smartmigration/readme.hints.md

# readme.hints.md

Internal development notes for `@push.rocks/smartmigration`. This file is not published to npm.

## Architecture

- The ledger is the source of truth. Two backends: mongo (a single document in smartdata's `SmartdataEasyStore` collection) and S3 (via `bucket.fastPut`/`fastGet`).
- Step execution is **sequential and registration-ordered**. The `from`/`to` fields exist for chain validation, not for DAG resolution.
- Drivers (`mongodb.Db`, `S3Client`) are exposed via `ctx`, not wrapped. smartmigration writes no SQL or S3 wrappers of its own.

## Locking caveats

- The mongo lock uses atomic `findOneAndUpdate` / `updateOne` operations against the `SmartdataEasyStore` collection and a background heartbeat that extends `lock.expiresAt` while steps are running.
- The S3 lock is best-effort (S3 has no conditional writes without versioning). S3-only deployments should not run multiple instances against the same ledger without external coordination.

## Why reuse SmartdataEasyStore?

- The collection already exists in the smartdata ecosystem and is a natural fit for singleton blobs keyed by `nameId`.
- Reusing it avoids introducing a second smartmigration-specific collection just to store one document per ledger.
- The lock path talks directly to the collection for atomic updates; the persisted document shape still mirrors EasyStore's `{ nameId, data }` layout.
- See `/mnt/data/lossless/push.rocks/smartdata/ts/classes.easystore.ts` for the reference document shape.

## Runtime notes

- `plan()` / `dryRun` resolve the same fresh-install shortcut as `run()`, so their `currentVersionAfter` value is predictive rather than just echoing the current ledger version.
- Successful resumable steps clear `checkpoints[stepId]`; failed steps leave checkpoints intact so the next run can resume.

## Testing notes

- Use `@push.rocks/smartmongo`'s `SmartMongo.createAndStart()` for an in-memory replica set.
- After `createAndStart()`, call `getMongoDescriptor()` and feed the result to `new SmartdataDb(...)`.
- Always call `db.close()` and `smartMongo.stop()` in test cleanup.