ADR 0046 — the installer fetches what it pins

The blocking question was where a container image comes from, and the version
that blocked assumed the machine might have no network. That assumption came
from the LAB: a scenario is a closed address space by design, which is what lets
two scenarios hold the same addresses without meeting. Production is not sealed
— a machine being adopted has a network, and one that does not is a machine
where very little works anyway.

So substrate.lock carries references, not payload: an image name and a digest,
fetched at apply time. A first node pulls from upstream because no mesh registry
exists yet; every node after that pulls from the mesh's own. The lab is the
exception and places images itself, the way it already places the host binary —
a property of a test environment, and letting it dictate the production design
would be the tail wagging the dog.

Pinned by DIGEST rather than tag. Reproducibility comes from pinning the
identity of a thing, not from carrying its bytes, which is what makes fetching
acceptable rather than a compromise.

ADR 0041 survives untouched, which was the point. "Copy it onto a machine and
run it" stays literally true — one binary, a few megabytes, which then fetches
what it was told to. Carrying images would have quietly redefined the property
that decision rests on.

Costs accepted and named: an apply can now fail because something is
unreachable, which a self-contained artifact could not, so it must fail legibly
— naming what it could not fetch and from where. And the lab needs a way to
place images into a machine that also has no container runtime, both of which
are lab-installation concerns and neither solved here.

Research 012's build-time-versus-apply-time reframing narrows accordingly: it
still holds for what a tailored installer contains, and no longer has to hold
for images.

02-DECISIONS/0046-the-installer-fetches-what-it-pins.md

@@ -0,0 +1,77 @@
+---
+status: accepted
+date: 2026-08-26
+deciders: jochen
+reconstructed: false
+extends: 0038-a-node-joins-by-linking-first.md
+---
+
+# 46. The installer fetches what it pins
+
+## Context
+
+Stage 2 of [the node host](../03-DESIGN/01-to-be/05-the-node-host.md) needs to raise the
+substrate, and a substrate service is a container, and a container needs an image. Where the
+image comes from had been blocking implementation.
+
+The blocking version of the question assumed the machine might have no network, which produced a
+bad trilemma: carry every image inside the artifact, fetch at apply time, or have something else
+place them first. Carrying them makes a three-megabyte binary into a multi-hundred-megabyte one
+and strains [ADR 0041](0041-the-host-depends-on-nothing.md).
+
+**The assumption was wrong, and it came from the lab.** A scenario is a closed address space by
+design — that is what lets two scenarios hold the same addresses without meeting. Production is
+not: a machine being adopted has a network, and one that does not is a machine where very little
+works anyway.
+
+## Decision
+
+**The installer fetches what the bundle pins.**
+
+`substrate.lock` carries **references, not payload** — an image name and a digest. At apply time
+the host fetches them.
+
+| Situation | Fetched from |
+|---|---|
+| a first node, no mesh yet | upstream, wherever the image ordinarily lives |
+| every node after that | the mesh's own registry |
+| **the lab** | **nowhere — the lab places them first** |
+
+**Pinned by digest, not by tag.** A tag moves; a digest does not. Reproducibility comes from
+pinning the identity of the thing, not from carrying its bytes — which is what makes fetching
+acceptable rather than a compromise.
+
+**The lab is the exception, and it is the lab's problem.** A sealed scenario cannot reach a
+registry, so the lab places images into a machine the same way it already places the host
+binary. That is a property of a test environment, and letting it dictate the production design
+would be the tail wagging the dog.
+
+## Consequences
+
+- **[ADR 0041](0041-the-host-depends-on-nothing.md) survives untouched.** *Copy it onto a
+  machine and run it* remains literally true: one binary, a few megabytes, which then fetches
+  what it was told to fetch. The alternative would have quietly redefined the property that
+  decision rests on.
+- **The bundle stays small and reviewable.** A list of pinned references is something a person
+  can read and check. A bundle containing images is not.
+- **An apply can fail because something is unreachable**, which a self-contained artifact could
+  not. That is the cost, it is accepted, and it must fail *legibly* — naming what it could not
+  fetch and from where, not "install failed".
+- **The lab needs a way to place images**, and the machine it places them into needs a container
+  runtime, which a sealed scenario cannot install either. Both are lab-installation concerns and
+  neither is solved here.
+- **The build-time-versus-apply-time reframing in
+  [research 012](../01-RESEARCH/012-the-minimum-viable-node/00-overview.md) narrows.** It still
+  holds for what a *tailored installer* contains — the missing pieces for a given machine — but
+  it does not have to hold for images, because fetching them is available and cheap. Recorded
+  because the reframing was general and is now qualified.
+- **Nothing here says what happens when a fetch is impossible on a real node.** An air-gapped
+  machine is not a case the mesh has, and if one appears this decision is what it revisits.
+
+## References
+
+- [ADR 0041](0041-the-host-depends-on-nothing.md) — the property this preserves.
+- [ADR 0038](0038-a-node-joins-by-linking-first.md) — the bundle this fills in.
+- [`07-the-substrate.md`](../03-DESIGN/01-to-be/07-the-substrate.md) — what the bundle pins.
+- [Research 012](../01-RESEARCH/012-the-minimum-viable-node/00-overview.md) — the reframing this
+  qualifies.