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.
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---
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topic: the mesh
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status: accepted
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status: superseded
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superseded-by: 0131-everything-on-the-mesh-speaks-to-the-broker-seat.md
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date: 2026-09-26
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deciders: jochen
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reconstructed: false
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@@ -4,11 +4,18 @@ 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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extends: 02-DECISIONS/0127-amqp-is-a-provision-not-the-bus.md
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extends: 02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md
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---
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# 128. The mesh bus is required, not ambient
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> **Pointer repointed, 2026-09-27.** This record was written extending
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> [ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) (superseded by [ADR 0131](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)), which
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> [ADR 0131](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md) has since superseded — AMQP is
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> not a provision at all. Nothing decided here changes; the frontmatter now rests on the live record,
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> and the citations below are read with that in mind.
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## Context
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[Design 29](../03-DESIGN/01-to-be/32-what-a-module-declares.md) opened by saying the bus is
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@@ -72,7 +79,7 @@ process in the path and nothing waiting on a bus account to create bus accounts.
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whose provider is the mesh itself.
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**A module may also provide a NATS server of its own, and that is a different interface.** Exactly
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as the AMQP broker provides `amqp` ([ADR 0127](0127-amqp-is-a-provision-not-the-bus.md)), a module
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as the AMQP broker provides `amqp` ([ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) (superseded by [ADR 0131](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md))), a module
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may run its own NATS and offer it as a backing service. That interface is **`nats`**; the mesh's
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own bus is **`mesh-bus`**; the two are never the same name, because a manifest that said `nats`
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could mean either and the difference is the whole architecture. The rule from 0119 decides which
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@@ -109,7 +116,7 @@ is legitimate: a private bus is a backing service, never a channel to another mo
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- [ADR 0125](0125-the-bus-is-the-only-broker.md) — superseded by 0119; its bootstrap argument is
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narrowed here to the case it supports.
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- [ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) — a broker as a backing service; this
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- [ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) (superseded by [ADR 0131](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)) — a broker as a backing service; this
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applies the same shape to the mesh's own bus and separates the two names.
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- [ADR 0043](0043-a-module-broker-account-is-scoped-by-emits-and-consumes.md) — authority from
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declarations, which this leaves untouched.
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@@ -4,14 +4,21 @@ status: accepted
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date: 2026-09-27
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deciders: jochen
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reconstructed: false
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extends: 02-DECISIONS/0127-amqp-is-a-provision-not-the-bus.md
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extends: 02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md
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---
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# 130. The predecessor is ending, and its broker goes with it
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> **Pointer repointed, 2026-09-27.** This record was written extending
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> [ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) (superseded by [ADR 0131](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)), which
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> [ADR 0131](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md) has since superseded — AMQP is
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> not a provision at all. Nothing decided here changes; the frontmatter now rests on the live record,
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> and the citations below are read with that in mind.
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## Context
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[ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) settled that the old broker is an ordinary
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[ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) (superseded by [ADR 0131](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)) settled that the old broker is an ordinary
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provider of the `amqp` provision rather than a compatibility module with an end date. It rejected
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giving it a retirement condition, and said why: *"its clients are not only the predecessor's, so the
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retirement condition describes a day that will not come."*
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@@ -0,0 +1,94 @@
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---
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topic: the mesh
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status: accepted
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date: 2026-09-27
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deciders: jochen
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reconstructed: false
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supersedes: 0127-amqp-is-a-provision-not-the-bus.md
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---
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# 131. Everything on the mesh speaks to the broker seat, and AMQP is not a provision
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## Context
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[ADR 0127](0127-amqp-is-a-provision-not-the-bus.md) settled the old broker as an ordinary provider
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of an ordinary provision, `amqp`, kept for whatever wanted a message broker of its own. The day the
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bus moved was the day that framing was tested, and it failed in a way that took the control plane
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down for an evening.
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Three things came out of the wreckage. **The protocol had leaked into the seat's contract**: for a
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module to hold `mesh-broker`, it had to provide what the seat delivers, and what it delivered was
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`amqp` — so the module that will carry the bus on NATS could not hold the seat that names the bus,
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while the module the mesh was leaving could. **A consumer of `amqp` is not asking for AMQP.** The two
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modules requiring it wanted the mesh's messaging — to emit an event, to hear a topic — and named the
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wire protocol only because that was the word available. **And AMQP and NATS are not interchangeable
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at the wire.** A provision named after a protocol can only ever be answered by that protocol, so once
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the bus is NATS an `amqp` provision has one possible provider, and it is the thing being retired.
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The operator's position, stated during the outage: modules depend on the broker *seat*, not on a
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protocol; AMQP is obsolete as anything the mesh's core knows about; a module that depends on `amqp`
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is wrong; and everything should reach the mesh's bus and be able to emit events and consume topics
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through it.
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## Decision
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**A module that needs messaging uses the mesh's bus, and the mesh's bus is whatever holds
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`mesh-broker`.** Emitting an event and consuming a topic go through the sdk, which is handed the
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bus by the mesh with the module's own credential. No manifest names a wire protocol to get it.
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**`amqp` is neither a provision nor a requirement.** Registration refuses a manifest that provides
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it or requires it. The `mesh-broker` seat delivers `mesh-bus`, and its holder is the module that
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provides `mesh-bus` — today the nats module, and only it.
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**The old broker's module and the two modules that required it leave the catalogue.** They are
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removed, not converted: one was a proof that a grant worked end to end, the other forwards mail off a
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queue, and both are re-done against the bus if wanted, as new modules under this record.
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**The controller's AMQP transport is deleted once every node reports on the new bus**, and the
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switch that selects a transport goes with it — one bus, so nothing to select.
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The predecessor's own broker is outside the mesh and not this record's concern
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([ADR 0130](0130-the-predecessor-is-ending-and-its-broker-goes-with-it.md)): what the predecessor's
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tooling loses when it stops is accepted there.
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## Options considered
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1. **Keep 0127: AMQP stays an ordinary provision with the old broker as its provider.** Rejected. It
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is what put the protocol into the seat's contract, it is why the seat could be left with no valid
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holder mid-change, and it keeps two transports in the control plane indefinitely for the benefit of
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two modules that did not want AMQP in the first place.
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2. **Bridge it: the old broker's module also provides `mesh-bus`, so both can hold the seat during the
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change.** Rejected. It makes the retiring broker a legitimate mesh bus for exactly as long as
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nobody removes the line, which in practice is forever, and it leaves `amqp` as a thing the core
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still knows the name of.
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3. **The seat is the dependency; the protocol is nobody's business but the holder's.** Adopted.
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## Consequences
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- **The change of holder is a handover, and it needs a command.** Nothing today moves a seat from
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one assignment to another as one act, and a seat the control plane dereferences cannot be empty
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in between — that emptiness is the outage this record comes from. The command takes a seat and the
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assignment taking it over. Designed and built before the cutover, under
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[28 — Building the bus](../03-DESIGN/01-to-be/28-building-the-bus.md).
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- **The seat's row moves to `mesh-bus` before the new holder registers, and that is safe.** The
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control plane composes its own bus address through the seat *by name*
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(`${seat:mesh-broker:…}`), and the overview derives holders by name; only registration and the
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provision-to-seat resolution read what a seat delivers. So the row can change under the current
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holder without unseating it, the new holder can then register its claim, and the handover happens
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when both are running. Verified in the code during the outage, not assumed.
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- **Registration gains two refusals**: a manifest providing `amqp`, and one requiring it.
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- **The `rollout check` stops saying the old broker stays.** It said so under 0127; it now lists
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unassigning it as the last step of the move.
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- **What got harder**: a third party that genuinely wants an AMQP broker on a mesh node runs one as
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any application module, with no provision and no seat, and nothing on the mesh routes to it. That
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is the cost of the mesh not knowing the word.
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## How this is checked
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| Rule | Checked by |
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|---|---|
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| No manifest provides or requires `amqp` | a registration test refusing each, naming this record; and a whole-catalogue test asserting no registered manifest names it |
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| `mesh-broker` delivers `mesh-bus`, and only a `mesh-bus` provider may hold it | the existing registration test for a delivering seat, with the row's value read from the store (mesh-controller#89) |
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| The seat's row can change without unseating the holder | a test composing the control plane's own address and the overview under a row that the current holder does not satisfy |
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| The rollout does not leave the old broker running | `rollout check` output, asserted in its test |
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| The AMQP transport is gone | the package does not compile with it referenced; the switch variable is refused as unknown at start |
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@@ -135,10 +135,11 @@ python3 00-META/checks/index.py fail if stale
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- **0116** — [The bus is built in five steps, and the protocol moves with it](0116-the-bus-is-built-in-five-steps.md)
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- **0119** — [A taken tunnel's predecessor is retired once the take is proven](0119-a-taken-tunnels-predecessor-is-retired.md)
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- **0125** — [The bus is the only broker](0125-the-bus-is-the-only-broker.md) *(superseded)*
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- **0127** — [AMQP is a provision, not the bus](0127-amqp-is-a-provision-not-the-bus.md)
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- **0127** — [AMQP is a provision, not the bus](0127-amqp-is-a-provision-not-the-bus.md) *(superseded)*
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- **0128** — [The mesh bus is required, not ambient](0128-the-mesh-bus-is-required-not-ambient.md)
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- **0129** — [A seat carries the protocol of its role](0129-a-seat-carries-the-protocol-of-its-role.md)
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- **0130** — [The predecessor is ending, and its broker goes with it](0130-the-predecessor-is-ending-and-its-broker-goes-with-it.md)
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- **0131** — [Everything on the mesh speaks to the broker seat, and AMQP is not a provision](0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)
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### Its tiers, from the bottom up
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