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
OVERPASS_URL	the Overpass API the map's roads come from; default https://overpass-api.de/api/interpreter, off switches maps off, otherwise https 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 residential) 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 and unclassified, 3 residential and living streets) and its points in tile units, simplified to about 1.5 m; gzipped when the car accepts it (20–30 KB a tile in a city before that). The roads come from OpenStreetMap through OVERPASS_URL, one request at a time and at most one a second, with a User-Agent naming this service, and are kept in memory for 30 days (up to 3000 tiles); the car keeps its own copy too. 400 for anything but zoom 14 or a tile outside it, 503 when Overpass is busy or unreachable (not cached, the car tries again a minute later), 404 when maps are off. The log says map tile fetched with the car's installation and the size, never the tile. Map data © OpenStreetMap contributors, ODbL.

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:

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

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

Container image

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.