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Issue 118 and ADR 0112 (proposed): a module definition names no node, no mesh and no path #112

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02-DECISIONS/0112-a-module-definition-names-no-node-mesh-or-path.md
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---
topic: what runs on it
status: proposed
date: 2026-09-25
deciders: jochen
reconstructed: false
extends: 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](../04-ISSUES/118-a-module-definition-decides-where-its-files-live/00-report.md) 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](0091-a-mount-is-declared-three-ways.md)); 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](0038-the-mesh-assigns-the-port.md):
the mesh assigns the machine-side port and the module says only what it needs, and three copies
became one fact. [ADR 0046](0046-a-module-configuration-is-its-assignments-not-its-manifest.md)
made the manifest identity and defaults, and the assignment's settings the configuration.
[ADR 0084](0084-which-provider-serves-a-consumer.md) 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](0046-a-module-configuration-is-its-assignments-not-its-manifest.md)). An
endpoint binding a public name to a port is one.
2. **Provisions, resolved by the mesh against a contract.** What other modules provide: a database,
a bucket, a vhost, a secret. Each comes with the contract the mesh and the module's
specification define. Where a provision has several providers, which one answers is part of the
assignment ([ADR 0084](0084-which-provider-serves-a-consumer.md)), so a module may take its
database from another node. Some have one provider per mesh by decision: a module's own secret
is a `secret` provision the controller mints and the vault records
([ADR 0085](0085-a-secret-is-a-provision.md), as amended). Whether the vault should instead
generate a secret against its contract is an open question, raised while reviewing this, and not
decided here.
3. **What the mesh generates or knows.** The credential the controller mints for each provision a
module takes ([ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md)), the ports it
assigns ([ADR 0038](0038-the-mesh-assigns-the-port.md)), facts about the machine.
**A directory is a provision, provided by the node's host.** A module requires one by name, such as
its configuration or its data. Its contract is the owner and mode it needs, including the owner its
image expects ([ADR 0107](0107-persistent-data-is-a-directory-bind-never-a-named-volume.md)). It
carries no persistence flag. A directory is kept while it holds anything
([ADR 0030](0030-data-outlives-the-mesh-that-declared-it.md), which refused a `keep` flag for good
reason), and data that is disposable is not a directory at all but a named volume (0107).
*Where* a directory 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](0100-a-node-in-use-is-adopted-before-it-is-converged.md)). A directory is
always provided on the module's own node, because a host path means nothing on any other.
**An operator's shared data stays an `access`** ([ADR 0051](0051-shared-data-is-the-operators.md)),
not a directory provision. 0051 rejected giving a directory an operator owner, because the mesh
must never create, chown or remove such data, and that stands. What changes is only where its
location is written: the module says it needs 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.
**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 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 the assignment's own configuration
colliding: a public name already taken is refused like any other singular thing.
## What this changes in earlier records
On acceptance, each of these is amended by a record of its own, not edited:
- [ADR 0051](0051-shared-data-is-the-operators.md): an access keeps its shape and its semantics; its
path moves from the definition to the assignment.
- [ADR 0091](0091-a-mount-is-declared-three-ways.md): a mount's host side is a resolved variable,
checked as resolved rather than as a path the definition declares.
- [ADR 0046](0046-a-module-configuration-is-its-assignments-not-its-manifest.md): settings are
addressed to an instance, not to a module on a node.
- [ADR 0084](0084-which-provider-serves-a-consumer.md): a provider is a (node, instance) pair, not a
(node, module) pair, so a consumer can name one of two instances on one node.
- The [glossary](../00-META/glossary.md): *provision* widens from "a service one module provides" to
include a directory the node's host provides, 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. The change is mechanical for most.
The design has to say how existing modules migrate without their data moving: an adopted or
already-running assignment is placed where its data already is.
- The controller resolves variables at assignment and refuses unresolved ones. The host provides
directories. The contributions file's format changes, and so does the SDK's reconcile loop that
reads it.
- Identity moves from the module to the instance, which touches logins, broker accounts, settings,
provider selection 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; whether the vault
generates secrets.
## 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, binding, secret, receives and grants entry, is a variable 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. |
| Installation resolves every variable | A resolution test with one variable unanswered: refused, naming the variable and its possible sources. |
| A directory is provided on its module's own node | A resolution test: an assignment placing a directory on another node is refused. |
| 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, 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 118](../04-ISSUES/118-a-module-definition-decides-where-its-files-live/00-report.md): the evidence
- [ADR 0038](0038-the-mesh-assigns-the-port.md): the same decision, for ports
- [ADR 0046](0046-a-module-configuration-is-its-assignments-not-its-manifest.md): configuration is the assignment's
- [ADR 0084](0084-which-provider-serves-a-consumer.md): which provider answers is the assignment's
- [ADR 0085](0085-a-secret-is-a-provision.md), [ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md):
secrets as a provision, and who mints what
- [ADR 0051](0051-shared-data-is-the-operators.md), [ADR 0107](0107-persistent-data-is-a-directory-bind-never-a-named-volume.md),
[ADR 0030](0030-data-outlives-the-mesh-that-declared-it.md): what a directory's contract carries, and what it must not
02-DECISIONS/README.md
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- **0087** — [A seeded file is created once, and what grows in it is not the mesh's](0087-a-seeded-file-is-created-once.md)
- **0091** — [A mount is declared, and there are three things it can be](0091-a-mount-is-declared-three-ways.md)
- **0099** — [A step that runs once names what it reads, and runs again when it changed](0099-a-step-that-runs-once-names-what-it-reads.md)
- **0112** — [A module definition names no node, no mesh and no path: everything it needs is resolved at assignment](0112-a-module-definition-names-no-node-mesh-or-path.md) *(proposed)*
### How it is built
04-ISSUES/118-a-module-definition-decides-where-its-files-live/00-report.md
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---
status: open
opened: 2026-09-25
located-in: []
fixed-by:
amended-design:
---
# 118 — A module definition decides where its files live on the machine
## What was observed
A review of where module code reads its files turned up a cross-cutting pattern. Every module
definition in the catalogue chooses, in its own manifest, where on the machine its files live.
Counted on the catalogue's `main`, 2026-09-25:
| where in the definition | host-path strings |
|---|---|
| directory and file resources | 257 |
| container mounts, host side | 230 |
| own secrets | 78 |
| bindings | 53 |
| env-files | 50 |
| secrets | 35 |
| container environment | 28 |
| accesses | 21 |
| receives, grants | 24 |
| everything else | 13 |
**789 host-path strings in 70 of the 71 definitions.** Mounts are checked: a container may not
mount a path its module never declared ([ADR 0091](../../02-DECISIONS/0091-a-mount-is-declared-three-ways.md)).
Nothing checks the same path where it is retyped as a value: an environment variable, an env-file
line, a literal in module code.
### Where that has already gone wrong
- **A provider that would provision nobody, silently.** One DNS provider mounts its grants
directory at a short path inside its container, then tells its provisioner to read the
contributions file at the host path, which does not exist in there. Nothing requires the
provision today, so it has not failed yet. When a consumer arrives, it will get no record, and
nobody will be told.
- **The mesh's own wire carries host paths into containers.** Each contribution names its
consumer's credential as "the file on this machine holding that consumer's credential", a host
path computed from the provider's grants directory. So every provider has to mount that directory
at the *identical* path, or it cannot read what it was given. Ten of the eleven providers with a
grants directory do. It is a convention nothing states or checks, and the eleventh is the
provider above.
- **The warning that would have caught it is lost in the SDK.** The controller always writes the
contributions file, even when empty, so a provider can tell "nothing asked" from "never written".
The SDK's reconcile loop treats an unreadable file as empty, and logs nothing.
- **Code carries copies with nothing checking them.** Several modules default a path in code when an
environment variable is unset. Five of those defaults disagree with the value their own manifest
sets. One of them is a host path used inside a container that does not mount it.
### And a module cannot be assigned to one node twice
Everything that identifies a running module is keyed by the module's name: its directories, its
container names, the login it presents to a provider, its broker account. Two assignments of one
module to one node would share every one of them. Assigning the same application twice is an
ordinary need: production beside staging, one site per customer, two instances of one service
configured differently, two stores of one engine.
## Why it matters beyond this instance
A definition that names machine paths is not portable between nodes. It cannot follow data onto a
second disk, or onto a machine being adopted with its data already in place, without editing the
module. It cannot run twice on one node. It keeps every path in two or three places with nothing
checking that they agree. The defects above are what that allows, and each was found by reading,
not by any check.
## Open questions
- Should a definition name any host path at all, or should every location come from the
assignment and the mesh?
- If a directory is something a module *requires* rather than *declares*, what is its contract:
ownership, mode, whether it is kept when the module goes?
- What identifies an assignment, if a module may be assigned to one node more than once?
- What would the contributions file carry instead of host paths, so a provider needs no
identical-path mount?