---
status: active
initiated: 2026-08-25
touches:
  - 02-DECISIONS/0002-everything-is-a-module.md
  - 02-DECISIONS/0017-modules-outside-the-core-are-grouped-by-domain.md
  - 02-DECISIONS/0036-a-node-is-a-managed-machine.md
  - 03-DESIGN/00-as-is/02-modules-and-manifests.md
  - 03-DESIGN/00-as-is/10-module-catalogue.md
  - 04-ISSUES/007-an-installed-package-is-not-a-capability/00-report.md
---

# 011 — The module graph

## What is being investigated

Whether the catalogue's missing structure is a **graph** — modules declaring what they need,
what they offer, and what they exclude — and what that replaces.

**The design is in [`proposal.md`](proposal.md): one kind of edge.** A module provides names
and requires names, and that single relation absorbs requiring a module, requiring a resource,
and the interface-and-adapter idea. An abstract name is legitimate **only where providers are
genuinely substitutable** — `terminal` passes, `database` does not, because a consumer speaking
Postgres does not speak MongoDB. The adapter is what creates an interface; without one there is
a **tag**, which describes and does not bind.
A **node provides names too**, which makes capability checking stop being a separate mechanism
and makes the host's own capability report an input to resolution rather than something a
person reads.

[`analysis.md`](analysis.md) measured the current catalogue. Its value to the design is two
lessons rather than its machinery — a field that means *depends on* should say so, and
placement does not belong in a manifest — and the rest is recorded as as-is evidence.

**Measured, and the premise was wrong for the existing system: the graph is not missing there.**
[`analysis.md`](analysis.md) — 126 manifests, 103 edges, no cycles, nothing dangling, and a
resolver that topologically sorts them, already called by the tool loader, the installer and
the delivery coordinator. What the effort assumed would need building is a thing to call.

What survives is narrower: three declarations that do not exist (`excludes`, a required node
capability, an interface with adapters), and two defects worth fixing whatever else is
concluded — `provider:` is a dependency edge that is not read as one, which makes the closure
for a working mesh come out without a database; and the resolver continues past a cycle and
past a missing dependency, contrary to ADR 0008.

[ADR 0017](../../02-DECISIONS/0017-modules-outside-the-core-are-grouped-by-domain.md) proposes
grouping modules by domain. [Research 005](../005-domain-grouping/analysis.md) measured that
proposal and found its evidence holds in exactly one place — reachability — which
[ADR 0037](../../02-DECISIONS/0037-the-host-applies-it-does-not-decide.md) has since absorbed
into the host. The measured case for domain grouping has therefore been consumed by a decision
taken for unrelated reasons, and what remains is fifty modules that co-change with nothing.

That leaves the original complaint unanswered: the catalogue records **what was installed**
rather than **what anything is for**, and nothing in the system can see a relationship between
two modules. Grouping asserts relationships. A graph records them.

## Why now

A proposal to split modules into *provisioning services* and *applications* was worked through
and abandoned in favour of one concept with facets, for a reason worth keeping:

- The split cannot be filed consistently. A git forge is consumed as a service *and* operated
  through a web interface. An analytics service grants tracking identity *and* is a dashboard
  somebody reads. An identity provider grants authentication *and* has an admin console.
- The operator's correction is the sharper form: **what runs on the machine is a supervised
  container, not something a user started.** That is a fact about *how a thing runs*, not about
  what kind of thing it is — so it is a facet, not a taxonomy.

Filing decisions that follow from nothing are the disease research 005 measured. A second
taxonomy would reproduce it.

So [ADR 0002](../../02-DECISIONS/0002-everything-is-a-module.md) survives, and the question
becomes what a module must be able to **declare**.

## The shape being investigated

Five declarations, of which two exist today.

| Declaration | Today | Notes |
|---|---|---|
| **requires a resource** — a database, a bucket | yes | provisioning, [ADR 0005](../../02-DECISIONS/0005-capabilities-are-provisioned-on-declaration.md) |
| **provides a resource** | yes | as above |
| **requires another module** | **no** | the dependency edge — the graph's substance |
| **excludes another module** | **no** | installing A makes B unavailable |
| **requires a node capability** | **no** | a graphical session, a container runtime, an architecture |

And one structural idea on top: **an interface module carries no implementation.** Adapters
provide it. An assistant interface with several model-provider adapters; a terminal interface
with several terminal adapters. A dependent names the interface and never an implementation.

**Prior art to measure against, not invent past.** This is a package manager's model, and the
platform's own package manager already has all of it: `depends` is the dependency edge,
`conflicts` is exclusion, and `provides` is the interface — several packages provide one
virtual name, and a dependent names the virtual one. That the design arrived at the same shape
independently is evidence for it. It is also a warning: dependency resolution, version
constraints, conflict handling and rollback are a long-solved and easily-botched problem, and
the effort should establish what to **delegate** rather than reimplement.

## Capabilities, and what may be installed

The operator's formulation: *system specs are capabilities, and capabilities unlock installable
modules — you cannot install a graphical application on a node with no display server.*

The question that follows is whether the mesh may install a capability. The effort's working
position, to be tested:

- **Intrinsic capabilities** — hardware, architecture, network position — are facts about a
  machine. They are detected, never installed, and a module requiring one it does not have is
  not unresolved but **impossible** on that node.
- **Provided capabilities** — a display server, a container runtime — are not a separate kind
  of thing at all. They are modules that provide a capability, and requiring one is an ordinary
  edge the graph resolves by installing it.

If that holds, "may the mesh install a capability" is not a policy question. It is dependency
resolution, and the only genuinely new thing is detection.

Which is where [issue 007](../../04-ISSUES/007-an-installed-package-is-not-a-capability/00-report.md)
bears directly: *an installed package is not a capability*. A provided capability is not real
because a package is present — it is real when it is present, running and working, and the
difference is exactly the class of fault this repository keeps recording.

## What it touches beyond the catalogue

The operator's assessment is that the machinery around a module is wanted and its integration
is not: **scheduled tasks, hooks, migrations, configuration and environment settings are worth
keeping; seeds are not; and the current integration is wrong enough to need a major refactor.**

That is a claim to test rather than adopt. Research 005 already found supporting evidence from
a different direction — that the densest apparent coupling in the catalogue is manifest
boilerplate churn, cross-cutting changes to the machinery applied N times — which is what an
integration being wrong looks like from the outside.

## Open questions

| Question | Why it is open |
|---|---|
| ~~What does the graph **delete**?~~ | **Answered for the design** in [`proposal.md`](proposal.md): the distinction between requiring a module and requiring a resource, the interface as a kind of thing, capability checking as a separate mechanism, domain grouping, and possibly tiers as a separate concept. *(For the existing system the answer was nothing, because it is already there — [`analysis.md`](analysis.md).)* |
| When two modules provide one name, who chooses? | The substance of the interface idea, and the proposal does not settle it: a node setting, a mesh setting, or an explicit pin. |
| What does a provider hand back? | A consumer requiring `postgres` needs credentials and an address; one requiring `terminal` needs a command. Same relation, different shape flowing across it. |
| ~~Where do domain modules fit?~~ | **Answered: they do not.** A `networking` module gathering a firewall, resolver and proxy under one name came from an older shape. There is no such thing to install — there is core infrastructure, a set of concrete modules named individually, not flavourable, with no grouping module in front of them. |
| Would the missing declarations be used? | Zero manifests declare exclusions or capabilities, which is equally consistent with *nobody needs them* and *nobody can express them*. Nothing measured separates those. |
| Should `provider:` become a real edge, or should the relationship be declared twice? | It names a module and means *depends on*. Reading it as an edge fixes the closure; the alternative is requiring the consumer to also list it under `dependencies:`, which is duplication a resolver already avoids elsewhere. |
| Should placement leave the catalogue? | A provision pins itself to a named node, in the manifest. Which node runs what is an inventory decision — tier 2 by the skeleton's own test — and having it in tier 4 means a second node cannot provide the mesh's database without editing the module that consumes it. |
| Where does resolution happen — mesh or platform package manager? | The mesh must model mesh-level edges. Whether it also resolves operating-system packages, or delegates, decides whether a solver has to be written. |
| Is an interface a module, or a name? | Arch makes it a name that packages claim. Making it a module gives it a manifest, an owner and a place to document the contract — and a thing with no implementation to install. |
| What does an exclusion mean for something already installed? | Refuse the install, or make the conflict visible and let it be decided. The second is a policy surface; the first is a package manager. |
| Does node adoption scan for capabilities, applications, or both? | The operator proposes scanning an adopted node and enabling what it finds. Under the working position above, the scan is for capabilities — but a machine with a terminal already installed is also a module already satisfied, and whether that is adoption or drift is undecided. |
| One installation image, or several? | Proposed: pre-built images carrying different capability sets, so a machine is adopted quickly. Several images bake capability sets at image time, which is the filing problem in a new form and reintroduces what detection exists to avoid. One image carrying the host and nothing else is [ADR 0038](../../02-DECISIONS/0038-a-node-joins-by-linking-first.md)'s *one binary installed by hand*, automated. The effort should settle which. |
| What happens to domain grouping? | [ADR 0017](../../02-DECISIONS/0017-modules-outside-the-core-are-grouped-by-domain.md) is still `proposed`. If the graph is the answer, 0017 is superseded rather than narrowed — its text is never edited. |