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

---
layer: to-be
status: designed
code: []
updated: 2026-08-27
decisions:
  - 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
  - 02-DECISIONS/0046-the-installer-fetches-what-it-pins.md
  - 02-DECISIONS/0047-the-bundle-may-carry-actions-the-link-may-not.md
  - 02-DECISIONS/0048-the-substrate-is-named.md
---

# The substrate

Tier 1. Defined the same way [the control plane](06-the-control-plane.md) 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](../../02-DECISIONS/0038-a-node-joins-by-linking-first.md)).

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](../../02-DECISIONS/0001-nodes-communicate-over-a-broker.md)) | 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** |
| anything else the mesh hosts | no | — | not substrate |

**The role and the product are both written**, here and everywhere
([ADR 0048](../../02-DECISIONS/0048-the-substrate-is-named.md)). 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](../../01-RESEARCH/006-mesh-from-scratch/00-overview.md) 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 0046](../../02-DECISIONS/0046-the-installer-fetches-what-it-pins.md)) |

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 0047](../../02-DECISIONS/0047-the-bundle-may-carry-actions-the-link-may-not.md)
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 0046](../../02-DECISIONS/0046-the-installer-fetches-what-it-pins.md)). 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](../../01-RESEARCH/011-the-module-graph/worked-provider.md):

```
0  Docker exists                        detected, or installed as a package
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 Docker must be running before anything else happens — and Docker 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](../../01-RESEARCH/012-the-minimum-viable-node/00-overview.md));
- a package, which needs the machine's own package manager and a network — both permitted by
  [ADR 0046](../../02-DECISIONS/0046-the-installer-fetches-what-it-pins.md).

So the host's bootstrap vocabulary is six 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](../../02-DECISIONS/0047-the-bundle-may-carry-actions-the-link-may-not.md)) — 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 0047](../../02-DECISIONS/0047-the-bundle-may-carry-actions-the-link-may-not.md). 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](05-the-node-host.md), 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.