ADR 0095: the control plane is the way to ask a module; issue 049 resolved; design 19 amended

2026-09-21 20:38:23 +02:00
parent acc9824949
commit 39a01b7f7e
5 changed files with 79 additions and 14 deletions
@@ -0,0 +1,60 @@
+---
+topic: the tiers
+status: accepted
+date: 2026-09-21
+deciders: jochen
+reconstructed: false
+extends: 02-DECISIONS/0047-a-module-runs-its-code-as-its-own-process-with-its-own-account.md
+---
+
+# 95. The control plane is the way to ask a module
+
+## Context
+
+A module serves tools over the broker under an account scoped to what it emits, consumes and
+serves ([ADR 0047](0047-a-module-runs-its-code-as-its-own-process-with-its-own-account.md)). A
+tool call is a request and a reply: the caller creates a reply queue and publishes to the serving
+module's request key, and no module's scope grants either — nor should it, since a module that
+only publishes events has no business declaring queues. So a module could serve tools and nothing
+in the mesh could call them
+([issue 049](../04-ISSUES/049-a-module-can-serve-tools-and-nothing-can-call-them/00-report.md)):
+not an operator at a terminal, not an agent acting for one.
+
+## Considered Options
+
+1. **Calling is a grant**: a module declares it may be asked, and a consumer is issued an
+   account that may create a reply queue and publish to that module's request key. Deferred: a
+   module-to-module call is the only caller that resembles what the mesh mints today, and none
+   asks for one yet.
+2. **The control plane is the way in.** Adopted. It holds a connection that may already, so a
+   person or an agent asks through it, and every question passes one process — which is where
+   an audit of who asked what belongs.
+
+## Decision
+
+`mesh-controller ask <module> <tool> [json]` publishes the request on the tool exchange under
+`<module>.<tool>`, with a private reply queue bound under its own name, waits for the answer
+whose correlation matches, and prints it as the module gave it. A tool that answered with an
+error has answered: the answer is printed and the exit status says so. A module that never
+answers is said to have not answered, with where to look.
+
+A module declares nothing about being asked: serving a tool is being askable through the control
+plane. A module-to-module call, if one is wanted, is a grant like any other and a later decision.
+
+## Consequences
+
+Anything with the control plane in reach can ask any module anything it serves. What got
+harder: nothing outside the control plane can, and the control plane's connection is one more
+thing on the path of every question — a cost accepted for the audit it buys.
+
+## How it is checked
+
+A tools-only bed asks a served tool through the control plane and asserts an answer arrived —
+an error, since the lab has no upstream and no token, which is an answer where a timeout would
+not be.
+
+## References
+
+- [issue 049](../04-ISSUES/049-a-module-can-serve-tools-and-nothing-can-call-them/00-report.md)
+- [ADR 0047](0047-a-module-runs-its-code-as-its-own-process-with-its-own-account.md)
+- [`03-DESIGN/01-to-be/19-the-module-protocol.md`](../03-DESIGN/01-to-be/19-the-module-protocol.md)
@@ -114,6 +114,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0079** — [The foundation seats are named after their servers](0079-the-foundation-seats-are-named-after-their-servers.md)
 - **0092** — [An operator delivers a pair credential, and the mesh never replaces it](0092-an-operator-delivers-a-pair-credential.md)
 - **0094** — [A module may hold several secrets from one provider, each a pair of its own](0094-a-module-may-hold-several-secrets-from-one-provider.md)
+- **0095** — [The control plane is the way to ask a module](0095-the-control-plane-is-the-way-to-ask-a-module.md)

 ### What runs on them, and how it gets there