hq/03-DESIGN/01-to-be/28-building-the-bus.md

layer, status, code, updated, decisions

layer	status	code	updated	decisions
to-be	in-progress	mesh-catalog modules/nats mesh-controller internal/catalogue mesh-lab scenarios	2026-09-26	02-DECISIONS/0116-the-bus-is-built-in-five-steps.md 02-DECISIONS/0106-the-bus-is-nats.md 02-DECISIONS/0119-amqp-is-a-provision-not-the-bus.md 02-DECISIONS/0118-a-module-declares-its-own-seats.md 02-DECISIONS/0074-the-wire-is-specified-not-the-types.md 02-DECISIONS/0079-the-foundation-seats-are-named-after-their-servers.md 02-DECISIONS/0100-a-node-in-use-is-adopted-before-it-is-converged.md 02-DECISIONS/0039-what-the-sdk-holds-and-refuses.md

28. Building the bus

The work of ADR 0116's five steps, in the order its dependencies allow, with what each ends at. Design 25 is the architecture and stays the authority on what is built; this document holds only the order, the sizes and the proofs, and it is wrong the moment it disagrees with design 25 rather than the other way round.

Each step ends at something runnable. A step that cannot name what its bed proves is not a step, and is divided further before it is started.

How this is built, and when it is run

Written as code with unit tests, committed per change, and taken to the lab once the pieces that would change the outcome are in place. The mistake this avoids is the one design 22 records: running a long bed against a mesh mid-transformation and debugging paths the next step deletes. Where a fault can be reasoned out of the code path, it is — reading, not running.

So the beds below are acceptance tests at the end of assembled work, not the tool for finding each bug, and a step's bed is run when that step is finished rather than while it is being written.

What the work is, measured

Counted 2026-09-26, non-test source only. The point of counting is that none of this is unknown territory: every piece has a shape already standing beside it.

Piece	Today	Size	Becomes
the controller's link	Go, one package	~1 800 lines	the same package on NATS
the host's link	Go, one package, mirroring the contracts rather than importing them	~1 000 lines	the same, on NATS
the tool runtime's client	TypeScript, one file	~390 lines	the same, on NATS
the sdk's messaging surface	TypeScript: messaging, events, tools, contracts, primitives	~360 lines across five	unchanged, see below
the broker module	the adopted AMQP broker: client, tools, provisioner, bootstrap, image, manifest	~340 lines of module code	the nats module, same shape
the beds	39 lab scenarios, including a broker bed, an adoption bed, a genesis bed and a store-window bed	—	four analogues and one new

Two measurements are worth stating on their own, because they change what the steps are.

The sdk speaks no AMQP, and never did. The word appears in its source three times, in three comments; its messaging module says in as many words that it "carries the contract, not a specific AMQP client build." ADR 0039 put the client in the runtime, and the payoff is collected here: no module is rebuilt for this change, and the sdk's own diff is three comments. That is the whole reason a bus can be replaced under a live mesh at all.

The wire therefore has three implementations, not two, and no suite pins any of them. ADR 0074 spoke of "the existing two implementations" — Go and TypeScript. Measured, the Go side is two separate packages that mirror rather than share (the host imports nothing, by ADR 0005), so the count is the controller's link, the host's link, and the runtime's client. And a search for conformance fixtures finds none anywhere in the four repositories: design 22's Phase 1.2 — the suite — has not been built.

This corrected the record. ADR 0116 said step 3's fixtures were recaptured on NATS. There is nothing to recapture, so step 3 builds the suite, and its first job is to pin the wire the mesh has before changing it — a suite written only against the new bus certifies whatever the new bus happens to do. A fact went stale while the decision stood, which is a progressive insight (02-DECISIONS/README.md): it is marked and dated in ADR 0116 itself rather than left to be discovered here.

The order the work actually allows

The five steps are chunks of capability; the build order is not simply 1 to 5, and pretending otherwise would put two beds where they cannot run. Three edges decide it:

• A specification precedes the implementations it governs. ADR 0074's whole argument is that agreement is specified and checked, not hoped for. So the wire's NATS binding is written before the three implementations are, even though it is step 3 — and its conformance half can only finish once two implementations exist to disagree.
• A mesh cannot be raised on a bus nothing speaks. A bed that raises a mesh on NATS from genesis — enrolling a node, holding a push while the store restarts, rolling out an upgrade — needs the controller and the host to speak NATS already. That is the implementations, and they arrive with step 3.
• Adoption needs the module and nothing else. Step 2 puts a correctly configured server into a running mesh that continues to ignore it, which depends on no link at all.

So step 1's bed proves the server, from genesis, configured — not a mesh living on it. The full genesis bed is step 4's, where it can first run.

This corrected the record too. ADR 0116 first attributed "a mesh raised on NATS from genesis" to step 1. That bed cannot run until the links exist, and a step whose proof cannot run is the exact failure the record was written to prevent — so step 1 now ends at the server standing, correctly configured and carrying nothing, and the full bed is named under step 4. The five steps, their names, their order and the single rollout are unchanged; only where two beds run has moved. Marked and dated in ADR 0116 as a progressive insight, with what the record said before.

step 1  module, genesis places it        ──┐
step 2  adoption into a running mesh     ──┤  neither needs a link
                                           │
step 3  the wire specified ──► three implementations ──► the suite
                                           │
step 4  the flows, and the full genesis bed
                                           │
step 5  the rollout

Step 1 — the module, and genesis raises it

Revised 2026-09-26 (ADR 0118, design 29). Tasks 1.3 and 1.4 said the controller composes every account and creates the four streams at genesis, from a fixed set. That is only the mesh's own half. A module declares seats with their protocols, so streams are created at registration and durable consumers at assignment — neither of which has happened at genesis. The fixed foundation set stays here; the derived machinery moves to step 3, where the declaration model it reads from is specified. Tasks 1.1 and 1.2, already done, are untouched by this: the module and its reload mechanism do not care what the configuration says.

Why here. Everything else needs a server to talk to, and genesis is where the foundation is defined. The mesh this is for will never travel this path — it is already running, and takes step 2 — but genesis is the definition every other path is measured against, and one that exists only on paper is wrong until there is a second mesh to find out.

• 1.1 the nats module: manifest, image, one container, its client, TLS and monitoring ports, JetStream on a named volume — the shape of design 25 §5, and the same shape the broker module beside it already has
• 1.2 the composed configuration as a directory resource, and the entrypoint that watches the one file and signals the server itself — design 25 §5's correction, kept inside the module because a container has no reload and a recreate would drop every connection the mesh has
• 1.3 the controller composes that file: accounts, permissions, TLS, JetStream — a user's permissions derived from its declaration and nothing else, over the three namespaces of design 29 §2, plus its own ack subject and its own inbox prefix (design 25 §4)
• 1.4 the mesh's own streams, created at genesis and asserted idempotently on start, by the controller as their only writer — the mesh's own, not all of them: a seat's streams are created when the module declaring it is registered, and a module's durable consumers when it is assigned, so this task is the fixed foundation set and 3.x carries the derived rest
• 1.5 genesis raises it as foundation, claiming the seat mesh-broker — the seat is the server's role, not the product. Already true of the controller and needed no change: it resolves the broker by seat ("that is where the broker is, whatever else the topology says") and names no broker module anywhere in its source. What remains is naming nats instead of the AMQP broker where a genesis module set is declared, which is scenario and installer configuration — carried with 1.6 rather than before it.
• 1.6 the genesis-broker bed — deferred: beds are run once, at the end, rather than per step (novox/hq design 22's rule, and the operator's instruction). Every claim step 1 makes is covered by a unit test or was demonstrated against the real server; what the bed adds is the claims that need a mesh.

Not done here, deliberately. The controller builds a module's broker credential as an amqps:// URL and defaults a portless genesis address to 5671. Those are correct until the rollout and must not move: steps 1 to 4 leave every node on AMQP (ADR 0116), so changing the credential's shape now would break the running bus to serve a bus nothing speaks yet. They change with the links, in step 3.

Done when. A mesh raised from nothing has the server standing with the streams asserted and every account and permission composed from the manifests; a user cannot publish outside its emits, subscribe outside its consumes, ack another user's delivery or subscribe another's inbox prefix; the monitoring port is refused from anything but the private network; a change to the composed file is live within one watcher interval without a restart, and the container is not recreated by it.

The permission checks belong here rather than later because the server enforces them itself — a plain client proves them, no link required — and they are the whole of what ADR 0043 asks for. No mesh traffic is on the bus yet; that is step 4's bed, not this one's.

Step 2 — adoption puts it in the seat

Why here. It needs only step 1's module, it is the path the mesh that exists will actually take, and it is what makes steps 3 and 4 safe to develop against a live mesh. A running mesh does not get a foundation module by being raised again; it adopts one in place (ADR 0100).

• 2.1 the server raised beside the existing broker on its own ports, carrying nothing — installer-side, from the upstream image
• 2.2 the nats module assigned, which recreates the container once, deliberately (see below), keeping its JetStream directory
• 2.3 the seat claim, and the resolver's refusal of a second holder mesh-wide — already true and now proved: the refusal is generic to any mesh-scoped seat, and three tests pin what matters for this one — a second bus anywhere is refused naming the seat, a different bus implementation is refused for the same reason (which is what lets the bus be replaced at all), and the AMQP broker no longer contends for it, so both run on one mesh
• 2.4 the adoption bed — deferred with the other beds

Adoption here is not a no-op, and pretending it would be is the trap. The host keeps an existing container only when its spec matches the declaration exactly (apply.go: existed && before.Spec == want && running → unchanged; anything else is rm -f and recreate). Genesis raises the server from the upstream image, because nothing has been built yet; the module declares the mesh-built artifact, which carries the entrypoint that reloads configuration in place. Those two specs differ, so assigning the module recreates the container.

That is correct, and it is ADR 0067's pivot exactly: raise a temporary thing, then reinstall it as an ordinary module. It is safe only because it happens while the bus carries nothing — which is what 2.1 means by "carrying nothing", and why step 2 comes before anything speaks NATS rather than after. One recreate, at the one moment it costs nothing.

After that, never again. The configuration is a directory mount rather than a file, so rewriting accounts does not change the container's spec and the entrypoint reloads the server in place. That is the whole point of task 1.2, and this is the moment it pays: every later account, permission or person's access change touches a running bus with connections on it.

Done when. A mesh already running has the server adopted, holding mesh-broker; a second assignment anywhere is refused at resolution — one per mesh; and every node is still on the old bus with nothing routed to the new one. That last check is the point of the step: adoption that quietly carried traffic would be step 5 arriving early and unrehearsed.

Step 3 — the protocol on NATS

Why here. The implementations cannot be written against an unwritten wire, and this is the step that decides what "agreeing" means for everything after it. It is the largest step and the one that pays for itself furthest away.

• 3.1/3.3 the fixtures — one directory in the sdk, read by each implementation's own runner rather than copied into either, because a fixture copied twice is two fixtures. The Go emitter and the runtime's NATS client both pass the first: every required header set, each value in the pinned shape, the subject derived the same way, and the payload the body alone.

  **The suite also had to settle what "byte-for-byte" can mean**, which ADR 0074 stated and
  nothing had yet had to implement. The envelope is exact — subject, required headers, names
  and formats — because that is what two implementations get wrong invisibly. The body is
  not: Go sorts a map's keys and JavaScript keeps insertion order, so identical bytes would
  commit every implementation to a canonical JSON encoder, to buy a property the mesh never
  uses. Read strictly it would have sent somebody writing one.
  
  Still to capture: a served tool call, a grant and its answer, and the contributions file —
  the other three ADR 0074 names.
  

• 3.2 design 19 rewritten from exchanges, queues and routing keys to the subjects and streams of design 25 §2–§3, per capability, with ADR 0074's model untouched: floor plus capabilities, an implementation legitimate when it claims less, identity from the sealed credential, dedup on x-event-id. Claims checked against a running server are marked verified in the text, so a reader can tell what was measured from what was reasoned. One limitation lifts with the transport: a module may now call another's tool, which issue 049 recorded it could not.

• 3.4 the controller's link on NATS

• 3.5 the host's link on NATS — mirroring, still importing nothing

• 3.6 the tool runtime's client on NATS, behind the unchanged sdk contract — round-tripped against a real server: a tool answered across two connections, a throwing handler reaching the caller as an error rather than a timeout, an event delivered once with its key, body, node and event id intact. Ships beside the AMQP client and is selected at the rollout, because steps 1 to 4 leave every node on AMQP.

  **A constraint it surfaced, recorded where somebody issuing a certificate will look.** The
  AMQP client pinned the exact certificate and switched hostname verification off, which is
  sound because a fingerprint is stronger than a name. The NATS client exposes no equivalent
  hook — its TLS options are PEM strings with no verify callback — so the pin still happens
  before dialling and the library's own name check happens beside it. **The bus's certificate
  must carry a subject-alternative name matching the address nodes dial it by**, or the
  connection is refused by a library error rather than by anything the mesh says.
  

• 3.7 the sdk's three stale comments, and nothing else in it — three lines, which is the whole of the sdk's diff for the bus change, and the measurement that predicted it

• 3.8 the declaration model of design 29: local names derived to subjects, the three namespaces, permissions computed from a declaration, and a manifest that contains no subject. Done in the controller's composer (permissions, streams, consumers), in the runtime's client (subjects derived from the credential, never named by a module), and as a catalogue test asserting all 72 manifests hold no subject — because the rule held by construction, and a rule held by construction is one a later field breaks quietly.

• 3.9 seats declared by modules — the manifest now carries seats (name, scope, accepts/emits/serves, retention) and uses, and registration refuses a mesh-* name, a duplicate declarer, an undeclared uses or claim, a seat with no protocol, a scope mismatch, and a holder that does not answer what its seat promises. Still to do: creating a seat's streams at registration and its holder's work-queue consumer at assignment, which need the JetStream client wired in.

  The refusal for an unknown claim *moved* rather than disappeared — the parser cannot judge
  it from one manifest any more, because another module may legitimately declare that seat,
  so it is registration's. The test that encoded the old rule was rewritten rather than
  deleted, and a second one pins the case the parser could not distinguish.
  
  **Done**: a seat's work queue is derived and created, and a holder's worker with it. The
  JetStream client behind them is wired and verified against a running server, which also
  completes 1.4's missing half — the pure `Asserter` had no implementation until now.
  

• 3.10 the ten seat renames — done in the controller's table, the ten manifests that claim them, the controller's own shipped manifests, and every test. Not a migration after all: a holding is derived at resolution, never stored, so nothing recorded points at an old name (recorded as a progressive insight on ADR 0118). A kept rename table tells a manifest written against an old name what it became, because a module lives in its own repository and may be registered long after the catalogue stopped using one.

  **A seat and the interface it delivers are different names.** The `git` seat became
  `mesh-git` while the `git` *provision* it delivers did not change, and the same for the
  package registry. A blanket replace got this wrong first and the failure read "the package
  registry is served on `<nil>`", which does not say "you renamed an interface" — so a test
  now pins every seat against the interface it delivers.
  

Done when. The fixtures are produced and consumed byte for byte by every implementation that claims the capability, and a module built before any of this serves its tools unchanged on the new runtime. The step is not done when the code runs — two implementations that disagree about an envelope do not fail to compile, they ignore each other while both keep running, which is the failure ADR 0074 exists to catch.

And the shared library gained nothing but the binding. A new transport is when the pressure to add conveniences is highest, and ADR 0039's rule does not bend for it: a helper that arrives with the bus is a review failure, not a detail. Code shared among a module's own features stays in that module.

Step 4 — the core speaks it

Why here. The links exist from step 3, so the flows that are not on the bus at all can move onto it, and the beds that need a mesh living on NATS can finally run.

• 4.1 the full genesis bed — a mesh raised on NATS from nothing and living on it: a node enrols over TLS with a claimed token and the enrolment user cannot read a declaration; a push is held while the store restarts and applies after, nothing lost or duplicated; a node that was away gets exactly the newest declaration and refuses a replayed older one by sequence; an upgrade rolls out to two nodes; an event dead-letters after max-deliver; and an enrolment held by a nak-with-delay cycle still reaches the enrolling node, proving the reply travels in the payload and not the transport field the consumer's ack has claimed. The server-enforced permissions were proved at step 1 and are not re-proved here
• 4.2 a build source's change reaches the builder over the bus, and the build that follows is the one the change asked for
• 4.3 an installation completes over the bus, with the same outcome as the path it replaces
• 4.4 a person's client: the account, the client that speaks the bus, and the tool surface over it (design 25 §7) — a module's tool invoked from another node and from a person, refused from an account that may not
• 4.5 reports and catch-up: a node that was unreachable catches up rather than losing them

Done when. Each converted flow is proved against the behaviour it replaced, and the full genesis bed is green. Observation is not in this step — heartbeats, conditions and key-value state are research 017's, that effort already reserves them for after the move, and a flow built ahead of its design would be rebuilt.

Step 5 — the rollout

Why here. It is the only step that moves a node's bus, and it moves every node's at once.

• 5.1 the cutover bed: a mesh on AMQP with a predecessor stand-in on the compatibility broker moves its bus in one rollout, every node reporting on NATS afterwards, the stand-in's own client still connected throughout
• 5.2 the rollout: accounts composed, then the controller, every host and every runtime together; every node confirmed heard before AMQP stops
• 5.3 the mesh's accounts removed from the compatibility broker, leaving the predecessor's users
• 5.4 the compatibility broker retires when its condition holds — no client connected for the period the operator sets

Done when. Every node reports on NATS and the predecessor's clients never noticed.

The through-line

The order is dependency, not preference. Steps 1 to 4 leave every node on AMQP, so the cost of being wrong is bounded until the last step: a step may be abandoned, or reordered after step 2, without a rollback. The server stands before anything speaks to it; the wire is specified before it is implemented three times; the flows move once there is something to move them onto; and the bus itself moves once, at the end, on one day.

What is deliberately not here

• Observation — research 017's, after the move, by its own design.
• Leaf nodes — design 25 §11 keeps this out of scope and says so; a leaf per machine is a later question, noted so it is not forgotten.
• The predecessor's world. It is AMQP, it cannot move, and it does not need to: its broker is the compatibility module until its last client is gone.

How this list is kept true

A task is ticked when its change is committed, not when it is written. A step is done when its bed is green, not when its tasks are ticked. If a step's tasks are all ticked and its bed has not run, the step is in progress and this document says so — that gap is the thing the whole shape is built to make visible.