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 resolved at assignment

Context

Issue 118 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.

The mesh has already decided this once, for ports. ADR 0038: the mesh assigns the machine-side port and the module says only what it needs, and three copies became one fact. ADR 0046 made the manifest identity and defaults, and the assignment's settings the configuration. ADR 0084 made which provider serves a consumer part of the assignment. Paths are the largest thing still left in the definition.

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 host paths in definitions, and have the mesh rewrite them per assignment. Rejected. It is string surgery on paths: deciding which strings are machine paths by their shape, which is the inference this repository has refused elsewhere. And the definition would still read as though it decided where things live.

3. A definition names variables, and the assignment resolves them. Chosen.

Decision

A module definition is node-agnostic and mesh-agnostic. It names no node, no mesh and no host path. Everything that makes a running instance this instance is a variable.

Installing a module on a node resolves every variable, or refuses. A refusal names each unresolved variable and what could answer it. Variables are answered from three sources:

1. The assignment's own configuration. Values chosen for this module on this node, carried as settings (ADR 0046). An endpoint binding a public name to a port is one.
2. Provisions, resolved by the mesh against a contract. A database, a bucket, a vhost, the occupant of a seat, another assignment, and a secret from the vault: a module's own password, internal token or external key is a secret provision like any other (ADR 0085). Each comes with the contract the mesh and the module's specification define. Which node answers one is part of the assignment (ADR 0084), so a module may take its database from another node.
3. What the mesh generates or knows. The credential it mints for each provision a module takes, which is how every provision is delivered, the vault's included (ADR 0048); the ports it assigns (ADR 0038); facts about the machine.

A directory is a provision. A module requires one by name, such as its configuration or its data, and the host on the node where the assignment runs answers it. Its contract is what the module needs from it:

• the owner and mode, including the owner the image expects (ADR 0107);
• whether it holds data that outlives the module (ADR 0030) or is disposable.

Where it is on the machine is the assignment's. A node has a default layout, and an assignment may place one directory elsewhere, on a second disk or where an adopted machine's data already is (ADR 0100). An operator's shared data (ADR 0051) is the same provision with the operator as owner: the module says it needs read or read-write access, and the assignment says where the data is. A directory is always answered on the module's own node, because a host path means nothing on any other.

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.

The mesh's own files carry no host path. Bindings, secrets and contributions are named relative to where the module receives them, so a provider reads what it was given without mounting anything 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 it instead: directories, container names, the login a consumer presents, broker accounts, a seat's holder. So one module may be assigned to one node more than once. What must stay singular stays so by a seat, or by the assignment's own configuration colliding: a public name already taken is refused like any other singular thing.

Consequences

• Every definition changes. 70 of 71 name host paths today. The change is mechanical for most and needs the design to say how existing modules migrate without their data moving: an adopted or already-running assignment is placed where its data already is.
• The control plane resolves variables at assignment and refuses unresolved ones. The host answers directory provisions. The contributions file's format changes, and so does the SDK's reconcile loop that reads it.
• Identity moves from the module to the assignment, which touches logins, broker accounts and every resource name.
• What got harder: a definition no longer says where a module's data is on a machine. 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 variable syntax; a node's default layout; whether a second instance of a module is supported from the first step or after the definitions have moved.

How it is checked

Rule	Checked by
A definition names no host path	A catalogue test fails on any absolute path outside the image side of a mount, with a declared list of exceptions that shrinks to empty as definitions move.
Installation resolves every variable	A resolution test with one variable unanswered: refused, naming the variable and its possible sources.
A provider reads what it was given without an identical mount	A provisioner test reading a contributions file whose credentials are named relative to where it is mounted.
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, no collision.

References

• Issue 118: the evidence
• ADR 0038: the same decision, for ports
• ADR 0046: configuration is the assignment's
• ADR 0084: which node answers is the assignment's
• ADR 0085, ADR 0048: a module's own secrets come from the vault; the mesh mints only the delivery credential
• ADR 0051, ADR 0107, ADR 0030: what a directory's contract carries