hq/02-DECISIONS/0112-a-module-definition-names-no-node-mesh-or-path.md

topic, status, date, deciders, reconstructed, extends

topic	status	date	deciders	reconstructed	extends
what runs on it	proposed	2026-09-25	jochen	false	0046-a-module-configuration-is-its-assignments-not-its-manifest.md

112. A module definition names no node, no mesh and no path: everything it needs is a requirement the mesh resolves

Context

Issue 119 found 789 host-path strings in 70 of the catalogue's 71 module definitions. Each definition chooses where on the machine its directories, mounts, bindings, secrets, env-files and received files live, and often repeats that path in an environment variable or in code. Mounts are checked against what the definition declares (ADR 0091); nothing checks the copies. The issue records what that has already allowed:

• a provider that would provision nobody without a word;
• a contributions file that carries host paths into containers, so every provider must mount its grants directory at the identical path;
• defaults in code that disagree with their own manifests;
• no way to assign one module to one node twice, because every identity is keyed by the module's name.

Paths are one case of a wider pattern. A module gets what it needs through at least six separate mechanisms today, each with its own syntax and its own failure modes:

• provisions, read through bindings;
• settings on the assignment (ADR 0046);
• ports the mesh assigns (ADR 0038);
• machine facts a manifest asks for;
• secrets, either minted or accepted from an operator;
• literals carried in the definition itself.

The mesh has already unified parts of this. Ports became the mesh's rather than the module's (0038), configuration became the assignment's (0046), and which provider serves a consumer became the assignment's choice (ADR 0084). What remains is the concept that joins them.

Considered Options

1. Keep host paths in definitions, and check that every copy agrees. Rejected. It checks the agreement of something that should not be there. A definition still could not follow its data to another disk, be adopted onto a machine whose data is already somewhere, or run twice on one node.

2. Keep the separate mechanisms, and add directories as a seventh. Rejected. It fixes paths and keeps the pattern that produced them: each mechanism is resolved, validated and refused differently, so a module author learns six systems and a reviewer checks six kinds of gap.

3. One concept: a module requires, and the mesh resolves every requirement against a contract. Chosen.

Decision

A module definition is node-agnostic and mesh-agnostic. It names no node, no mesh and no host path. Everything a module needs is a requirement: a name, a contract saying what the module may read from it, and which kind of provider answers it.

Installing a module on a node resolves every requirement, or refuses. A refusal names each unresolved requirement and what could answer it, all at once.

There are four kinds of provider, and the set is closed:

provider	answers	today's mechanism it replaces
another module	a database, a bucket, a vhost, a secret, a route	provisions and bindings
the node's host	a directory, a port, facts about the machine	resource paths, ${port:}, ${machine:}, facts
the mesh	the module's identity and names, and the delivery of every answer	derived logins and generated names; the controller's delivery
the operator, through the assignment	a value a person chooses that is not secret: a public name, a greeting, a number of workers	settings, carried literals

A module provider is chosen as ADR 0084 and ADR 0110 say: a pin, then the holder of a seat that delivers the provision; for a provision no seat delivers, co-location and then the only one. A host provider is always the module's own node, because a host path or a port means nothing on any other. An operator value is the assignment's, or the requirement's default, or unresolved.

A person's value stays cheap. An operator requirement's contract is a type and, optionally, a default. It needs no provider module, no grant and no credential.

Every secret is a secret requirement, answered by the vault, with no exception by kind (ADR 0113). An external API key an operator chooses is no different: the operator delivers it to the vault (ADR 0092), and the module requires a secret like any other. A provider that needs a secret for a consumer requires it from the vault, like any consumer, and answers with resources and data. The mesh carries every answer back.

A directory is a host provision. Its contract is the owner and mode the module needs, including the owner its image expects (ADR 0107). It carries no persistence flag. A directory is kept while it holds anything (ADR 0030, which refused a keep flag for good reason), and data that is disposable is not a directory but a named volume (0107). Where it is on the machine is the assignment's. A node has a default layout, and an assignment may place a directory elsewhere: on a second disk, or where an adopted machine's data already is (ADR 0100).

An operator's shared data stays an access (ADR 0051), not a directory. 0051 rejected giving a directory an operator owner, because the mesh must never create, chown or remove such data, and that stands. Only where its location is written changes: the module requires read or read-write access, and the assignment says where the data is.

Inside a container, a module sees its own paths. The definition says where the image expects each directory. The mesh mounts the assignment's location there. No host path is ever a value a process reads, and the mesh's own files (answers, contributions) name nothing by host path, so a provider needs no mount at a machine-identical path.

An assignment has an identity of its own: an instance name, defaulting to the module's name. Everything keyed by the module's name today is keyed by the instance instead: directories, container names, the login a consumer presents, broker accounts, a claim's holder, the settings an assignment carries, and a provider's identity. So one module may be assigned to one node more than once. What must stay singular stays so by a claim, or by an operator value colliding: a public name already taken is refused like any other singular thing.

Genesis is not an exception. It raises the vault first and asks it for the foundation's secrets, so the foundation's requirements are answered the same way as everything else (ADR 0113).

What this changes in earlier records

On acceptance, each of these is amended by a record of its own, not edited:

• ADR 0046: settings become operator requirements, addressed to an instance rather than to a module on a node.
• ADR 0038: a port becomes a host requirement. What 0038 decided is unchanged; it is the first case of this rule.
• ADR 0051: an access keeps its shape and its semantics; its path moves from the definition to the assignment.
• ADR 0091: a mount's host side is a resolved requirement, checked as resolved rather than as a path the definition declares.
• ADR 0084: a provider is a (node, instance) pair, not a (node, module) pair, so a consumer can name one of two instances on one node.
• ADR 0049: a login is built from the instance, which gets a short form under the same rules as a slug, so it still fits the tightest backend.
• To-be 26: a seat's holder is an instance.
• The glossary: provision widens from "a service one module provides" to a requirement answered by any of the four providers, and instance is added. Neither lands while this record is only proposed, because the glossary is the authority on the words in use, not on words under review.

Consequences

• Every definition changes. 70 of 71 name host paths today, and most use at least three of the mechanisms this replaces. The change is mechanical for most. The design has to say how existing modules move without their data moving: an adopted or already-running assignment is placed where its data already is.
• The controller resolves every requirement at assignment and refuses unresolved ones. The host answers directories and ports. The settings, placeholders, facts and bindings that exist today are retired as separate mechanisms, once nothing uses them.
• Identity moves from the module to the instance, which touches logins, broker accounts, settings, provider selection and every resource name. A login already has a 20-character limit (ADR 0049), which a node and an instance name will strain. The design must answer that before a second instance is possible.
• What got harder: a definition no longer says where a module's data is on a machine, or what a setting's value is. The assignment does, and plan shows it. That is the point, and it is also a real loss of at-a-glance legibility, which the overview has to give back.
• Not decided here: the syntax a definition reads a requirement's fields with; a node's default layout; the order in which the mechanisms are retired. To-be 27 proposes all three.

How it is checked

Rule	Checked by
A definition names no host path	A catalogue test: the host side of every mount, and every resource location, is a requirement rather than an absolute path. A declared list of exceptions shrinks to empty as definitions move.
No host path is a value a process reads	A catalogue test: every absolute path in a container's environment or env-files lies on the container side of one of its mounts, or is declared the image's own. A second test finds literal paths in module code used as fallbacks for an environment variable.
A definition names no node and no mesh	The parser has no field that names a node; a node is named only in an assignment. A catalogue test finds no domain name in any definition value.
Every requirement has one of the four providers	The parser refuses a requirement whose provider kind is not one of the four.
Installation resolves every requirement	A resolution test with one requirement unanswered: refused, naming it and what could answer it.
A host requirement is answered on its module's own node	A resolution test: an assignment placing a directory or a port on another node is refused.
A module can run twice on one node	A resolution test assigning one module twice under two instance names: two directories, two logins, two containers, separate settings, no collision.
A public name already taken is refused	A resolution test: a second instance asking for a public name the first holds is refused, naming the first.
An adopted assignment is placed where its data is	An adoption test: the directory resolves to the data's existing location, and nothing is moved.

References

• Issue 119: the evidence
• ADR 0038, ADR 0046, ADR 0084: the parts already unified
• ADR 0110: which module provider answers
• ADR 0113: the vault makes every secret, and how answers travel
• ADR 0092: the operator as a provider
• ADR 0051, ADR 0107, ADR 0030: what a directory's contract carries, and what it must not