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

layer, status, code, updated, decisions

layer	status	code	updated	decisions
to-be	designed		2026-08-27	02-DECISIONS/0019-how-this-repository-works.md 02-DECISIONS/0037-the-node-host.md 02-DECISIONS/0036-a-node-and-how-it-joins.md 02-DECISIONS/0048-the-substrate-and-the-control-plane.md 02-DECISIONS/0037-the-node-host.md 02-DECISIONS/0048-the-substrate-and-the-control-plane.md 02-DECISIONS/0049-connectivity.md 02-DECISIONS/0037-the-node-host.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 0036).

The test, applied:

	control plane needs it	can it grant itself one?
a relational store — PostgreSQL	its own state lives there	no — provisioning needs the store	substrate
a message bus — LavinMQ	it reaches nodes over it (ADR 0001)	no — it cannot grant itself a virtual host	substrate
an object store — MinIO	artifacts and blobs it delivers	no — it needs a bucket to hold them	substrate
an image registry — the OCI registry	images it delivers to nodes	no — it needs a repository	substrate
an identity provider	only if it delegates authentication	—	conditional, below
ingress — Traefik	not to start; only to be reached by name	— it grants itself one afterwards	not substrate (ADR 0049)
anything else the mesh hosts	no	—	not substrate

The role and the product are both written, here and everywhere (ADR 0048). The role is what the argument turns on — the test above works on roles, and would give the same answers for a different store. The product is what actually gets installed and pinned, and a design that names only the role does not record that the choice was ever made.

The dependency is on the protocol, not the product: AMQP for the bus, S3 for the object store, the OCI protocol for the registry. That is what keeps the naming safe rather than a commitment that cannot be revisited — replacing one is a substrate migration, not a redesign. The store is the exception, and the exception matters: the provisioning model uses databases, roles and schemas as PostgreSQL means them, so it is the one member that is not a swap.

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

substrate.lock holds what must exist before the control plane runs — which is a smaller set than the substrate, and the difference is easy to miss. It is the only place in the mesh where versions are pinned by hand rather than resolved.

Being substrate and being in the bundle are two different questions:

	is it substrate?	must it precede the control plane?
PostgreSQL	yes — the control plane's own state lives in it	yes — there is nowhere to put that state otherwise
LavinMQ	yes — it cannot grant itself a virtual host	not established — see below
MinIO	yes — it cannot grant itself a bucket	no — nothing is delivered before the mesh exists
the OCI registry	yes — it cannot grant itself a repository	no — the first node fetches upstream (ADR 0048)

The three on the bottom rows are substrate by role and ordinary by delivery: by the time they are wanted there is a control plane, and it provisions them the way it provisions anything. That keeps the bundle to roughly one image rather than four, which is what makes it small enough for the review ADR 0037 requires.

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 0048). 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 — docker or podman — or installed
1  PostgreSQL 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  LavinMQ, MinIO, the registry, and    the ordinary path
   everything else are provisioned

Only PostgreSQL is raised from the bundle, for the reason in The pinned bundle above — the rest of the substrate is wanted only once there is a control plane to provision it.

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

Which runtime is detected, not chosen (ADR 0037): a machine that already has one keeps it. On a machine with none, the control plane names the package, because what it is called differs per system. 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 0048.

So the host's bootstrap vocabulary is six shapes: package, container, file, directory, service, and action. All six are built (05-the-node-host.md stage 2), so nothing in this bootstrap is blocked on the host any longer.

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 0037) — 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 bus must precede the control plane. The bundle table marks this not established, and it is the one row that could still move. The control plane reaches nodes over AMQP, but at step 4 there is exactly one node and it is the local machine — so whether LavinMQ is needed to start or only to reach a second node depends on whether the control plane's own contexts talk to each other over the bus. If they do, LavinMQ joins PostgreSQL in the bundle and the bootstrap grows a step; if they do not, it is provisioned like anything else. This is a question about the control plane's internal shape, not about the substrate, which is why it is not answered here.
• Whether the host can do step 2. Resolved by ADR 0037. 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.