jschoubben
7b4916e9ec
The architecture 0117 opened needs a module to offer a service as a role on the bus — one holder, addressed by what it does. A closed table in the controller cannot express that: a capability a module contributes would require changing the mesh itself. But 0110 closed the set for a good reason — nothing could say what seats a mesh had, and the hand count came out at eleven of thirteen. That argues for enumerable, not hardcoded, and 0110 weighed free-form against a fixed table without considering a third option: closed at any moment and derived from the catalogue. A derived list cannot drift, which is how the count broke. So: the mesh's seats stay the mesh's, reserved by the mesh- prefix so the prefix is the rule and there is no list to maintain; ten seats are renamed to restore 0079's convention; everything 0110 decided about what a seat IS survives untouched. Design 29 carries the declaration model: three namespaces, subjects derived from local names so a manifest survives the wire changing, queues never declared, five relationships (the job and state shapes 0041 had no room for), and the build-publish-deploy lifecycle with hard, soft and build-time dependencies distinguished. 0041 gets a progressive insight: "no per-consumer setup, only a subscription" was a fact about a topic exchange, and a JetStream durable consumer is a real object someone creates. WBS 1.3/1.4 were wrong and say so: streams come at registration and consumers at assignment, so only the foundation set belongs at genesis.
131 lines
8.1 KiB
Markdown
131 lines
8.1 KiB
Markdown
---
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topic: the tiers
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status: accepted
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date: 2026-09-26
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deciders: jochen
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reconstructed: false
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supersedes: 02-DECISIONS/0110-a-seat-is-a-module-assignment-from-a-closed-set.md
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---
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# 118. A module declares its own seats; the mesh reserves its own
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## Context
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[ADR 0110](0110-a-seat-is-a-module-assignment-from-a-closed-set.md) closed the set of seats. Its
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evidence was strong and still is: nothing could answer *which seats does this mesh have, and who
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holds each*. Answering it meant reading every manifest in two repositories and then the
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controller's own code, and when that enumeration was done by hand while writing the record, **it
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reported eleven claims where there were thirteen.** The fix was a table in the controller, and
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adding a seat became a decision.
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What that table cannot express is the architecture [ADR 0117](0117-the-bus-is-the-only-broker.md)
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opened. With one bus and no private brokers, a module offering a service to other modules offers
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it as **a role on the bus**: a set of subjects, exactly one holder, addressed by what it does
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rather than by which module or node provides it. A telegram sender, a licensing master, anything
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a mesh might want one of. Under a closed table, adding any of those means editing the controller
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— so a capability contributed by a module would require a change to the mesh itself, which is the
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coupling the module system exists to prevent.
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**The two requirements look opposed and are not.** 0110 needs the set *enumerable*. The
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architecture needs it *extensible*. Those conflict only if enumerable means *written down in one
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place by hand* — which is exactly the property that let the count drift in the first place.
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## Considered Options
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1. **Keep the closed table, add each new seat by decision.** Rejected. Every capability a module
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contributes would need a change to the controller and a record before it could be offered, and
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the mesh would carry the names of services it does not itself implement.
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2. **Free-form seats, as before 0110.** Rejected for 0110's own reason, unchanged: nothing can
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say what a mesh has, and a name invented at a claim site is a name nobody can explain later.
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3. **A set that is closed at any moment and derived rather than maintained**, with the mesh's own
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seats reserved by name. Adopted. 0110 weighed options 1 and 2 and never considered this one.
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## Decision
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**A seat may be declared by a module, and the set of seats a mesh has is derived: the mesh's own,
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plus those declared by every module it has registered.** The set is still closed — a seat named
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nowhere is refused — but it is computed from the catalogue rather than written in the controller.
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Everything 0110 decided about what a seat *is* stands untouched: one holder at its scope; a
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definition says which seats a module *can* hold and an assignment says which it *does*; holding
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one may deliver a provision; a seat makes a role singular, never a module.
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**Enumeration is a query, not an inventory.** The catalogue knows every registered manifest, so
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"which seats does this mesh have, and who holds each" is answered by asking it. This is a
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stronger answer than the table gave, not a weaker one: a derived list cannot drift from reality,
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and drift is how the hand-made count came out at eleven of thirteen.
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**The mesh's own seats are reserved by prefix.** Every seat the mesh itself defines is named
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`mesh-*`, and a module declaring any `mesh-*` name is refused at registration. The prefix *is*
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the reservation rule — no list of reserved names to maintain, and no way for the mesh's own
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namespace to be colonised by a manifest. This requires renaming the seats that drifted from
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[ADR 0079](0079-the-foundation-seats-are-named-after-their-servers.md)'s convention: `the-catalogue`
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becomes `mesh-catalog`, `git` becomes `mesh-git`, and the node-scoped `the-build-machine`,
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`the-dns-port`, `the-intrusion-prevention`, `the-packet-filter`, `the-private-network`,
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`the-resolver-configuration`, `the-showcase` take the same prefix.
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The mesh's seats stay the mesh's for a reason that does not apply to a module's: **the mesh's own
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code looks them up by name.** The resolver *is* the thing that finds the store. `mesh-store` is
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not a convention the controller follows, it is an identifier the controller dereferences.
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**A declared seat carries a protocol.** A module declaring a seat says what may be sent to it,
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what it emits, and what it serves. The holder must satisfy it; a module may not claim a seat whose
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protocol it does not implement. Callers declare that they use the *seat*, never the module, so
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replacing the implementation changes nothing for any caller.
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**A seat is for a role; an event stays addressed to its emitter.** The two are not
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interchangeable and the choice is not stylistic. An event is *this happened to me* — the emitter's
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identity is the meaning, which is why the envelope carries source, node and time
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([ADR 0042](0042-the-shape-of-an-event-on-the-wire.md)); routing it through a role would erase the
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provenance an audit needs. A seat is *this capability, whoever provides it* — where not knowing
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the holder is the point. Publish an event when the fact is about you; declare a seat when you are
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offering something another module could offer instead.
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**Two modules declaring the same seat name is refused at registration**, second one loses.
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Registration is the last moment the mesh can still say no, and a seat name meaning two different
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protocols is the failure nobody could diagnose afterwards.
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## Consequences
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- **The controller's seat table stops being the set** and becomes the mesh's own reserved entries.
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Resolution reads the catalogue for the rest.
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- **Ten seats are renamed.** A rename is a migration, not an edit: existing assignments hold the
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old names, so the change carries a mapping and is applied once, and the lab beds that name seats
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are updated with it.
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- **A `uses` naming an undeclared seat is refused at registration**, which is where 0110's
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guarantee lands under this model — the same refusal, at the same moment, from a derived set.
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- **Adding a capability stops requiring a decision record.** That is a real loss of governance and
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the intended trade: the argument for a seat's existence moves into the module that declares it,
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where it is reviewed as part of the manifest. The mesh's own seats keep the old bar.
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- **[ADR 0041](0041-events-are-a-relationship.md)'s machinery claim is already stale** for a
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different reason, and is corrected in place there under the rule in
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[`README.md`](README.md) — a progressive insight: on JetStream a subscription is a durable
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consumer, a real object someone must create.
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- **What got harder:** a seat's protocol is now a compatibility surface between modules that do
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not know each other. Changing one breaks callers already bound to it, and nothing here says how
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that is versioned. It is the first thing to answer in the design, and the thing most likely to
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hurt later rather than now.
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## How it is checked
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- **The overview answers, and is right.** A command lists every seat, its scope, its protocol and
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its holder, derived from the catalogue — and a test asserts the count against a fixture mesh,
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because an enumeration nobody checks is how thirteen became eleven.
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- **`mesh-*` is refused to a module.** A registration test: a manifest declaring `mesh-anything`
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is refused, naming the prefix as the reason.
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- **An undeclared seat is refused.** A registration test on `uses`, and a resolution test that
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nothing reaches runtime unresolved.
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- **A second declarer loses.** A registration test: two manifests, same seat name, the second
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refused and the first untouched.
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- **A holder must satisfy the protocol.** A claim whose module does not serve what the seat
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declares is refused at assignment, not discovered when a caller times out.
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## References
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- [ADR 0110](0110-a-seat-is-a-module-assignment-from-a-closed-set.md) — superseded here; its
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requirement is kept and only its mechanism replaced.
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- [ADR 0117](0117-the-bus-is-the-only-broker.md) — one bus, which is what makes a role addressable.
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- [ADR 0079](0079-the-foundation-seats-are-named-after-their-servers.md) — the naming convention
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the reserved prefix restores.
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- [ADR 0041](0041-events-are-a-relationship.md) — the event half of the boundary drawn here.
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