# The Rust rewrite

Status: **done** (2026-10-18). The service is `ping-collector`
([rust/README.md](../rust/README.md)); Python keeps model training,
evaluation and the archive backfills. Phases 1-3 replaced the Python
collector, the forecast + notifier and the website/API one at a time, each
checked against the Python side on production data before the switch;
phase 4 removed the Python service code. The migration tooling
(`scripts/collector_parity.py`, `scripts/api_parity.py`,
`scripts/install_collector.sh` with its cutover/rollback modes, the
`spikes/rust-forecast` spike) is in git history before the cleanup commit.

The assessment below was written before phase 1 and is kept as the record of
why and how.

## Short version

- **Feasible.** The risky part, serving the scikit-learn forecast from Rust with
  no train/serve skew, works: the spike reproduces the pandas feature matrix
  and the HistGradientBoosting regression output bit for bit (classifier heads
  to the last ulp).
- **Training stays Python.** There is no Rust HGB that trains like
  scikit-learn, and the evaluation harness (`scripts/evaluate_variants.py`) is
  where model work happens. A "Rust rewrite" means a Rust service (collectors,
  API, forecast serving, ntfy) plus a Python offline job (monthly retrain and
  evaluation) that exports models for Rust.
- **Performance is not a reason.** The forecast takes 158 ms per station in Python
  and 0.25 ms in Rust, but it runs once an hour. Load tests showed the site
  is bandwidth-bound once caches are hit.
- **Real reasons, if any:** one process instead of leader-elected uvicorn workers
  (the root of per-worker state bugs like `/scraping/status`), a fraction of the
  memory (324 MB today), typed parsing and SQL, one deployable binary, and the
  same stack as proplan (axum + sqlx).
- **Recommendation:** no big-bang rewrite. If Rust is wanted, start with the
  collector + notifier daemon (phase 1 below), which stands alone and can be
  abandoned without loss. The cheap Python alternative is at the end.

## What exists (Python, `src/`)

| Area | LOC | Fate in Rust |
| --- | ---: | --- |
| Collectors: RID hourly, HII/ThaiWater, RID dams, Open-Meteo | 2,500 | port; typed serde payloads |
| Web API (20 routes), schemas, health, metrics | 2,300 | port with identical JSON (golden-file tests) |
| DB adapters: Postgres, SQLite, MySQL, InfluxDB, VictoriaMetrics | 1,300 | Postgres only via sqlx, existing 13 tables, no data migration |
| ntfy notifications; Matrix alerting (CLI path) | 1,050 | port ntfy; Matrix only if still used |
| ML serving: predict, features, rain/dam/HII fill | ~1,300 | port; parity-tested (spike) |
| ML training + evaluation | ~1,700 | **stays Python** |
| CLI (`main.py`) | 650 | small clap CLI (`fill-gaps`, test cycle) |
| `static/dashboard.html` (164 KB) | | unchanged, served as a static file |
| tests | 3,900 | Rust unit tests + parity fixtures + API golden files |

## Target shape

One binary, one process (tokio):

- tasks: RID / HII / dams / rain collectors on their schedules; forecast
  precompute when new readings land; ntfy notifier. Shared state is an
  `Arc<RwLock<..>>`, so there's no leader election, no status hand-off file and
  no executor sizing.
- axum + tower-http: same routes and JSON, compression (gzip/brotli also covers
  the 257 KB river GeoJSON), static dashboard, CORS; `utoipa` if `/docs` stays.
- sqlx against the existing schema. Python and Rust can run against the same
  database during migration.
- models: `train_all()` also exports each bundle (`models/flood_<station>.json.gz`,
  1.2-2.3 MB), `scripts/retrain.sh` promotes it. Rust reloads changed files on
  its next run. A bundle whose `feature_names` Rust cannot build falls back to
  the heuristic, as Python does.

## The permanent cost: the Python/Rust model contract

Every feature added in `features.py` must be ported to Rust in the same change,
or the new bundle is refused. That has to be enforced by CI: Python writes a parity
fixture per station (as `export_fixture.py` does), and a Rust test asserts
bit-identical features and regression output. The spike found two traps that
silently flip tree branches (HGB thresholds often equal real data values):
serde_json's default float parsing (1 ulp off; enable `float_roundtrip`) and
pandas' Kahan-compensated rolling sums/means (replicated in `rolling_kahan()`).
This slows model iteration, which is the most active part of the repo.

## Benefits and costs

| For | Against |
| --- | --- |
| Single process: no leader election, no per-worker state | Two languages forever; parity CI is mandatory |
| Memory ~324 MB -> est. 60-80 MB (mostly tree arrays; less with u32/u16 nodes) | Weeks of work with little user-visible change |
| Typed RID/HII parsing, compile-checked SQL | Re-introduces bugs into recently hardened code (tz, alerts, security) |
| One static binary; no venv on the server for the API | Server still needs Python + sklearn for the monthly retrain |
| Same stack as proplan | Feature work = Python + Rust change every time |

## Plan (each phase ships on its own)

| Phase | Scope | Status |
| --- | --- | --- |
| 0 | Spike: P.1 features + HGB inference | done, bit-identical |
| 1 | `ping-collector`: RID, HII, dams, Open-Meteo | in production since 2026-10-17 23:09 (Python: `COLLECTION_ENABLED=false`) |
| 2 | Forecast crate + ntfy | in production since 2026-10-18 15:08 (`FORECAST_IN_COLLECTOR=true`); live check on production data: 48/48 rows identical, 96/96 probabilities bit-identical |
| 3 | Website + read API in `ping-collector`, Matrix like ntfy | in production since 2026-10-18 16:40 (`WEB_LISTEN`, `water-monitor.service` disabled); on production data 53 of 53 compared routes identical to FastAPI (`scripts/api_parity.py`) |
| 4 | Cleanup: Python service code removed | done 2026-10-18 |

0. **Spike: done.** P.1 features + HGB inference, bit-identical.
1. **Collector daemon** (`rust/collector`, ~1.5k LOC incl. tests): RID with the
   fixed semantics (store late readings, retry while behind, re-fetch
   yesterday's holes), HII, dams, Open-Meteo, into the same tables. The Python
   API runs with `COLLECTION_ENABLED=false` and re-forecasts when the newest
   RID reading in the database advances. ntfy moved to phase 2: the outlook
   alert needs the forecasts, which are still Python. Cutover and rollback
   were one command each (`scripts/install_collector.sh`, since removed).
2. **Forecast crate + ntfy** (`rust/forecast`, run by `ping-collector` with
   `FORECAST_IN_COLLECTOR=true`): all stations, HII/rain/dam inputs, heuristic
   fallback, probabilities and stages, `forecast_history`, `forecast_current`
   (served by the API) and `notification_state`. Parity: fixtures from the
   Python production path (CI: P.1 model; locally all 15), a live comparison
   script, and 751 notifications replayed word for word. Porting found a
   Python serving bug, fixed first: forecast rain (`rain_fc24`) was always
   NaN at serving time (docs/FLOOD_FORECASTING.md section 5). The ntfy change:
   ThaiWater mirror readings keep level alerts and the feed check current
   while RID lags at night. The Python API then only serves.
3. **Website + read API** (`rust/collector/src/web`, axum): every FastAPI
   route with the same parameters, JSON fields and order, status codes,
   FastAPI-style errors (`{"detail": ...}`, pydantic's 422 list), cache
   headers, gzip, CORS, Umami API tracking and the stale-while-revalidate
   response caches (now with byte budgets). `/docs` is the same Swagger UI;
   its OpenAPI document is generated from the route table whose parameter
   declarations also do the validation. One process serves, collects,
   forecasts and notifies (`WEB_LISTEN`), so the status file, the leader
   election and the `forecast_current` hand-off between processes are gone
   in effect. Checked by `scripts/api_parity.py`, which runs `ping-collector
   serve` next to the FastAPI app on the same database and compares every
   route (query values vary per run so FastAPI's caches answer fresh);
   `--cutover-api` refuses to switch on a difference. Intended differences:
   `/health` counts ThaiWater telemetry of the same gauges while RID lags (as
   the notifier); `/scraping/status` adds the collector version and the last
   forecast run; `/forecast` serves an old stored forecast marked
   `X-Data-Stale` instead of recomputing (same process); `/sensors/thaiwater`
   is cached for a minute; database errors serve the last good response
   where FastAPI answered `[]`. Matrix: the notifier's transitions go to the
   room too (per station, outlook, feed; not the per-topic summaries), with
   their own `matrix:` state keys seeded from ntfy's, replacing the
   `run.py --alert-check` cron job that repeated alerts every 15 minutes.
   Python keeps training, evaluation and the monthly retrain timer.
4. **Cleanup** (done): FastAPI/uvicorn, the Python collector, notifier, Matrix
   alerting, CLI (`run.py`), the multi-backend adapters (SQLite, MySQL,
   InfluxDB, VictoriaMetrics), demo databases, Docker/Grafana files and the
   migration scripts are gone, about 17k lines. The Python environment is the
   training stack (pandas, scikit-learn, SQLAlchemy/psycopg2): `src/ml`, the
   backfill clients/stores (`src/hii_collector.py`, `src/rid_reservoir.py`)
   and `src/ml/predict.py` as the reference implementation the Rust forecaster
   is tested against. PostgreSQL only; the Python database tests run against
   a throwaway schema (`TEST_DATABASE_URL`, as the Rust ones). One installer,
   `scripts/install.sh`.

Decided before phase 3 (2026-10-18): Matrix stays and behaves like ntfy;
`/docs` stays; SQLite dev mode goes; the monthly retrain stays on the same
machine (Python, `water-monitor-retrain.timer`).

## Cheaper alternative (stay Python)

Most of the operational gain comes from taking collection out of the web
workers: run the collector as its own systemd unit (one process) and the
uvicorn workers as read-only API. That's about a day of work. It removes leader
election and per-worker state, but doesn't bring the memory, typing or
single-binary benefits.