hq/03-DESIGN/01-to-be/07-the-substrate.md

layer, status, code, updated, decisions

layer	status	code	updated	decisions
to-be	designed		2026-08-26	02-DECISIONS/0030-the-repository-structure.md 02-DECISIONS/0037-the-host-applies-it-does-not-decide.md 02-DECISIONS/0038-a-node-joins-by-linking-first.md

The substrate

Tier 1. Defined the same way the control plane is, because the same gap applied: the word was load-bearing and unpinned.

The definition

The substrate is what the control plane consumes and cannot grant itself.

Every module that needs a database asks the control plane's provisioning for one. The control plane needs a database too — and it cannot ask itself, because it is not running yet. That circularity is not an awkwardness to work around; it is the definition. Anything on the wrong side of it must be raised some other way, and the other way is the bundle the host carries (ADR 0038).

The test, applied:

	control plane needs it	can it grant itself one?
a relational store	its own state lives there	no — provisioning needs the store	substrate
a message bus	it reaches nodes over it (ADR 0001)	no — it cannot grant itself a virtual host	substrate
an object store	artifacts and blobs it delivers	no — it needs a bucket to hold them	substrate
an image registry	images it delivers to nodes	no — it needs a repository	substrate
an identity provider	only if it delegates authentication	—	conditional, below
anything else the mesh hosts	no	—	not substrate

What that resolves

Four or five? Research 006 asks whether the identity provider is a substrate service, and the test answers it conditionally — which is the honest answer rather than a number.

• If the control plane delegates authentication, it cannot serve anybody before the provider exists, and it cannot grant itself a client. Substrate.
• If it authenticates natively, the provider is an ordinary hosted service like any other. Not substrate.

So the count follows from a design decision that has not been taken, and the record should say that rather than assert four.

Why not "important infrastructure". An identity provider, a mail server and an analytics service are all infrastructure by any ordinary reading, and none of them are substrate — the control plane starts and runs without them. Important is not the test; the control plane cannot obtain it is.

What the substrate is not

• Not tier 0. The host raises the substrate; it is not part of it. The host carries the declaration that brings the substrate up, and depends on nothing.
• Not the control plane. These are services with no knowledge of the mesh. A store does not know what a node is.
• Not a place for logic. The skeleton is explicit: tier 1 is declarations only, no logic of its own. A substrate service is an upstream image, pinned, with configuration.
• Not privileged. The substrate is provisioned from by the control plane and grants nothing on its own initiative.

The pinned bundle

The substrate is what substrate.lock contains, and this is the only place in the mesh where versions are pinned by hand rather than resolved.

Why pinned: the bundle is applied when no mesh exists, so nothing can resolve a version, ask a registry, or check a constraint. What the host carries must already be exact.

Why references and not payload: the bundle names images by digest and the host fetches them (ADR 0046). A first node is a real machine with a network; the sealed case is the lab, and the lab places images itself.

Reproducibility comes from pinning the identity of a thing rather than carrying its bytes, which is what keeps the bundle small enough for a person to read and check.

Raising it

The order, from research 011:

0  a container runtime exists           detected, or installed as a package
1  the store runs                       pulled by digest, from the bundle
2  a database is created in it          an action, run locally
3  the control plane's schema applied   an action, against that database
4  the control plane starts             and only now is there a mesh
5  everything else is provisioned       the ordinary path

Step 0 is easy to leave out and it is where several things meet. A substrate service is a container, so something must run containers before anything else happens — and a container runtime is a package, not a container. It is:

• what the host's capability detection already reports, and the first use of that report by something other than a person;
• adopted rather than installed when the machine already has one with configuration somebody chose (research 012);
• a package, which needs the machine's own package manager and a network — both permitted by ADR 0046.

So the host's bootstrap vocabulary is five shapes: package, container, file, directory, service, and action. Files, directories and services exist; the rest do not yet.

Steps 2 and 3 happen before there is a mesh to do them, which is why provisioning is part of the bootstrap rather than a service consumers use later. They are actions the bundle declares and the host runs (ADR 0047) — so the host's vocabulary grows by one shape rather than by one resource type per substrate service.

Open

• Whether identity is the fifth. Above; it follows from a decision not yet taken.
• Whether the host can do step 2. Resolved by ADR 0047. A service running on this machine is part of this machine, so the scope was never in question — the real question was whether the host must learn what a database is, and it must not. The bundle declares an action; the host runs it and verifies it, and what a database means stays with the module that provides one.
• Whether one host can raise all four. The claim under stage 2 of the node host, never proved. If it is false, the tier boundary moves.
• How the substrate is updated once a mesh exists. Pinned by hand at bootstrap; afterwards the control plane could deliver it like anything else, and nothing says whether it does.