mg4/server/README.md

# Nils' MG4 — car server

A small Go service at car.k8s.stinnesbeck.com: the API the car writes to. Data lives in
InfluxDB 2; Grafana reads it from there for the dashboards (`../k8s/grafana/`). The car sends
its live data to ABRP itself; the server does not talk to ABRP (removed in 2.4.0).

```
car ──live telemetry───▶ server
    ──daily chunks─────▶   │
    ──charging sessions▶   ▼
                       InfluxDB 2 ◀── Grafana
```

## Configuration

Environment only; the server refuses to start when a required value is missing.

| Name | Notes |
|---|---|
| `INFLUX_URL`, `INFLUX_TOKEN`, `INFLUX_ORG` | required; the token needs read/write on the bucket |
| `INFLUX_BUCKET` | default `mg4` |
| `INGEST_TOKEN` | required, at least 24 characters: `openssl rand -hex 32` |
| `APPROVAL_PASSWORD` | optional, at least 12 characters; switches pairing on (see below) |
| `PUBLIC_URL` | the address a phone uses, for the pairing QR code; derived from the request if empty |
| `FORGE_URL`, `FORGE_REPO`, `FORGE_TOKEN` | optional; app updates (below). The token only needs to read the repository |
| `MAP_TILES_URL` | a TileJSON naming the vector tiles the map's roads come from; default OpenFreeMap's `https://tiles.openfreemap.org/planet`, `off` switches maps off, otherwise https only |
| `MAP_CACHE_DIR` | where fetched map tiles are kept across restarts (the manifest mounts a volume there); empty keeps them in memory only |
| `LISTEN_ADDR` | default `:8080` |

## Endpoints

The ingest endpoints accept `Authorization: Bearer <token>` with either the shared `INGEST_TOKEN`
or a paired car's own token. A paired token only works for the installation id it was paired
with (403 otherwise); the shared token works for any.

- `POST /v1/telemetry` — `{ installationId, samples: [...] }`, up to 500 samples in ABRP's `tlm`
  field names and units (`utc` in epoch seconds; flags as 0/1 or booleans). Unknown fields are
  dropped, known ones bounded: a value out of range is dropped on its own, and a sample without
  a plausible time (before 2001, or more than an hour ahead) is dropped whole. The rest is stored
  and the answer is `200 {stored, droppedCount, dropped}`, even when nothing was usable, since
  the car would only send it again.
- `PUT /v1/consumption/daily` — one delta chunk of a day's consumption.
- `PUT /v1/charging-sessions/{sessionId}` — a finished charging session (AC or DC, as far as the car was awake) with its curve.
- `GET /v1/consumption/history/{installationId}` — the car reads its own totals back.
- `GET /v1/health` — `{"status":"ok"}`.

A malformed request is a 400 naming the field; a database failure is a 500 without detail.

## App updates

`GET /v1/app/releases` and `GET /v1/app/assets/{release}/{asset}/{name}`, with the car's token:
the repository's releases, read from the forge with `FORGE_TOKEN`, in GitHub's field names
with download URLs pointing back at this server, and their attachments streamed through. The
repository stays private and the forge token never reaches the car; redirects away from the
forge are not followed. The car verifies everything itself (`OTA.md`). 404 when unset.

Each check and each download is logged with the car's installation ID ("shared token" for the
shared ingest token): `update check` (how many releases, the newest pre-release), `update
download started` (file, size), then `update handed out` (bytes sent, time taken) or `update
download cut short`. The forge's address and token never appear in the log.

## Map roads

`POST /v1/map/roads` `{z, x, y}`, with the car's token: the drivable roads (motorway to minor
streets) of one web-mercator tile at zoom 14, for the app's "Grid with map" style. The car names
the tile, not its position, and in the body, so it stays out of access logs. The answer is
`{z, x, y, extent: 4096, ways: [{c, p: [x, y, …]}]}`: per road its class (0 motorway and trunk,
1 primary and secondary, 2 tertiary, 3 minor streets) and its points in tile units, simplified to
about 1.5 m; gzipped when the car accepts it (10–15 KB a tile in a city before that).

The roads are fetched **only for the tiles a car asks for**: the OpenStreetMap vector tile
(OpenMapTiles schema) of that square from the TileJSON at `MAP_TILES_URL` (by default
OpenFreeMap: free, no key, no request limits; its template moves with each weekly build and is
looked up every 6 hours), decoded by a small reader of our own (`mvt.go`, the `transportation`
layer only). Each tile is fetched once and kept for 30 days in `MAP_CACHE_DIR` (and the last 300
in memory); at most two fetches run at once. A request waits up to 6 s; a slower fetch goes on
and is kept, and the car hears 503 with `Retry-After: 10` and gets the tile on its next try.
400 for anything but zoom 14 or a tile outside it, 503 when the tile host fails (not cached),
404 when maps are off. The log says `map tile fetched` with the size, never the tile.
Map data © OpenStreetMap contributors (ODbL), tiles © OpenMapTiles, served by OpenFreeMap.

## Settings from a phone

The ABRP credentials are long to type on the car. With pairing on, the car (with its own token)
opens `POST /v1/settings/start` `{installationId}` and shows `verificationUriComplete` as a QR
code. The phone fills in the form at `/settings?code=…` (ABRP API key and user token, charging
price, usable capacity; empty fields are left alone) and sends it with the approval password.
The car polls `POST /v1/settings/poll` `{deviceCode}`: 202 pending, 200 with the values (handed
out once, `Cache-Control: no-store`), 410 expired or unknown. A paired token only reaches its own
installation's request. The values live in memory only, at most ten minutes, and are never
stored, logged or put back into the page.

## Pairing a car by QR code

So nobody types a token in the car. Modelled on the OAuth device authorization grant (RFC 8628);
on only when `APPROVAL_PASSWORD` is set.

1. The car calls `POST /v1/pair/start` `{installationId}` and shows the returned
   `verificationUriComplete` as a QR code, plus the short `userCode` (e.g. `K7F-3QX`).
2. The phone opens `/pair?code=…`, the owner checks the code and approves with the approval
   password. Five wrong passwords from one address lock that address out for a minute; other
   addresses are unaffected (the address is the last `X-Forwarded-For` entry, which Traefik sets).
   An address may hold three open requests; a car starting again replaces its own.
3. The car polls `POST /v1/pair/poll` `{deviceCode}`: 202 pending, 200 with its own token
   (handed out once), 403 denied, 410 expired (after 10 minutes) or unknown.

Each car gets its own 43-character token; only its SHA-256 is stored (measurement `device` in
the same bucket), and the token is only handed out, and only accepted, once the device is
persisted, so a restart never loses a paired car. Manage cars with the approval password:

```sh
curl -H "Authorization: Bearer $APPROVAL_PASSWORD" https://car.k8s.stinnesbeck.com/v1/devices
curl -X POST -H "Authorization: Bearer $APPROVAL_PASSWORD" https://car.k8s.stinnesbeck.com/v1/devices/<id>/revoke
```

## Storage

One bucket, `installationId` tag on every point:

| Measurement | Timestamp | Tags | Fields |
|---|---|---|---|
| `telemetry` | sample `utc` | — | the ABRP fields that were sent |
| `consumption_chunk` | `recordedAt` | `date` | km, kwh, hours, socDrop, sohPercent |
| `charging_session` | `startedAt` | `sessionId` | endedAtMs, duration, energy, cost, start/end SOC |
| `charging_curve` | point time | `sessionId` | powerKw, socPercent |
| `device` | paired at | `deviceId` | tokenHash, installationId, revoked |

A point with the same measurement, tags and timestamp replaces the earlier one, so the car's
retries are idempotent: a cached chunk is re-sent with its original `recordedAt`. Days and the
lifetime total are summed from the chunks when read. Numbers are always written as floats,
because InfluxDB fixes a field's type on first write.

## Development

```sh
nix develop          # from the repository root: Go, govulncheck, InfluxDB 2
cd server
go test ./...
govulncheck ./...
```

## Container image

```sh
podman build -t git.stinnesbeck.com/nils/mg4-server:<version> .
podman push git.stinnesbeck.com/nils/mg4-server:<version>
```

The build runs `go vet` and the tests. The image is a static binary on distroless (about
11 MB), running as UID 65532 on port 8080.