Merge pull request 'ADR 0135: a module version prepares its state before it runs' (#161) from decision/0133-0134-migrations-and-deploy-facts into main

2026-09-28 09:57:47 +00:00
parent 74ba3ff1d4 476cda417d
commit 017b1401e4
5 changed files with 175 additions and 17 deletions
@@ -1,10 +1,11 @@
 ---
 topic: what runs on it
-status: accepted
+status: superseded
 date: 2026-09-28
 deciders: jochen
 reconstructed: false
 extends: 02-DECISIONS/0052-a-step-that-runs-once-before-a-container.md
+superseded-by: 02-DECISIONS/0135-a-module-version-prepares-its-state-before-it-runs.md
 ---

 # 133. A module owns its migrations, and the mesh owns when they run
@@ -0,0 +1,153 @@
+---
+topic: what runs on it
+status: accepted
+date: 2026-09-28
+deciders: jochen
+reconstructed: false
+supersedes: 0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md
+---
+
+# 135. A module version prepares its state before it runs
+
+## Context
+
+[ADR 0133](0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md) settled who runs a
+module's migrations and when, and it said so in the wrong vocabulary. It put the declaration on a
+*container* — "a container may declare steps to run before it" — and derived the scope of the work from
+the *machine*. Both are wrong at the level a module author works at, and the second is wrong on the
+facts.
+
+**A container is one resource kind the host applies.** A module has code, state and a version; whether
+its artifact is an image, a bundle or something later is the mesh's business. The module-facing
+vocabulary for a module's own code already exists and has nothing to do with a container runtime: a
+module declares **entrypoints** — this file is my tools, this file is my provisioner — and the mesh runs
+them. A manifest that says "run this container with these arguments, and here are the volumes and
+environment again" has an author writing down the machine's business twice.
+
+**And the scope is not the machine's to decide, because the mesh already decided what a state is.** A
+consumer is a module *on a machine* (migration 0015, from
+[issue 022](../04-ISSUES/022-one-credential-per-node-per-provision-not-per-module/00-report.md)):
+the mesh derives a login per consumer and the provider creates a database owned by exactly that login
+([ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md)). So a module on three machines is three
+consumers, three credentials and three databases. There is no shared state for two machines to race over,
+and ADR 0133's central caveat — that a module's migrations must take a lock because two machines might
+migrate at once — describes a situation the mesh does not currently produce.
+
+That correction makes the whole "level" question HAL answered with stages disappear: the scope of
+preparation is the scope of the state, and the mesh knows it.
+
+What the earlier record got right and this one keeps: the module owns the work, the mesh owns the moment,
+the gate is the guarantee, migrations stay forward-only, and none of it can be inferred from inside an
+artifact. What produced it also stands — the control plane was replaced with a build carrying a migration,
+nothing applied it, and for three quarters of an hour every build was refused by the store with one line
+that reached only whoever was waiting on a reply
+([issue 133](../04-ISSUES/133-the-control-planes-schema-is-migrated-at-birth-and-never-again/00-report.md)).
+
+## Decision
+
+**A module version declares an entrypoint that prepares its state.** One name in the manifest, in the
+same vocabulary as the entrypoints it already declares for its tools and its provisioner. No container,
+no command line, no environment, no mounts — those are how a machine runs the module's code, and the
+module already said that once.
+
+**The mesh runs it as it runs that module's own code, to completion, in the module's own context.** Every
+binding, credential and setting the module's code would receive, because it *is* the module's code. How a
+machine does that is the host's business and stays there: for an image artifact it is the step
+[ADR 0052](0052-a-step-that-runs-once-before-a-container.md) already defines, and a later kind of artifact
+changes the host, not the manifest.
+
+**Preparation gates the version.** A version whose preparation did not succeed does not run — anywhere.
+Since the rollout already sends machines one at a time and stops at the first that does not take a
+version, a preparation that fails stops the rollout there, leaving every other machine on the version
+that works.
+
+**Preparation is scoped to the state, and the mesh derives that scope.** State the mesh provisions is per
+consumer — a module on a machine — so preparation happens once per consumer. State the module keeps on
+the machine is per machine, which is the same answer. A module that holds an exclusive seat has one of
+itself, so its preparation happens once by definition. No level, no election, no cross-node ordering, and
+no lock obligation invented for a race the mesh does not create.
+
+**Once per version per state.** A version bump attempts preparation once against each state it has; the
+module's own runner decides there is nothing to do, which is what a runner with a version table does
+anyway. A retry after a partial failure runs it again, so the work is the module's to make safe against
+that — the one obligation no design can remove.
+
+**Forward-only and additive.** Preparation runs while the previous version is still serving, so a
+migration that removes or renames what the old code reads breaks the mesh in the window between the two.
+
+**Declared, never inferred.** The control plane cannot see inside an artifact, so a module that ships
+migrations and declares no entrypoint is not refusable at registration. It breaks on its first upgrade,
+and this record says so rather than implying a check that cannot exist.
+
+## Options considered
+
+1. **A container declares steps before it** — [ADR 0133](0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md).
+   Superseded, not because the mechanism is wrong but because the *declaration* is in the wrong place: it
+   makes every module author restate the machine's arrangement, and it ties a module's own lifecycle to
+   one resource kind. The host-side mechanism it named is retained and is now an implementation detail.
+2. **Each module prepares itself when it starts** — what the catalogue does today. Rejected: a schema
+   failure becomes a crash loop rather than a stop, nothing in the declaration says the module has a
+   state to prepare, and the version serves the moment it starts rather than after the state is right.
+3. **The mesh applies migrations itself**, with a driver and a version table per store type. Rejected:
+   the mesh would have to know one store from another, hold another module's credentials and reach a
+   machine with them, which [ADR 0005](0005-the-node-host.md) forbids. It is also what forces a stage
+   system: something central has to decide where the work happens.
+4. **A hook lifecycle** — pre-build, post-build, pre-deploy, post-deploy. Rejected: a declaration is a
+   desired state reconciled forever, so there is no deploy moment to hook. "Pre-deploy" is exactly this
+   record; pre- and post-build are what a recipe and the artifact list already are; "post-deploy" names
+   nothing that happens.
+5. **A declared level** — once per module, or once per assignment. Rejected: the mesh already knows what a
+   state is, so asking an author to choose is asking them to restate a fact the mesh holds, with a chance
+   of contradicting it.
+6. **Record a preparation level per module in the store.** Rejected for the reason ADR 0133 gave and this
+   record keeps: gating makes the invariant true by construction, and a level is a second account of the
+   same fact.
+
+## Consequences
+
+**An author's whole contract is one line, once.** Write the migration in the module's code, name the
+entrypoint that runs it, and every later version rolls out as: build, prepare, run — with nothing
+per-version to remember and nothing about the machine to restate. That is the property this exists for.
+
+**Three hand-written steps in the catalogue collapse**, and the control plane's own migrate step stops
+repeating its server's environment and mounts.
+
+**The catalogue's self-preparation becomes the exception to remove.** One shape, and the mesh's own
+control plane is not an exception either.
+
+**A module scaled across machines with one shared state is not expressible**, and this record does not
+make it so. The mesh gives each consumer its own state; a deliberately shared one is a different
+provision model, and the place the "once, mesh-wide" question would genuinely return. Named here so it is
+a decision when it happens rather than a surprise.
+
+**There is still no readiness-gated step.** Only an action carries `verify`; nothing declares that a
+service answers, so preparation that must happen *after* something is serving — seeding through its own
+API — remains unexpressible.
+
+**Genesis keeps its own action.** At birth there is no control plane to derive anything, which is what
+[ADR 0067](0067-genesis-is-a-pivot.md) says about that moment.
+
+## How this is checked
+
+- **The composition carries the preparation, in the module's own context.** A test on a node's composed
+  declaration: a version declaring a preparation entrypoint is preceded by it, and what it is given
+  equals what the module's own code is given — asserted equal rather than written twice, which is the
+  drift the superseded shape invited.
+- **A preparation that fails stops the version.** The host does not go past a step that did not complete,
+  and the rollout stops at the first machine that did not take a version. Both are existing behaviours
+  with existing tests; the test for preparation asserts the two together — the machine does not run it,
+  and the machines after it are left alone.
+- **Once per version per state.** A test that a second convergence of the same version prepares nothing,
+  and that a new version prepares again.
+- **The mesh's own control plane declares one.** The case that failed on 2026-09-28 is the case the tests
+  cover, rather than a case a comment says is covered.
+
+## References
+
+- [ADR 0133](0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md) — what this supersedes, and why
+- [ADR 0052](0052-a-step-that-runs-once-before-a-container.md) — the host-side step that implements it for an image artifact
+- [ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md), [issue 022](../04-ISSUES/022-one-credential-per-node-per-provision-not-per-module/00-report.md) — a consumer is a module on a machine, which is what makes the scope derivable
+- [ADR 0018](0018-a-picture-is-read-from-what-runs.md) — a digest is the record that something happened
+- [ADR 0005](0005-the-node-host.md) — the control plane decides and never touches a machine
+- [ADR 0134](0134-the-mesh-says-what-it-applied.md) — what makes a failed preparation visible
+- [issue 133](../04-ISSUES/133-the-control-planes-schema-is-migrated-at-birth-and-never-again/00-report.md) — the failure that produced both records
@@ -214,7 +214,8 @@ python3 00-META/checks/index.py            fail if stale
 - **0120** — [A roster fact carries its format as a template: the mesh owns the data, the module owns the format](0120-a-roster-fact-carries-its-format-as-a-template.md)
 - **0121** — [A system seat is named for its scope, and a module may define its own](0121-a-system-seat-is-named-for-its-scope-and-modules-define-their-own.md)
 - **0122** — [A seat is data the controller owns, and a rename is a database update](0122-a-seat-is-data-a-rename-is-a-database-update.md)
- **0133** — [A module owns its migrations, and the mesh owns when they run](0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md)
+- **0133** — [A module owns its migrations, and the mesh owns when they run](0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md)  *(superseded)*
+- **0135** — [A module version prepares its state before it runs](0135-a-module-version-prepares-its-state-before-it-runs.md)

 ### How it is built

@@ -12,7 +12,7 @@ decisions:
  - 02-DECISIONS/0112-a-module-definition-names-no-node-mesh-or-path.md
  - 02-DECISIONS/0083-one-push-leaves-the-mesh-consistent.md
  - 02-DECISIONS/0129-a-seat-carries-the-protocol-of-its-role.md
-  - 02-DECISIONS/0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md
+  - 02-DECISIONS/0135-a-module-version-prepares-its-state-before-it-runs.md
  - 02-DECISIONS/0134-the-mesh-says-what-it-applied.md
 ---

@@ -260,13 +260,15 @@ queue.
 and publishes it last-per-subject. A node that was away gets exactly the current one, never a
 queue of superseded ones, and a replayed older one is refused by sequence.

-**A version that needs its store prepared prepares it first.** A container may declare steps to run
-before it — the same container, to completion, with different arguments — and the mesh derives them as
-run-once resources placed ahead of it, so a schema change and the code that needs it arrive together or
-not at all. The module owns what the step does; the mesh owns when it runs, and refuses to start the
-container if it failed ([ADR 0133](../../02-DECISIONS/0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md)).
-Per node, because a node converges without waiting on its neighbours; a step that must happen once
-mesh-wide belongs to a module holding a seat, which is what one holder on record already means.
+**A version prepares its state before it runs.** A module version may declare an entrypoint that brings
+its state to the shape that version needs — the same vocabulary as the entrypoints it declares for its
+tools and its provisioner, and nothing about how a machine runs it. The mesh runs that entrypoint as it
+runs the module's own code, to completion, in the module's own context, and a version whose preparation
+did not succeed does not run: the rollout stops at the first machine that did not take it
+([ADR 0135](../../02-DECISIONS/0135-a-module-version-prepares-its-state-before-it-runs.md), superseding
+[ADR 0133](../../02-DECISIONS/0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md)).
+Once per state, and the mesh derives what a state is: a consumer is a module on a machine, so what the
+mesh provisions is per consumer and preparation is too. No level to choose, and no race to lock against.

 **Applying is reported to a role.** The host applies and reports to the `mesh-controller` seat —
 not to an address it was given at genesis. Held and retried while the store restarts
@@ -469,9 +471,9 @@ billing existing under that name.

 - **A manifest holds no subject.** A catalogue test: no manifest contains a string matching the
  subject grammar. The rule is worthless if it is followed by convention.
- **A derived step is the container it precedes.** A composition test: the step's image, environment,
-  volumes and network equal that container's, so the two cannot drift — which is the failure the
-  hand-written kind has, three times over in the catalogue today.
+- **A preparation is given what the module is given.** A composition test: what the preparation
+  entrypoint receives equals what the module's own code receives, asserted rather than written twice —
+  which is the drift a hand-written step invites, three times over in the catalogue today.
 - **A convergence that changed nothing says nothing.** Two identical reports, one emitted fact: what is
  guarded against is a fact per minute per machine, which is a stream nobody reads.
 - **Permissions are exactly the three namespaces.** A composition test per module: the derived
@@ -55,10 +55,11 @@ server starts, which means the old binary briefly runs against the new schema. A
 removes or renames something would break the running control plane in the window between the two.

 **A hand-written step is one the next module forgets**, which is why this fix is not where the matter
-ends: [ADR 0133](../../02-DECISIONS/0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md)
-makes the step derived — a container declares what must run before it, and the mesh composes the gated
-resource — so the control plane stops being the only module that had to remember. The same record
-settles the level question HAL answered with stages, and
+ends: [ADR 0135](../../02-DECISIONS/0135-a-module-version-prepares-its-state-before-it-runs.md) makes it
+derived and puts it where an author works: a module version declares an entrypoint that prepares its
+state, and the mesh composes the gated work from it, so the control plane stops being the only module
+that had to remember. That record also settles the level question HAL answered with stages — a consumer
+is a module on a machine, so the scope of preparation is the scope of the state — and
 [ADR 0134](../../02-DECISIONS/0134-the-mesh-says-what-it-applied.md) answers the second open question
 below: what a machine applied, and what it refused, become facts on the bus rather than a line in a log.