# ADR-008: Plugin migration ownership (per-plugin chains from the cutover)

- **Status:** ACCEPTED
- **Date:** 2026-07-10
- **Deciders:** cproudlock
- **Supersedes:** the "Migration strategy (resolved)" section of ADR-004

## Context

ADR-004 resolved a Phase 7B footgun by folding every bundled plugin's tables
into the single core Alembic chain (migration `7c04_fold_plugin_schema`), and
later core migrations (`7d04`..`7d16`) kept adding plugin schema directly to the
core chain. At the time this was the safe choice: bundled-plugin baselines and
the core baseline had both been creating the same tables, so
`flask plugin upgrade-all` would collide with `flask db upgrade`.

That resolution left the per-plugin Alembic engine
(`shopdb/plugins/migrations.py`, `shopdb/plugins/alembic_template.py`, the
`alembic_version_<plugin>` version tables, and `flask plugin upgrade-all`) fully
built but unused for the bundled plugins. The framework is the product (per the
project's charter); a plugin that cannot own its own schema is not really a
plugin. Sister sites that adopt or fork a plugin need its schema history to
travel with the plugin, not be entangled in the host's core chain. Keeping every
future plugin table change in the core chain also means the core chain grows
without bound and a plugin can never be cleanly removed.

The blocker ADR-004 worried about (double table creation) only exists while a
plugin's own migration tries to CREATE tables the core chain already created.
That is avoidable: history is immutable, so the tables already built by the core
chain stay owned by the core chain; only NEW schema needs a new home.

## Decision

Ownership splits at a fixed cutover, the current core-chain head.

1. **Core chain owns history through its head.** The core Alembic chain
   (baseline `68b3947ae14f` .. head `7d16_directoryemployees`) remains
   authoritative for every table that exists at the cutover, including the
   bundled-plugin tables it created. Those migrations are immutable and are not
   rewritten. `flask db upgrade` continues to reproduce the full schema.

2. **Plugin chains own plugin schema going forward.** From the cutover forward,
   any change to a plugin's schema lands as
   `plugins/<name>/migrations/versions/000N_*.py` in that plugin's own chain,
   never in the core chain. The core chain is reserved for core tables.

3. **Every table-owning bundled plugin gets a `0001` anchor.** Each such plugin
   carries a migration chain whose first revision is a stamp-only no-op:
   `upgrade()` does nothing because the core chain already created the tables.
   The anchor exists so the plugin chain has a base that
   `flask plugin upgrade-all` can stamp into the per-plugin version table
   `alembic_version_<plugin>`. Blueprint-only plugins that own no tables get no
   chain.

4. **Deploy and upgrade sequence.** A deploy runs `flask db upgrade`
   (core chain, creates everything through the head) then
   `flask plugin upgrade-all` (stamps every plugin anchor and applies any later
   per-plugin migrations). The same two commands upgrade an existing install;
   both are idempotent. The registry (`instance/plugins.json`) tracks each
   plugin's applied revisions in `migrations_applied`.

5. **Table-ownership registry.** `PLUGIN_TABLE_OWNERS` in
   `shopdb/plugins/alembic_template.py` is the explicit map of which tables each
   plugin owns; it is kept in sync with the plugins' `__tablename__` declarations
   and pinned by `tests/test_plugin_migrations.py`.

External (out-of-tree) plugins per ADR-003 already shipped their own chains;
this ADR brings the bundled plugins onto the same model, so there is one rule
for all plugins.

## Consequences

### Positive

- A plugin's schema history travels with the plugin. Adopting or forking sites
  get the plugin's migrations, not a slice of someone else's core chain.
- The core chain stops accreting plugin schema; it stays about core tables.
- A plugin can be evolved (or, with its own downgrade, removed) independently.
- The long-built per-plugin Alembic engine is finally exercised on every deploy,
  so it cannot silently rot.

### Negative / cost

- Two migrate commands per deploy instead of one. Documented in `docs/DEPLOY.md`
  and `docs/UPGRADE.md`; both are idempotent so the cost is one extra safe call.
- A plugin author must now put new tables in the plugin chain and register them
  in `PLUGIN_TABLE_OWNERS`, rather than autogenerating into the core chain. The
  `flask plugin new` guidance and this ADR spell that out.
- The cutover is a discontinuity: tables created before it are core-owned,
  tables created after it are plugin-owned. The line is the core-chain head at
  this ADR's date, recorded here so it is unambiguous.

### Neutral

- No schema changes and no data migration: the anchors are no-ops. A fresh
  install and an existing install converge to the same state.

## Alternatives considered

1. **Keep everything in the core chain (status quo per ADR-004).** Simplest
   operationally but defeats the plugin-as-product goal: plugin schema cannot
   travel, the core chain grows without bound, and plugins can never be cleanly
   removed. Rejected.
2. **Rewrite history so plugin tables move out of the core chain into plugin
   `0001` CREATE migrations.** Would make each plugin chain self-contained from
   empty, but breaks the immutability rule, forces every existing site to
   re-run a rewritten chain, and re-introduces the exact double-creation footgun
   ADR-004 fixed. Rejected.
3. **Anchor that CREATEs tables with `IF NOT EXISTS` guards.** Lets a from-empty
   install build plugin tables from the plugin chain, but then two chains both
   claim the same tables and drift can diverge silently. The no-op anchor keeps
   a single authoritative creator (the core chain) for cutover-era tables.
   Rejected.

## References

- ADR-003 (plugin distribution; external plugins already ship chains)
- ADR-004 (deployment topology; this ADR supersedes its migration-strategy note)
- `shopdb/plugins/alembic_template.py` (`PLUGIN_TABLE_OWNERS`, shared env runner)
- `shopdb/plugins/migrations.py`, `shopdb/plugins/cli.py` (`upgrade-all`)
- `plugins/<name>/migrations/` (per-plugin chains and `0001` anchors)
- `tests/test_plugin_migrations.py` (ownership + chain + idempotency guards)
- `docs/DEPLOY.md`, `docs/UPGRADE.md`, `docs/PLUGINS.md` (deploy sequence)
