# MIGRATIONS — async vs sync policy

CoreScope's ingestor applies schema/data migrations inline at boot in
`cmd/ingestor/db.go`. Every migration that runs synchronously blocks the
ingestor from accepting packets until it returns. On a dev DB that's
milliseconds; at prod scale (1.9M+ observations, 80K+ adverts, 2600+ nodes
on Cascadia) it can pin the boot for minutes and trigger restart loops —
the "upgrade broke prod" failure class (#791, #1483, and others).

## The rule

**Any new `CREATE INDEX`, `ALTER TABLE`, or data-rewriting `UPDATE`/`DELETE`
in a migration file MUST do ONE of the following:**

### Option 1 — Run via `Store.RunAsyncMigration` (preferred for backfills)

```go
// Scheduled in OpenStore() AFTER the *Store is constructed.
if err := s.RunAsyncMigration(ctx, "my_migration_v1",
    func(ctx context.Context, db *sql.DB) error {
        _, err := db.ExecContext(ctx, `CREATE INDEX IF NOT EXISTS ...`)
        return err
    }); err != nil {
    log.Printf("[migration/async] scheduling failed: %v", err)
}
```

- The migration is recorded as `pending_async` in the `_async_migrations`
  table **immediately** — the ingestor boots and starts ingesting.
- `fn` runs in a goroutine; the WaitGroup is shared with the rest of the
  ingestor (`Store.WaitForAsyncMigrations()` waits for everything).
- On success the row flips to `done`; on error/panic to `failed` with the
  error message captured.
- Idempotent: rows in `done` state short-circuit; `failed`/`pending_async`
  rows are retried on the next boot.

Reference implementations: `Store.BackfillPathJSONAsync` (path_json
backfill) and the converted `obs_observer_ts_idx_v1` index build in
`OpenStore`.

### Option 2 — Annotate as preflight-cheap

Some migrations are genuinely cheap at any scale (e.g. `ALTER TABLE ADD
COLUMN`, `CREATE INDEX` on a table you know is bounded to a few thousand
rows). Annotate the migration block with a comment **on the line
immediately above the migration block** so the preflight gate recognises
the opt-out:

```go
// PREFLIGHT: async=true reason="ALTER ADD COLUMN — O(1) sqlite operation"
if r := db.QueryRow("SELECT 1 FROM _migrations WHERE name = 'foo_v1'"); ...
```

The reason MUST be a real one-line justification you can defend in
review. "It's fine" is not a reason.

### Option 3 — Opt out per PR

If the migration is genuinely safe and you don't want to add an inline
annotation, put a single line in the PR body:

```
PREFLIGHT-MIGRATION-SCALE: <30s N=80K verified on Cascadia staging snapshot
```

This must include both `<30s` and `N=<some scale>` so a reviewer can
challenge the measurement.

## The gate

`~/.openclaw/skills/pr-preflight/scripts/check-async-migrations.sh` runs
on every PR via the preflight orchestrator. It greps the diff for new or
modified migration blocks (files matching `cmd/ingestor/db.go`,
`cmd/ingestor/maintenance.go`, `internal/dbschema/**`, `**/migrations/**`,
`**/*.sql`, plus any Go file touching `CREATE INDEX` / `ALTER TABLE` /
`CREATE UNIQUE INDEX`). For each hit it requires one of the three
opt-outs above. Hard-fail (exit 1) — no warning-only mode.

## Concurrency model

CoreScope runs **one ingestor process** per deployment (`cmd/ingestor/`,
single binary, single `*Store`). There is no cluster mode, no leader
election, no second writer. SQLite is opened with `SetMaxOpenConns(1)`
and a 5s `busy_timeout`; all writes (live MQTT ingest + async migration
goroutines + maintenance backfills) serialize through the one connection
in a single process.

What this means for async migrations:

- **No cross-process race** to worry about. Two ingestor instances
  running against the same DB is not a supported deployment shape.
- **Within a single process**, concurrent `RunAsyncMigration(name=X)`
  callers race the initial `SELECT status` → `UPDATE/INSERT` step. The
  current implementation re-schedules `fn` on a pending/failed row so a
  duplicate caller may legitimately re-run it; once status is `done` all
  further calls short-circuit. See
  `TestRunAsyncMigration_ConcurrentSameNameSerialized` for the contract.
- **`fn` runs concurrently with live ingest writers.** Because
  `MaxOpenConns=1`, a long `CREATE INDEX` will serialize behind / ahead
  of insert batches via SQLite's busy-timeout. This is acceptable for
  index builds (the boot path is unblocked, which was the whole point),
  but it means long migrations DO add latency to live writes. Document
  expected runtime in the `reason=` annotation and prefer batched/chunked
  fn implementations for multi-minute work (see `BackfillPathJSONAsync`
  for the canonical batched pattern with inter-batch `time.Sleep`).

## Scale budgets

Per-migration target: **<30s** at current prod scale (Cascadia: ~2,600
nodes, ~80K observations; previous prod snapshot: ~1.9M observations).

Worked example (#1483, `obs_observer_ts_idx_v1`): composite index build
on `observations(observer_idx, timestamp)`. At ~1.9M rows the sync build
pinned ingestor boot for several minutes → restart loop. Converted to
async via `RunAsyncMigration` in `OpenStore` so boot returns immediately
and the index materializes in the background; the existing `_migrations`
short-circuit at the top of the migration block ensures DBs that already
completed the sync v3.8.3 build do NOT re-run it through the goroutine
path on subsequent boots.

If you cannot meet the <30s budget, document the expected upper bound
and operator runbook expectation (e.g. "index build expected ~10 min on
a 5M-row table; ingestor remains responsive; monitor via
`SELECT status, error FROM _async_migrations WHERE name = ...`").

## Why this exists

Pattern that keeps repeating:

1. Author writes `CREATE INDEX foo ON observations(...)` in a migration.
2. Local dev DB has ~100 rows. Migration returns in 1ms. CI is green.
3. Reviewer focuses on plan correctness, not scale.
4. Ship.
5. Prod boots, sqlite scans 1.9M rows, the ingestor sits at `[migration]
   Adding index...` for 8 minutes, healthcheck times out, container
   restarts, loops.
6. Operator pages. Hotfix. Apology.

The gate doesn't try to detect table size (undecidable from a diff). It
enforces **annotation discipline**: every author who adds a migration
must consciously decide which bucket it falls into and write that down.
That is the cheapest possible intervention that breaks the cycle.