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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.
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layer, status, code, updated, decisions
| layer | status | code | updated | decisions | |||
|---|---|---|---|---|---|---|---|
| to-be | designed | 2026-09-25 |
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23 — Choosing a provider
A provision is named for what the consumer's code is coupled to
(ADR 0027):
postgres-database, not database. That decides what kind of provider satisfies a requirement.
It does not decide which provider, and the mesh runs more than one of most kinds.
Why there is a choice at all
Node-specific services delivered to the mesh is the design, not an exception. Every control-capable node runs its own relational store, its own cache, its own object store; a single node may run several relational stores, each raised by the module that needed a particular engine or version. The one provision that is single today — the identity provider — already serves applications whose home is another node. So for a given provision name there are usually several providers, one per node, and they are not interchangeable: each holds different data and lives in a different place. A consumer bound to the wrong one reads the wrong database or takes a network hop it did not need.
Naming the kind is therefore only half of "how a consumer gets what it needs". The other half is which provider, and it has two shapes: consume a provider, or carry your own.
Consuming a provider
A provider is not a mesh-wide singleton. It is identified by the node it runs on together with the module that provides it — a (node, module) pair. A consumer's requirement resolves to one such provider, and which one is part of the assignment, not the manifest (ADR 0046): the same module, assigned twice, may be served by two different providers.
The default is co-location. A consumer that names no provider is served by the provider of that provision on its own node. This is the ordinary case and is meant to need nothing said — a module that wants a database wants, almost always, the database on the machine it runs on. A mesh that happens to run exactly one provider of a kind is simply the case where co-location and "the only one there is" name the same thing; that is there happens to be one, not a mesh-wide scope written into the provision.
Coupling to data is named. The exception to co-location is a consumer coupled to a particular provider's contents: two modules that must share one database, or a consumer that must reach a provider on a different node. That coupling is exactly what may not be guessed, so the assignment names the provider. Naming it is also what makes a later move safe — the mesh knows the binding is to that provider and not to whichever one is nearest.
A seat names the mesh's one provider of a kind. Where a seat delivers the provision, its holder answers for it when several providers exist and the consumer named none. That is not picking: the choice was made once, mesh-wide, by assigning the holder, rather than once per consumer by naming it (ADR 0118 (superseding ADR 0110), 26 — The seats). A named provider still wins over the seat, because a consumer coupled to particular contents has said so.
Ambiguity is refused, never resolved by picking. If several providers of a kind exist, none is named, none is co-located, and no seat delivers it, the requirement is unsatisfiable and is refused with the candidates shown — the same stance ADR 0027 took against a confidently-wrong match, applied to the instance rather than the dialect. A wrong answer delivered quietly costs more than a refusal.
Carrying your own
A module need not consume a provider at all. It may carry its own instance of an engine inside its own composition — reachable only on the module's own network, publishing no host port, and not declared as a provision. Nothing else in the mesh can see it or bind to it, and it cannot collide with anything on a well-known port. To resolution it does not exist; it is an internal part of the module, like any other container the module runs.
This is legitimate but it is the exception, and the design says when: only when a genuine engine fork or a pinned server version makes the shared provider unusable. A module written against a customised engine, or one that needs an extension the node's provider does not carry, has no choice but to carry its own. A module that merely pins an old image of an ordinary engine does not — the version on a compose file is the server's, and the application talks to a newer shared server perfectly well once its data is migrated in. The rule is share by default; embed only when a fork or a version forces it. Most of the per-module stores that exist in the mesh being migrated onto are the first kind wearing the second's clothes, and consolidate onto the node's provider.
The distinction is worth stating because the two cases look identical from outside — a module with a database either way — and the mesh must be able to tell them apart to reason about either. A consumed provider is a binding the mesh records, rotates and can move. An embedded instance is a private detail the mesh does not manage and must not mistake for a provider.
What is not settled here
A provider that moves between nodes must keep its identity, so that a consumer's recorded choice does not silently rebind to a different provider that inherited its place. That is a property the provider lifecycle must supply, and this document names it as a requirement rather than describing its mechanism.