jschoubben
cbcbba8099
**0027 — provisions.** A module written against PostgreSQL could be matched to a provider of SQL Server, resolve as satisfied, and fail on its first query. The name said the role, so nothing distinguished engines. Refusing on ambiguity could not help: with one provider of each name nothing is ambiguous. Enforced at parse rather than documented, because the old naming was the documentation. **0028 — the substrate.** 0006 admits an object store on the grounds that it cannot grant itself a bucket. That answers the second half of the test and assumes the first: the control plane does not need one. Verified — no S3 client in mesh-control, and internal/builder/registry.go records the deliberate choice to put artifacts in the OCI registry as content-addressed blobs. The row was inherited from the system being replaced, where an object store distributed module tarballs, and was never re-tested against the definition above it. So an object store is an ordinary module, and a mesh with nothing needing one runs none. Migrating it is module work, not substrate work. 0028 also states what 0006 left unsaid: a substrate service and a module of the same product are different instances. The substrate is raised from the bundle before any mesh exists, so it is not in the module graph — a workload depending on it would depend on something the graph cannot see, cannot rotate a credential for, and cannot move, and would put workload data in the store the control plane keeps its own state in. Both records were found by reading code against design rather than design against itself, which is the review that should have happened sooner.
110 lines
5.5 KiB
Markdown
110 lines
5.5 KiB
Markdown
---
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topic: what runs on it
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status: accepted
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date: 2026-08-31
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deciders: jochen
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reconstructed: false
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extends: 02-DECISIONS/0009-modules-and-the-graph.md
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---
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# 27. A provision names what the consumer is coupled to, not the role it plays
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## Context
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Provisions are named after roles. The catalogue and every test fixture built so far say:
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```
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provides: database
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requires: database
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```
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**Nothing distinguishes one engine from another.** A module requiring `database` is satisfied by
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any module providing `database`, so a module written against PostgreSQL can be matched to a
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provider of Microsoft SQL Server, resolve as satisfied, deploy, and fail on its first query.
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**The mesh runs several engines** — PostgreSQL, Microsoft SQL Server, MariaDB, and others behind
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products that expose their own. This is not a hypothetical collision.
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**The failure is in the direction that hides.** Resolution *succeeds*. Nothing is refused, nothing
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is logged, and the breakage surfaces later as an error inside an application, on a machine, with
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nothing connecting it back to a match made elsewhere by something that thought it had done its
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job. **A wrong answer delivered confidently costs more than a refusal**, and the whole point of
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refusing on ambiguity ([ADR 0009](0009-modules-and-the-graph.md)) was to not do this.
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**How it got in:** every test written for the resolver had exactly one provider of each name, so no
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mismatch was expressible and none was caught. The fixtures agreed with the design. That is the same
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fault as [`04-ISSUES/005`](../04-ISSUES/005-pipeline-test-harness-unbuildable/00-report.md)'s
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imagined output and [`019`](../04-ISSUES/019-a-comment-asserting-a-fact-about-a-machine/00-report.md)'s
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unchecked comment, at the level of a name rather than a line.
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## Considered Options
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1. **Keep role names; let the operator assign correctly.** The mesh would refuse ambiguity when two
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providers exist, so a person picks. **Rejected.** It makes correctness depend on somebody
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knowing that the module they are assigning speaks a particular dialect — which is exactly the
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knowledge the provisioning model exists to remove. And with one provider of each name, nothing
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is ambiguous and nothing is asked.
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2. **A role name plus a `flavour:` or `engine:` qualifier**, matched as a second field.
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**Rejected.** Two fields that must agree is a constraint the resolver has to enforce and a
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manifest author has to remember, to express something one field already can. The name is the
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contract; splitting it invites a requirement that names a role and forgets the qualifier, which
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then matches everything again.
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3. **The name says what the consumer is coupled to.** **Adopted.**
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## Decision
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**A provision is named for the thing a consumer's code is written against.**
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```
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provides: postgres-database
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requires: postgres-database
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```
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**The test is whether the consumer can tell the difference.** If swapping the provider would break
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the consumer, the name must say which provider — because a match that breaks the consumer is not a
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match. If the consumer genuinely cannot tell, a role name is correct and better.
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| provision | | why |
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|---|---|---|
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| `postgres-database`, `mssql-database` | **specific** | applications are written against a dialect; a swap breaks them |
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| `route` | **role** | the consumer wants its name reachable and does not care what proxies it |
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| `resolver` | **role** | the consumer wants names to resolve |
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| `artifact-store` | **role** | the consumer fetches by digest over a protocol, and nothing else |
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**`database` is not a provision and may not be provided.** There is no context in which an
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application talks to a generic database: it talks to PostgreSQL or it talks to SQL Server. A name
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that cannot be true of any real consumer should not be expressible.
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**This is about coupling, not about products.** Two providers of `postgres-database` — a container
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on this node and a managed instance elsewhere — are interchangeable and *should* both match. What
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may not be interchangeable is what the consumer's queries are written in.
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## Consequences
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**Every manifest that names a database changes.** Doing this now costs a rename across a handful of
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examples. Doing it after modules are migrated costs it across all of them, plus every deployment
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that resolved against the old name.
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**Wrong requirements now fail loudly, and at the right moment.** A module requiring
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`postgres-database` where only `mssql-database` is provided is unsatisfiable, so it is **refused at
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resolution** with both names visible — rather than deployed and broken later. This is the property
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that was lost, restored.
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**Generic role names are still right, and the rule says when.** This does not push specificity
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everywhere; it puts it exactly where a consumer is coupled. Naming `route` after a particular proxy
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would be the same error in the other direction, and would prevent a swap that genuinely changes
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nothing.
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**It is checked, not merely stated** ([`00-META/how-we-build.md`](../00-META/how-we-build.md) §5).
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A manifest providing a name known to be engine-generic is refused, naming what to say instead.
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Without that, this record is a convention, and a convention is what the previous naming was.
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## References
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- [ADR 0009](0009-modules-and-the-graph.md) — provisions, and refusing on ambiguity
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- [`03-DESIGN/01-to-be/07-the-substrate.md`](../03-DESIGN/01-to-be/07-the-substrate.md) — *the
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provisioning model uses databases, roles and schemas as PostgreSQL means them*, which is this
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record's point made about the substrate before it was made about modules
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