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jschoubben 784b487bf9 ADR 0131: everything on the mesh speaks to the broker seat, and AMQP is not a provision

Taken during the outage of 2026-09-27, when the protocol leaked into the seat's
contract: to hold mesh-broker a module had to provide amqp, so the module that
will carry the bus could not hold the seat that names the bus, while the module
being retired could. Supersedes 0127. Modules depend on the seat and reach the
bus through the sdk; no manifest provides or requires amqp; the old broker's
module and the two modules that required it leave the catalogue; the AMQP
transport is deleted once every node reports on the new bus.

Design 28 step 5 rewritten under it: the seat handover becomes its own task and
is built first, because the seat the control plane dereferences cannot be empty
in between — that emptiness was the outage. The cost note now carries what was
measured rather than what was assumed.

0128 and 0130 extended 0127; each now rests on 0131 with a dated note and
changes nothing it decided. Every other citation of 0127 names its replacement.
records.py still fails on 0120/0112, which predates this branch.

2026-09-27 23:03:05 +02:00

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Glossary — the words this repository uses, and the ones it stopped using

One name per thing. This page is the authority; where an older record says something else, that record is being superseded, not this page. It exists because the terms kept drifting in conversation — control plane / controller / master / hub for one thing, substrate / foundation for another — and a mesh you cannot name precisely is a mesh two people describe differently.

The mesh and its machines

node — a machine in the mesh. There are 0..n of them, and each runs the host agent. A node is just a machine that has joined; being one implies nothing about what it runs.
control-node — the one node that also holds the mesh-controller seat. There is exactly one per mesh. "control-node" is not a separate kind of machine — it is a node that additionally runs the controller (and, today, the foundation). Lose it and the other nodes keep running what they were last told; they simply cannot be told anything new.
~~master / slave~~, ~~hub / peer~~ — not used. The relationship is controller and nodes, and no node is subordinate: a node applies declarations on its own and survives the control-node dying.

What runs the mesh

controller — the component that decides what each node should be, holds the mesh's records, and tells nodes over the broker. Replaces "control plane" (borrowed from networking's control-plane/data-plane, and opaque here).
mesh-controller — the module that runs the controller. It claims the mesh-controller seat at mesh scope, which is what makes it singular. Replaces the module name mesh-control. (The git repository has been renamed mesh-control -> mesh-controller on the forge; the module, container and image it produces are mesh-controller.)
foundation — the store and the broker, raised at genesis before any module system exists. Replaces "substrate" (a biology metaphor that landed for no one). The foundation is not a third thing beside the store and broker — it is those two, named together.
store — the one postgres server. It holds the controller's own context databases (inventory, identity, licences — a context owns its store, ADR 0008) and every module's own database. One server, many databases — never one shared "mesh database".
bus — the mesh's own nervous system: NATS, one per mesh, carrying every link the mesh has — control, declarations, builds, events, tool calls (ADR 0106). A module reaches it by requiring mesh-bus (ADR 0128); one that does not require it has no account on it. Held by the mesh-broker seat, which is named after the role rather than the server, so the server can change without the seat doing so.
the deprecated broker — the lavinmq module. It was the mesh's bus and is not any more. It keeps running as an ordinary provider of the amqp provision, for modules that need a message broker of their own the way something needs a database (ADR 0127 (superseded by ADR 0131)) — no seat, not foundation, never raised at genesis, and a mesh that never installs it is complete.

Say the deprecated broker, not "the compatibility broker" (it serves the mesh's own modules, not only the predecessor's) and not "the AMQP broker" (naming it after a protocol invites describing the bus by contrast with it, which is backwards: the bus is the mesh's nervous system and this is a module).

What the mesh stores and serves

package — what code resolves when it is compiled: an npm/cargo/pypi dependency, by version. Served by the package-registry (gitea). Only a builder talks to it.
artifact — what the mesh delivers to a machine to install and run: an OCI image, by digest. Served by the artifact-store (distribution). Every node pulls from it.
These are two protocols, not one store being weak — see ADR 0075.

How modules relate to the mesh

seat — a named role at a scope (node / site / mesh), held by a module assignment, from a closed set the mesh defines: a claim naming a seat outside the set is refused. A seat may deliver a provision, and its holder is then the mesh's answer for it when several modules provide it (ADR 0126 (superseding ADR 0110)). The set, with who holds each seat, is the overview of what a mesh has (26 — The seats). A seat has a capacity: a capacity-1 seat is exclusive (one holder); a higher-capacity seat is a bench (several holders coexist).
claim — a module taking a spot on a seat. claims: [{name, scope}] in a manifest. A mesh-scoped exclusive claim is how the mesh says "there is one of me". A foundation seat is named after the server it guards: the mesh-controller, postgres and lavinmq modules claim the mesh-controller, mesh-store and mesh-broker seats (ADR 0079).
provision — a service one module provides and others require; the mesh resolves a provider and wires the two with an endpoint and a credential. A provision is a service you offer, a seat is a role you occupy, and the two meet where a seat delivers a provision: occupying the seat is what makes a module the provider of it.

How this page is kept

A new name for an existing thing lands here first, in the same change that introduces it in code. A record under 02-DECISIONS/ keeps whatever word it was written with — those are immutable — so a term retired here may still appear there, and the mapping above is how to read it.

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