---
topic: what runs on it
status: accepted
date: 2026-08-31
deciders: jochen
reconstructed: false
extends: 02-DECISIONS/0009-modules-and-the-graph.md
---

# 27. A provision names what the consumer is coupled to, not the role it plays

## Context

Provisions are named after roles. The catalogue and every test fixture built so far say:

```
provides: database
requires: database
```

**Nothing distinguishes one engine from another.** A module requiring `database` is satisfied by
any module providing `database`, so a module written against PostgreSQL can be matched to a
provider of Microsoft SQL Server, resolve as satisfied, deploy, and fail on its first query.

**The mesh runs several engines** — PostgreSQL, Microsoft SQL Server, MariaDB, and others behind
products that expose their own. This is not a hypothetical collision.

**The failure is in the direction that hides.** Resolution *succeeds*. Nothing is refused, nothing
is logged, and the breakage surfaces later as an error inside an application, on a machine, with
nothing connecting it back to a match made elsewhere by something that thought it had done its
job. **A wrong answer delivered confidently costs more than a refusal**, and the whole point of
refusing on ambiguity ([ADR 0009](0009-modules-and-the-graph.md)) was to not do this.

**How it got in:** every test written for the resolver had exactly one provider of each name, so no
mismatch was expressible and none was caught. The fixtures agreed with the design. That is the same
fault as [`04-ISSUES/005`](../04-ISSUES/005-pipeline-test-harness-unbuildable/00-report.md)'s
imagined output and [`019`](../04-ISSUES/019-a-comment-asserting-a-fact-about-a-machine/00-report.md)'s
unchecked comment, at the level of a name rather than a line.

## Considered Options

1. **Keep role names; let the operator assign correctly.** The mesh would refuse ambiguity when two
   providers exist, so a person picks. **Rejected.** It makes correctness depend on somebody
   knowing that the module they are assigning speaks a particular dialect — which is exactly the
   knowledge the provisioning model exists to remove. And with one provider of each name, nothing
   is ambiguous and nothing is asked.

2. **A role name plus a `flavour:` or `engine:` qualifier**, matched as a second field.
   **Rejected.** Two fields that must agree is a constraint the resolver has to enforce and a
   manifest author has to remember, to express something one field already can. The name is the
   contract; splitting it invites a requirement that names a role and forgets the qualifier, which
   then matches everything again.

3. **The name says what the consumer is coupled to.** **Adopted.**

## Decision

**A provision is named for the thing a consumer's code is written against.**

```
provides: postgres-database
requires: postgres-database
```

**The test is whether the consumer can tell the difference.** If swapping the provider would break
the consumer, the name must say which provider — because a match that breaks the consumer is not a
match. If the consumer genuinely cannot tell, a role name is correct and better.

| provision | | why |
|---|---|---|
| `postgres-database`, `mssql-database` | **specific** | applications are written against a dialect; a swap breaks them |
| `route` | **role** | the consumer wants its name reachable and does not care what proxies it |
| `resolver` | **role** | the consumer wants names to resolve |
| `artifact-store` | **role** | the consumer fetches by digest over a protocol, and nothing else |

**`database` is not a provision and may not be provided.** There is no context in which an
application talks to a generic database: it talks to PostgreSQL or it talks to SQL Server. A name
that cannot be true of any real consumer should not be expressible.

**This is about coupling, not about products.** Two providers of `postgres-database` — a container
on this node and a managed instance elsewhere — are interchangeable and *should* both match. What
may not be interchangeable is what the consumer's queries are written in.

## Consequences

**Every manifest that names a database changes.** Doing this now costs a rename across a handful of
examples. Doing it after modules are migrated costs it across all of them, plus every deployment
that resolved against the old name.

**Wrong requirements now fail loudly, and at the right moment.** A module requiring
`postgres-database` where only `mssql-database` is provided is unsatisfiable, so it is **refused at
resolution** with both names visible — rather than deployed and broken later. This is the property
that was lost, restored.

**Generic role names are still right, and the rule says when.** This does not push specificity
everywhere; it puts it exactly where a consumer is coupled. Naming `route` after a particular proxy
would be the same error in the other direction, and would prevent a swap that genuinely changes
nothing.

**It is checked, not merely stated** ([`00-META/how-we-build.md`](../00-META/how-we-build.md) §5).
A manifest providing a name known to be engine-generic is refused, naming what to say instead.
Without that, this record is a convention, and a convention is what the previous naming was.

## References

- [ADR 0009](0009-modules-and-the-graph.md) — provisions, and refusing on ambiguity
- [`03-DESIGN/01-to-be/07-the-foundation.md`](../03-DESIGN/01-to-be/07-the-foundation.md) — *the
  provisioning model uses databases, roles and schemas as PostgreSQL means them*, which is this
  record's point made about the substrate before it was made about modules