exists.
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**1 — profile and inventory.** The host runs on a machine, detects what it can do, and reports
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what it is. No control plane, no declarations, no network. Verifiable immediately: the lab's
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`place:` gains its first implementation, and a raised scenario finally contains something.
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**2 — apply, from the bundle.** The host applies `substrate.lock` with no mesh present. This is
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the first node's path, and it is the claim the skeleton's Move 1 rests on and has never proved:
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that one host can raise the substrate alone.
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The first vocabulary is bounded by something the lab makes unavoidable: **a scenario is a
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closed address space**, so a resource that must be fetched cannot be applied there at all. So
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stage 2 begins with what needs no network — files, directories, service state — and the types
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that need artifacts wait on where those come from, which is open in
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[`02-scenario-declaration.md`](02-scenario-declaration.md).
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**3 — link and store.** The node connects, receives declarations, and holds what it applied.
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**4 — enrolment.** The one genuinely new mechanism in
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[ADR 0039](../../02-DECISIONS/0039-the-link-is-the-security-boundary.md); everything else there
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is configuration of what already runs.
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Stage 1 is deliberately the smallest useful thing. The lab currently raises **empty machines**
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([`00-as-is/11`](../00-as-is/11-the-lab.md)) because `place:` has nothing to place; stage 1 ends
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that, and every later stage is tested by a lab that already works.
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## How it is verified
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**The lab is the harness.** A scenario places a host on a machine and asserts what it did —
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against a real hypervisor, with the boundary never mocked
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([ADR 0034](../../02-DECISIONS/0034-a-test-defends-a-decision.md)).
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Each decision above owes a test:
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| Decision | What asserts it |
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| 0037 — the host never queries the mesh database | no database client in the dependency tree; a dependency-direction lint failing on an upward import |
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| 0038 — one behaviour, two sources | the same code path raises a first node and joins a second |
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| 0039 — a node holds no shared credential | a raised node's store contains no credential to any service |
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| 0036 — disconnection is a situation | a node cut off and returned reconciles without being re-adopted |
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| 0008 — a failed step fails the apply | an apply with a failing step reports failure |
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## Open
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- **Whether one host can raise the substrate alone.** Move 1 assumes it. Stage 2 tests it, and
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if it is false the tier boundary moves.
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- **What the host carries versus what it finds.** It manages `wg`, `nft`, `pacman`, `docker`;
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it does not contain them, and how it obtains one it lacks is undecided —
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[04-ISSUES/007](../../04-ISSUES/007-an-installed-package-is-not-a-capability/00-report.md).
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- **Six vocabularies.** Zero dependencies, but the host must still know what a peer, a rule, a
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package, a unit, a container and a dataset *are*. Nothing has measured that surface, and it is
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the residue of the question [`host-size.md`](../../01-RESEARCH/006-mesh-from-scratch/host-size.md)
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answered.
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- **Rescue.** [ADR 0038](../../02-DECISIONS/0038-a-node-joins-by-linking-first.md) suggests it is
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a node whose local state is discarded so the mesh re-derives it, and does not decide it.
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- **What may expire.** An identity needing refresh to stay valid would make a laptop fail for
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being a laptop ([ADR 0036](../../02-DECISIONS/0036-a-node-is-a-managed-machine.md)).
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