ADR 0188: a provider declares what it derives for each consumer, and the mesh tells both ends

Issue 124: a value the mesh's own rule produced reached neither end as a
statement. The object store's provisioner derived each consumer's bucket in
its own code; all three consumers transcribed the rule into their own
definitions, one of them wrong, and each of the three also named the machine
it happens to run on.

A served value may now name the consumer the mesh is serving. Design 27
amended; issue 124 resolved.

02-DECISIONS/0188-a-provider-declares-what-it-derives-for-each-consumer.md

@@ -0,0 +1,130 @@
+---
+topic: the mesh
+status: accepted
+date: 2026-10-02
+deciders: jochen
+reconstructed: false
+extends: 02-DECISIONS/0048-a-provider-creates-the-credential-the-mesh-minted.md
+---
+
+# 188. A provider declares what it derives for each consumer, and the mesh tells both ends
+
+## Context
+
+An arrangement between a consumer and a provider is delivered entirely by the mesh. Where the
+provider is, which port it answers on, what name the consumer must present, where its password
+is — each arrives as a fact the consumer reads from its binding, or as `${bound:…}` filled into a
+file before the declaration leaves the control plane. The provider invents none of it and hands
+none of it back ([ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md)).
+
+One kind of value escapes that. Where the **provider names the resource** — a bucket, a database,
+a vhost — the name is derived from the consumer, per consumer, and the mesh has no way to carry
+it. `serves` is a literal block in the provider's definition: the same values for every consumer.
+A provisioner's contract takes a provision and returns nothing. So a value the mesh's own rule
+produced reaches neither end as a statement; it is recomputed at one end and transcribed at the
+other.
+
+The object store is the instance ([issue 124](../04-ISSUES/124-a-consumer-cannot-be-told-what-its-provider-derived/00-report.md)).
+Its provisioner normalises the login the mesh minted into a bucket name and creates, checks and
+removes exactly that; the rule lives in twenty lines of the module's own TypeScript. Its three
+consumers each write the answer into their own definition by hand. Two transcribed it correctly;
+one named a predecessor's bucket, and would have authenticated successfully and been refused on
+every object, which reads like a credential fault and is not one.
+
+Even corrected, the transcriptions are wrong in a second way. Each is `mesh-<node>-<slug>`, so
+each **names the machine the module happens to run on today** — a definition stating a fact about
+one installation, which [ADR 0155](0155-a-definition-names-no-installation-and-how-that-is-checked.md)
+forbids and whose check does not catch because the name is not a domain. Move any of the three to
+another machine and its configuration points at a bucket its key cannot open.
+
+The shape is not the object store's. A database provisioner that prefixed names, a queue provider
+that scoped vhosts, any provider that derives a resource from who is asking: each forces the
+consumer to reproduce somebody else's rule and keep it in agreement by hand.
+
+## Decision
+
+**1. A served value may name the consumer the mesh is serving.** A `serves` block, which is
+literal today, may interpolate the mesh's own statement of who the consumer is:
+
+- `${consumer:as}` — the identity the mesh minted for this consumer, exactly as the login it is
+  told to present ([ADR 0049](0049-a-consumers-identity-fits-the-tightest-backend.md));
+- `${consumer:as:dns}` — the same identity written as a DNS label.
+
+Nothing else. **The mesh learns no protocol here; it spells its own name in an alphabet it already
+knows.** The identity is the mesh's, minted by the mesh, already capped at twenty characters
+because of what an S3 access key accepts; `dns` is that same name with its separator written `-`
+instead of `_`, which is the whole of the difference between the mesh's identifier alphabet and
+the one buckets, vhosts and hostnames use. A provider that needs a prefix or a suffix writes it
+around the placeholder, because a served value is a string.
+
+The rejected alternative is **the provider returning values from provisioning** — the natural
+channel, since the provider is what derived them. It is rejected for three reasons, in order of
+weight. It inverts the delivery the mesh is built on: a grant would carry data the provider wrote
+rather than only data the mesh minted, and [ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md)
+removed exactly that second path once already. It makes a consumer's declaration incomplete until
+its provider's reconcile loop has run, so a consumer could not be composed before a provider
+answered — a bootstrap order the mesh does not have and does not want. And it puts the rule where
+nothing can check it: a value that arrives from a running process cannot be refused at resolution,
+only discovered wrong later, which is the failure this record exists to end.
+
+**2. The mesh resolves it once, per consumer, and tells both ends from the one resolution.** At the
+moment a consumer's declaration is composed, the mesh knows exactly who the consumer is. There, and
+only there, the placeholders are filled. The result reaches:
+
+- the **consumer**, as the served facts in its binding file and as `${bound:<provision>:<key>}` in
+  any file it writes — unchanged mechanisms, carrying one more key;
+- the **provider**, as `serves` on that consumer's entry in its contributions file, so the
+  provisioner is *told* the name rather than recomputing it.
+
+**The provider stops deriving in code and starts declaring.** One statement, filled once, delivered
+to both ends: the two cannot disagree, because there is no second computation to disagree with.
+
+**3. A served value stays settled before it is per-consumer.** Settings still compose into `serves`
+([ADR 0174](0174-a-node-varies-a-module-through-settings-and-kept-regions-never-an-edit.md)), and the consumer
+placeholders are filled after that, so an operator may set a prefix and the mesh still derives the
+rest. A `${consumer:…}` naming a fact or an alphabet the mesh does not have is refused when the
+definition is parsed, with what it may say.
+
+**4. A consumer may no longer name the resource its provider derives.** With the value delivered,
+a literal in a consumer's definition is not merely redundant — it is the one thing that can
+disagree with what the provider will actually create. The three object-store consumers lose their
+hand-written bucket names in this change.
+
+## Consequences
+
+- One more thing a definition may say, and one less thing a module may be wrong about. The
+  vocabulary grows by a placeholder; the catalogue loses three literals that named this
+  installation's control node.
+- A provider's naming rule becomes readable in its definition instead of in its source. `minio`'s
+  `bucketFor` goes; the manifest says `"bucket": "${consumer:as:dns}"` and the provisioner uses
+  what it is given.
+- A provider that already serves consumers keeps serving them: the derived value equals what the
+  code derived, so no bucket, database or login changes name. This is a change of **who says it**,
+  not of **what is said**.
+- The mesh now holds a rule in another system's alphabet — one rule, `dns`, stated once. A second
+  alphabet is a decision, not an addition: the cost of each is that the mesh must be right about
+  somebody else's naming, and that cost is only worth paying where the mesh already mints the name.
+
+## How this is checked
+
+- A served value naming an unknown fact or alphabet is refused at parse, with the list of what it
+  may say — tested on both halves of the message.
+- Resolving a consumer whose provider derives a value puts that value in the consumer's binding
+  file, in its `${bound:…}` substitutions, and in the provider's contributions entry for that
+  consumer — one test asserting the three agree, because agreeing is the whole point.
+- Two consumers of one provider on one machine get two different derived values, and neither gets
+  the other's.
+- A catalogue-wide test refuses a consumer definition that writes a literal where its provider
+  derives: the provider's `serves` names the key, so the catalogue can say which definitions
+  transcribe one.
+- `dns` is checked against the identity the mesh actually mints, not against an invented string:
+  the test derives an identity with `ConsumerIdentity` and asserts the label it becomes.
+
+## References
+
+- [issue 124 — a consumer cannot be told a value its provider derived for it](../04-ISSUES/124-a-consumer-cannot-be-told-what-its-provider-derived/00-report.md)
+- [ADR 0048 — a provider creates the credential the mesh minted, and seals nothing](0048-a-provider-creates-the-credential-the-mesh-minted.md)
+- [ADR 0049 — a consumer's identity fits the tightest backend](0049-a-consumers-identity-fits-the-tightest-backend.md)
+- [ADR 0174 — a node varies a module through settings and kept regions, never through an edit](0174-a-node-varies-a-module-through-settings-and-kept-regions-never-an-edit.md)
+- [ADR 0155 — a definition names no installation, and how that is checked](0155-a-definition-names-no-installation-and-how-that-is-checked.md)
+- [design 27 — a module requires, the mesh resolves](../03-DESIGN/01-to-be/27-a-module-requires-the-mesh-resolves.md)

02-DECISIONS/README.md

@@ -188,6 +188,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0185** — [A control plane behind its seat's row serves what it can](0185-a-control-plane-behind-its-seats-row-serves-what-it-can.md)
 - **0186** — [A ban list never holds a neighbour, and the mesh's own bans are its own wherever they hang](0186-a-ban-list-never-holds-a-neighbour.md)
 - **0187** — [A dead tracker is not the machine's failure](0187-a-dead-tracker-is-not-the-machines-failure.md)
+- **0188** — [A provider declares what it derives for each consumer, and the mesh tells both ends](0188-a-provider-declares-what-it-derives-for-each-consumer.md)
 
 ### Its tiers, from the bottom up
 

03-DESIGN/01-to-be/27-a-module-requires-the-mesh-resolves.md

@@ -2,7 +2,7 @@
 layer: to-be
 status: in-progress
 code: [mesh-controller internal/catalogue]
-updated: 2026-09-30
+updated: 2026-10-02
 decisions:
   - 02-DECISIONS/0112-a-module-definition-names-no-node-mesh-or-path.md
   - 02-DECISIONS/0155-a-definition-names-no-installation-and-how-that-is-checked.md
@@ -14,6 +14,7 @@ decisions:
   - 02-DECISIONS/0084-which-provider-serves-a-consumer.md
   - 02-DECISIONS/0046-a-module-configuration-is-its-assignments-not-its-manifest.md
   - 02-DECISIONS/0038-the-mesh-assigns-the-port.md
+  - 02-DECISIONS/0188-a-provider-declares-what-it-derives-for-each-consumer.md
 ---
 
 # 27 — A module requires, the mesh resolves
@@ -206,6 +207,22 @@ name when nothing sets it. That is the contract half of this design's operator p
 the placeholder allows: the definition says which values reach which requirement, and nothing else
 does. *How it is checked:* the unit tests named in issue 173, and the plan comparison that closed it.
 
+*A provider says once what it derives for each consumer (2026-10-02,
+[ADR 0188](../../02-DECISIONS/0188-a-provider-declares-what-it-derives-for-each-consumer.md),
+[issue 124](../../04-ISSUES/124-a-consumer-cannot-be-told-what-its-provider-derived/00-report.md)):*
+where a provider **names the resource** it gives each consumer — a bucket, a database, a vhost — the
+name is derived per consumer, and a literal `serves` block could not carry it. A served value may
+now name the consumer the mesh is serving: `${consumer:as}`, the identity the mesh minted, and
+`${consumer:as:dns}`, that same identity written as a DNS label. Nothing else — **the mesh learns no
+protocol here; it spells its own name in an alphabet it already knows.** Settings are laid on first,
+so an operator may still set a prefix and the mesh derives the rest. The mesh fills it at the one
+moment it knows who the consumer is, and the one filled value reaches both ends: the consumer, as
+its binding's served facts and as `${bound:<provision>:<key>}` in any file it writes; the provider,
+as `derived` on that consumer's entry in its contributions file, so its provisioner is told the name
+rather than recomputing it. A consumer that writes the derived value into its own definition instead
+of asking for it is refused, naming the placeholder to use. *How it is checked:* the unit tests in
+ADR 0188's "how this is checked", each run against the unchanged controller first.
+
 ## How a definition reads what was resolved
 
 **One form, naming a requirement and a field of its contract.** A definition that needs the database's
@@ -214,7 +231,9 @@ name in a configuration file writes the same thing: the requirement's name and t
 controller fills it at resolution.
 
 This one form replaces the placeholders that exist today, one per mechanism: bound values, secrets,
-ports and machine facts.
+ports and machine facts. It subsumes the consumer placeholder too — a value a provider derives is
+read by the consumer exactly as any other field of the contract is, and `${consumer:…}` is only
+how the *provider* states the rule.
 
 **The seat placeholder stays, for the controller alone.** The controller composes its own
 declaration and reaches the store and broker it made before any module existed, so it cannot be

04-ISSUES/124-a-consumer-cannot-be-told-what-its-provider-derived/00-report.md

@@ -1,9 +1,9 @@
 ---
-status: located
+status: resolved
 opened: 2026-09-26
-located-in: [mesh-controller internal/catalogue/declaration.go, mesh-sdk src/provisioner, mesh-catalog modules/minio]
-fixed-by:
-amended-design:
+located-in: [mesh-controller internal/catalogue, mesh-sdk src/provisioner, mesh-catalog modules/minio]
+fixed-by: 02-DECISIONS/0188-a-provider-declares-what-it-derives-for-each-consumer.md
+amended-design: 03-DESIGN/01-to-be/27-a-module-requires-the-mesh-resolves.md
 ---
 
 # 124 — A consumer cannot be told a value its provider derived for it, so it transcribes one
@@ -63,3 +63,27 @@ compares it to what the provider will actually create. The one wrong instance wa
 - What would have caught the wrong instance? A test that resolves a consumer's grant and compares the
   bucket in its own configuration against the one the provider would create is a check that could
   exist today, for any interface, without the mechanism above.
+
+## Answered, 2026-10-02 — [ADR 0188](../../02-DECISIONS/0188-a-provider-declares-what-it-derives-for-each-consumer.md)
+
+The channel is the provider's own `serves` block, which may now name the consumer the mesh is
+serving: `${consumer:as}` and `${consumer:as:dns}`. The mesh fills it once, where it knows who the
+consumer is, and delivers the one filled value to both ends — the consumer's binding and its
+`${bound:…}` substitutions, and the provider's contributions entry, so a provisioner is told the
+name rather than deriving it. Each open question above, answered:
+
+- **Should a provider return values from provisioning?** No. It would make a grant carry data the
+  provider wrote, make a consumer's declaration wait on its provider's reconcile loop, and put the
+  rule where nothing can refuse it. The reasoning is in the record.
+- **Or should `serves` say a value is derived?** Yes, and the mesh performs the derivation — but it
+  learns no protocol doing it. The only fact is the identity the mesh itself minted, in one of two
+  alphabets it already knows.
+- **Should a consumer that names the resource be refused?** Yes. A consumer's file that already
+  contains the value the mesh is about to derive for it is refused at resolution, naming the
+  placeholder to write instead. That is the check this report asked for, and it is exact rather than
+  heuristic: a derived value carries the identity minted for this consumer on this machine, which
+  nothing else would spell out.
+
+minio's `bucketFor` is gone; its manifest serves `"bucket": "${consumer:as:dns}"`. The three
+consumers' hand-written bucket names are gone with it — each of them also named the machine the
+module happens to run on, which is the second thing wrong with a transcription.