hq/03-DESIGN/01-to-be/27-a-module-requires-the-mesh-resolves.md

---
layer: to-be
status: proposed
code: []
updated: 2026-09-25
decisions:
  - 02-DECISIONS/0112-a-module-definition-names-no-node-mesh-or-path.md
  - 02-DECISIONS/0113-a-provider-makes-what-it-provides-and-the-mesh-carries-it.md
  - 02-DECISIONS/0110-a-seat-is-a-module-assignment-from-a-closed-set.md
  - 02-DECISIONS/0111-a-build-source-is-on-the-git-seat-or-external.md
  - 02-DECISIONS/0084-which-provider-serves-a-consumer.md
  - 02-DECISIONS/0046-a-module-configuration-is-its-assignments-not-its-manifest.md
  - 02-DECISIONS/0038-the-mesh-assigns-the-port.md
---

# 27 — A module requires, the mesh resolves

**One concept for everything a module needs.** A module definition states what it requires. Each
requirement has a contract and a kind of provider. Installing the module on a node resolves every
requirement, or refuses and says why. Nothing else reaches a module: no path it chose, no setting
beside the model, no literal it carries.

This replaces six mechanisms that grew separately: provisions read through bindings, settings,
assigned ports, machine facts, minted or accepted secrets, and literals in the definition. Each
resolved, validated and failed in its own way ([ADR 0112](../../02-DECISIONS/0112-a-module-definition-names-no-node-mesh-or-path.md),
[issue 118](../../04-ISSUES/118-a-module-definition-decides-where-its-files-live/00-report.md)).

## A requirement

A requirement has three parts:

| part | is |
|---|---|
| name | what the module calls it, unique within the module |
| contract | the fields the module may read, and what each promises: a type, whether it is secret, and anything the provider must honour |
| provider kind | which of the four kinds of provider answers it |

**A contract is shared, not per module.** A database's contract is the database's, whoever requires
it. The mesh knows each contract, and a provider is checked against the one it claims to answer. A
module's own specification may narrow a contract (a password of at least this length, a directory
owned by this user) and never widen it.

## The four kinds of provider

The set is closed, like the seats. A fifth kind is a decision, because each kind is a place an answer
can come from and a reviewer has to know every one.

| provider | answers | resolved by | replaces |
|---|---|---|---|
| **a module** | a database, a bucket, a vhost, a route, a secret | the rule below | provisions and bindings |
| **the node's host** | a directory, a port, a fact about the machine | always the module's own node | resource paths, assigned ports, machine placeholders, facts |
| **the mesh** | the module's identity and names | the controller | derived logins, generated names |
| **the operator** | a value a person chooses | the assignment, else the requirement's default | settings, carried literals |

### A module provider

Which module answers, in order:

1. **a pin**: the assignment names a provider, because this consumer is coupled to that provider's
   contents ([ADR 0084](../../02-DECISIONS/0084-which-provider-serves-a-consumer.md));
2. **the holder of a seat** that delivers the provision, where one does. Co-location does not apply
   to these: the seat is the mesh's one answer for everyone ([ADR 0110](../../02-DECISIONS/0110-a-seat-is-a-module-assignment-from-a-closed-set.md),
   [26 — The seats](26-the-seats.md));
3. **the provider on the consumer's own node**, for a provision no seat delivers;
4. **the only provider** in the mesh;
5. otherwise **refused**, naming the candidates, or the unheld seat.

The provider makes what it provides and answers with its contract's fields. The mesh carries the
answer back to the consumer, sealing every secret field to the consumer's node
([ADR 0113](../../02-DECISIONS/0113-a-provider-makes-what-it-provides-and-the-mesh-carries-it.md)). The vault
is a module provider like any other: it holds the `mesh-vault` seat and generates the secrets it
provides.

### The node's host

The host answers what only a machine can: where a directory is, which port is free, what the machine
is. It is always the module's own node, because none of these means anything elsewhere.

**A directory.** The contract is an owner and a mode, and the owner the image expects where it has
one. There is no persistence flag. A directory is kept while it holds anything, and data that may be
lost is a named volume, not a directory ([ADR 0030](../../02-DECISIONS/0030-data-outlives-the-mesh-that-declared-it.md),
[ADR 0107](../../02-DECISIONS/0107-persistent-data-is-a-directory-bind-never-a-named-volume.md)).

*Where* a directory is on the machine is the assignment's:

- **a node's default layout**, a root per node with one directory per instance beneath it, used when
  the assignment says nothing;
- **a placement**, where the assignment puts one directory elsewhere: on a second disk, or where an
  adopted machine's data already is ([ADR 0100](../../02-DECISIONS/0100-a-node-in-use-is-adopted-before-it-is-converged.md)).

**An operator's shared data** is an access, as before ([ADR 0051](../../02-DECISIONS/0051-shared-data-is-the-operators.md)):
never created, owned or removed by the mesh. The module requires read or read-write access. Where
the data is, is an operator value on the assignment.

**A port** is what [ADR 0038](../../02-DECISIONS/0038-the-mesh-assigns-the-port.md) already decided: the
module says which port its software uses, and the host answers with where the machine put it.

**A fact** is something the machine knows: its name on the private network, the names of the mesh's
machines. Each fact has a contract like anything else.

### The mesh

The mesh answers who the module is: its login, its broker account, the names it is known by. These
are derived by the mesh so every party agrees by construction, and no provider may make them
([ADR 0049](../../02-DECISIONS/0049-a-consumers-identity-fits-the-tightest-backend.md)).

### The operator

A value a person chooses: a public name for an endpoint, a greeting, how many workers to run.

**It must stay cheap.** An operator requirement's contract is a type and, optionally, a default. It
needs no provider module, no grant and no credential. If asking a person for a value took more than
that, module authors would route around it, and the literals this replaces would come back.

**A secret operator value**, such as an external API key, is held by the vault as an
operator-delivered value ([ADR 0092](../../02-DECISIONS/0092-an-operator-delivers-a-pair-credential.md)),
never stored as a setting.

**An endpoint** is an operator value inside a route requirement: the public name is chosen on the
assignment, and the route provider answers. A public name already held by another assignment is
refused, like any other singular thing.

## How a definition reads what was resolved

**One form, naming a requirement and a field of its contract.** A definition that needs the database's
host in an environment variable, the directory's location on the host side of a mount, or the public
name in a configuration file writes the same thing: the requirement's name and the field. The
controller fills it at resolution.

This one form replaces the placeholders that exist today, one per mechanism: bound values, secrets,
ports, machine facts and seats. The seat placeholder the controller uses to reach its own foundation
stays, because the controller cannot be a consumer of a foundation it made before any module
existed. It is the controller's own and no module uses it.

**A secret field reaches a process as a file**, as today ([ADR 0086](../../02-DECISIONS/0086-a-secret-reaches-a-process-as-a-file.md)).
Reading one into a plain value, such as an environment variable, is refused when the definition is
parsed, unless the definition declares the exception 0086 allows, with its reason.

## An instance

An assignment has an identity: an **instance name**, which defaults to the module's name. Everything
keyed by the module's name today is keyed by the instance: directories, containers, the login it
presents, its broker account, the seats it holds, its settings and its identity as a provider.

So a module may run twice on one node, under two instance names. What must stay singular stays so:
by a seat, or by an operator value colliding, as with a public name.

**A login still has to fit the tightest backend**, which is twenty characters today
([ADR 0049](../../02-DECISIONS/0049-a-consumers-identity-fits-the-tightest-backend.md)). A node name and
an instance name will not fit in full. The instance therefore gets a short form alongside the module's
slug, under the same rules as a slug. This has to be settled before a second instance is allowed.

## Genesis

The one exception. Before any provider exists, genesis answers the foundation's own requirements
itself: the store's and broker's credentials, the vault's own access, and the root secrets. It seals
them to the operator key as it does now ([ADR 0085](../../02-DECISIONS/0085-a-secret-is-a-provision.md)).
After genesis, nothing is answered except by a provider.

## Refusing

Installation refuses when any requirement is unresolved, and **says everything at once**. For each
requirement it names what is missing and what would answer it:

- an unheld seat, and which modules could hold it;
- no provider, and which modules could provide it;
- an operator value with no default, and that the assignment must give it;
- a provider that has not answered yet, and which one.

The last one is a state, not a failure. A consumer waiting for its provider is shown as waiting, and
nothing is delivered until the answer arrives ([ADR 0113](../../02-DECISIONS/0113-a-provider-makes-what-it-provides-and-the-mesh-carries-it.md)).

## What this retires

| mechanism | becomes |
|---|---|
| provisions read through bindings | a module requirement; its answer is the contract's fields |
| settings on an assignment ([ADR 0046](../../02-DECISIONS/0046-a-module-configuration-is-its-assignments-not-its-manifest.md)) | operator requirements, addressed to an instance |
| a port the mesh assigns | a host requirement |
| machine facts and machine placeholders | host requirements |
| a secret the controller mints | a secret the vault makes ([ADR 0113](../../02-DECISIONS/0113-a-provider-makes-what-it-provides-and-the-mesh-carries-it.md)) |
| paths in resources, mounts, bindings, secrets and received files | host directory requirements, placed by the assignment |
| literals carried in a definition | operator requirements with defaults |

Each is retired only once nothing uses it. Until then both are accepted, and a catalogue test lists
the definitions still using the old form. That list shrinks to empty, and then the old form is
removed from the parser.

## Phases

Each phase ends at a check that holds, so none of them leaves a mechanism half-replaced.

1. **Resolution and the new form.** The controller resolves requirements from the four providers,
   refuses as above, and fills the one form. Old mechanisms keep working beside it. *Ends when* a
   definition written entirely in the new form installs on a lab machine.
2. **Providers answer.** The SDK harness answers with contract fields, the controller carries answers
   back and seals them, and the vault holds its seat and generates. *Ends when* the analytics and DNS
   providers answer their consumers and the broker's bootstrap step is removed.
3. **Definitions move.** Every catalogue definition is rewritten, adopted and running assignments
   placed where their data already is. *Ends when* the list of definitions using an old form is
   empty, and the old forms are removed.
4. **Instances.** The instance name and its short form, and everything keyed by it. *Ends when* one
   module runs twice on one lab machine with two public names.

## How it is checked

| Rule | Checked by |
|---|---|
| Every requirement has one of the four provider kinds | The parser refuses any other. |
| A definition names no host path, node or mesh | The catalogue tests of [ADR 0112](../../02-DECISIONS/0112-a-module-definition-names-no-node-mesh-or-path.md). |
| A module provider is chosen by pin, seat, co-location, only one, refusal | Resolution tests for each step, and for a second provider on a consumer's own machine when a seat delivers the provision. |
| A host requirement is answered on its own node | A resolution test placing one elsewhere: refused. |
| An operator value needs no provider | A resolution test: a requirement with a default resolves with no module assigned anywhere. |
| A secret field reaches a process as a file | The parser refuses a secret field read into a plain value, unless the definition declares the 0086 exception with a reason. |
| Refusal names everything at once | A resolution test with three unresolved requirements of different kinds: one refusal naming all three. |
| The old forms retire | The catalogue test listing definitions still using one. It must be empty before a form is removed. |

## Not settled here

- The exact spelling of the one form. It must name a requirement and a field and nothing else.
- The layout a node's default root uses beneath it, beyond one directory per instance.
- Whether a module provider's answer can change without the provider being asked, for example a
  provider moving. The rule so far is that it cannot, and moving is re-resolving.
- A contract registry: where contracts live, and how a new provision gets one. Today contracts are
  implicit in each provider's served fields.