--- layer: to-be status: designed code: [] updated: 2026-08-27 decisions: - 02-DECISIONS/0037-the-host-applies-it-does-not-decide.md - 02-DECISIONS/0053-one-control-plane-and-no-failover.md - 02-DECISIONS/0030-the-repository-structure.md - 02-DECISIONS/0015-mesh-brokers-nodes-host-agents-think.md - 02-DECISIONS/0055-the-control-plane-is-the-node-coordinating-contexts.md --- # The control plane Tier 2. The term appears seventy-nine times across this repository and was defined nowhere, which is `how-we-build` §5 failing on this repository's own vocabulary. This document defines it. It does **not** design the contexts inside it; those are open in [research 006](../../01-RESEARCH/006-mesh-from-scratch/00-overview.md). ## The definition > **The control plane is everything that needs to know about more than one node.** That is the whole test, and it is not arbitrary — it follows from [ADR 0037](../../02-DECISIONS/0037-the-host-applies-it-does-not-decide.md). The host applies and does not decide *because deciding needs knowledge the machine does not have*. So the line falls exactly there: | Question | Whose | |---|---| | write this file, with this content, with this mode | the **host** — one machine | | which nodes should run the store | the **control plane** — needs every node | | is this unit running | the **host** — one machine | | which peers belong in this node's overlay | the **control plane** — needs every node | | what does this machine have installed | the **host** reports; the control plane **records** | | has this node been unreachable for a week | the **control plane** — nobody else is watching | A useful consequence: **anything a single machine could answer alone is not the control plane's.** If it needs no second node, putting it here is a mistake, and the tier rule will not catch it because the dependency direction is still correct. ## What is inside it **Seven contexts and one interface** ([ADR 0055](../../02-DECISIONS/0055-the-control-plane-is-the-node-coordinating-contexts.md)) — each one earning its place by the test above rather than by being ours: | | | needs to know about more than one node because | |---|---|---| | **inventory** | nodes, modules, assignments, versions | that *is* the mesh-wide fact | | **config** | settings, secrets, and deriving them onto nodes | it derives **onto nodes** | | **connectivity** | overlay, resolution, exposure, filtering, certificates — **specified in full in [`08-connectivity.md`](08-connectivity.md)** | who peers with whom; which node is reachable | | **provisioning** | resource grants between modules | consumer and provider may be on different nodes | | **delivery** | source to artifact to node | it targets nodes | | **observability** | health, logs, metrics, alerts | *unreachable for a week* is nobody else's to notice | | **identity** | agents, humans, services, authorisation | credentials follow an agent's node bindings and modality | | **api** | the one interface every surface speaks to | — it is an interface, not a context | **What is deliberately not here.** `work`, `knowledge` and `stream` are **mesh-hosted applications** — first-party, shipped with everything else, and running on the mesh the way anything else does. A task does not need to know a node exists, and *being ours does not make something infrastructure*. `ai` is folded into `config`: a provider licence is an ordinary grant. **`record` is an open question rather than an eighth entry.** Contexts integrate through it ([ADR 0045](../../02-DECISIONS/0045-a-context-owns-its-store.md)), which makes it load-bearing, and [research 006](../../01-RESEARCH/006-mesh-from-scratch/skeleton.md) leaves *where it lives* unresolved — putting it in the substrate risks recreating the circularity the tier design just removed. Listing it here would settle by naming what has not been settled by arguing. **These are contexts, not services.** They are separate in the sense that matters — each owns its own store, and they integrate through the record rather than by reading one another ([`how-we-build`](../../00-META/how-we-build.md) §4). They are not separate deployables, and [research 011](../../01-RESEARCH/011-the-module-graph/00-overview.md) records why that constraint is load-bearing: a single surface can compose them only while there is one interface in front of them. ## What it is not - **Not the thing that changes machines.** It decides; the host applies. It never reaches into a node except through the host. - **Not a surface.** Tier 3 is how people and agents reach it. It has one interface; the surfaces are what speak to that interface. - **Not the substrate.** It *runs on* tier 1 — PostgreSQL, LavinMQ, MinIO, an OCI registry ([ADR 0048](../../02-DECISIONS/0048-the-substrate-is-named.md)) — and cannot start without them, which is what makes them a lower tier. - **Not privileged on a node.** It has no more access to a machine than the declaration vocabulary allows ([ADR 0039](../../02-DECISIONS/0039-the-link-is-the-security-boundary.md)). ## It is also a consumer The property that makes tier 2 unlike the others: **the control plane has requirements of its own.** It needs a PostgreSQL database, an AMQP virtual host, and a bucket — the same things any module needs, granted the same way. That is the circularity the tiers exist to resolve rather than hide: the control plane cannot provision its own database, because it is not running yet. So its **store** is raised from the bundle the host carries, before there is a control plane to ask ([ADR 0038](../../02-DECISIONS/0038-a-node-joins-by-linking-first.md), [research 011](../../01-RESEARCH/011-the-module-graph/worked-provider.md)). Its virtual host and its bucket are **not** in the bundle — by the time they are wanted there is a control plane to grant them. Whether the bus must come first is [open](07-the-substrate.md#open), and it turns on whether these contexts talk to each other over it. ## Where it runs **On nodes, like anything else.** It is not a place outside the mesh; it is modules the mesh hosts, assigned to nodes by the same mechanism as everything else. **One node runs it, and nothing takes over** ([ADR 0053](../../02-DECISIONS/0053-one-control-plane-and-no-failover.md)). The node is assigned, never elected — no promotion, no quorum, no split brain. That is sound rather than merely cheap, because the design already tolerates the control plane being absent by construction: a node reconciles from **its own** store ([ADR 0043](../../02-DECISIONS/0043-a-declaration-is-an-ordered-list-of-owned-resources.md)) and never needed to ask anybody to hold the state it was last given. So the control plane being down is not a new failure mode — it is [ADR 0036](../../02-DECISIONS/0036-a-node-is-a-managed-machine.md)'s ordinary disconnected situation, happening to every node at once. **What is lost is change, not operation.** The honest half: this node is a single point of failure, recovery is restore rather than failover, and **certificate renewal is the clock** — an outage outlasting a renewal window expires every public name. ## Open - ~~**The contexts themselves.**~~ **Decided** — seven, by [ADR 0055](../../02-DECISIONS/0055-the-control-plane-is-the-node-coordinating-contexts.md). What remains open is narrower and named there: **where the record lives**, which research 006 leaves unresolved because the substrate is the one place it must not go. - **How far it may be split.** One deployable today. Splitting a context out costs the single interface a surface depends on ([research 011](../../01-RESEARCH/011-the-module-graph/00-overview.md)). - ~~**How many run, and what a node does without one.**~~ **Resolved** by [ADR 0053](../../02-DECISIONS/0053-one-control-plane-and-no-failover.md). What remains is measurement: nothing reports how long the control plane has been unreachable, or how close a certificate is to expiry — both needed for restore-not-failover to be a plan rather than a hope. - **What the interface is.** One interface is stated; its shape, and whether it is request, subscription or both, is not ([research 011](../../01-RESEARCH/011-the-module-graph/worked-provider.md)).