# MQTT API

The program is both an MQTT **subscriber** (remote control) and **publisher** (status + capture events). The
MQTT channel is the `MqttControlChannel` implementation of `IControlChannel`
([src/mqtt/MqttControlChannel.cpp](../src/mqtt/MqttControlChannel.cpp), Eclipse Paho C++). It is used only when
MQTT is enabled; otherwise a `NullControlChannel` runs (publishes dropped, auto-sweep mode). All topics are
namespaced by tower name:

```
GGS/FWT/<tower_name>/...
```

`<tower_name>` comes from `config.ini` (`General.tower_name`). All messages use **QoS 1**. Published messages
are **retained**.

## Connection

- Broker URI = `Network.zkms_server_ip` from `config.ini`; client ID = the tower name.
- Auth: `mqtt_user` / `mqtt_pw` (preferring `$FGC_MQTT_USER`/`$FGC_MQTT_PW`); `clean_session = true`,
  keep-alive 20 s, `set_automatic_reconnect(true)`.
- Connect timeout 5 s. On connection loss Paho auto-reconnects; the channel re-subscribes on reconnect.
- **The program no longer exits if MQTT is unavailable** — it logs a warning and continues in degraded mode.
  Use `--no-mqtt` to disable MQTT entirely (null channel).

## Subscribed topics (inbound — remote control)

Subscribed on (re)connect; handled in `MqttControlChannel::message_arrived()`
([src/mqtt/MqttControlChannel.cpp](../src/mqtt/MqttControlChannel.cpp)).

| Topic | Payload | Parsed as | Effect |
|-------|---------|-----------|--------|
| `GGS/FWT/<tower>/target_HDG` | integer heading as string, e.g. `"137"` | validated with `stoi`, stored as **string** | Sets the yaw target used in ControlCode 1 (converted to encoder counts via `[Motor]`) |
| `GGS/FWT/<tower>/ControlCode` | integer as string, `"0"` or `"1"` | `stoi` → int | Selects the capture mode (see below) |

Invalid (non-integer) payloads are caught and logged; the previous value is kept.

The main loop consumes these via `get_sub_data()`, which returns a snapshot and **clears the "available"
flags** so each update is acted on once (`MqttControlChannel::poll()`).

### ControlCode semantics

| ControlCode | Behaviour ([CaptureScheduler](../src/core/CaptureScheduler.cpp)) |
|-------------|---------------------------------------------------|
| `0` | **Automatic sweep.** Walk the scan grid (ping-pong): `MOVE` to the next `(yaw,pitch)` waypoint, wait for standstill, trigger cameras. |
| `1` | **Directed.** `MOVE` yaw to `target_HDG` (pitch held), wait for standstill, trigger. |

When a ControlCode message arrives, the program echoes the current code back on the StatusCode topic.

## Published topics (outbound)

| Topic | When | Payload | QoS / Retain |
|-------|------|---------|--------------|
| `GGS/FWT/<tower>/StatusCode` | At startup (`"0"`); whenever a ControlCode message is received (echoes the code) | integer as string | 1 / retained |
| `GGS/FWT/<tower>/CamEvent` | After each image is saved | JSON object (below) | 1 / retained |
| `GGS/FWT/<tower>/Env` | Once per fresh ambient sensor sample (see `[Env] period_ms`) | JSON object (below) | 1 / retained |

### CamEvent payload

Built by `MqttControlChannel::publishCamEvent` from a `CamEvent`:

```json
{ "fwt":"ExampleTower", "cam":"RGB", "hdg":1373, "pit":300, "time":1719312345678 }
```

| Field | Type | Meaning |
|-------|------|---------|
| `fwt` | string | Tower name (`config.ini` `tower_name`) |
| `cam` | string | Camera label: `RGB`, `ACR`, or `NIR` (by camera index) |
| `hdg` | int | Gimbal yaw heading **× 10** (one decimal place encoded as integer) at capture time |
| `pit` | int | Gimbal pitch elevation **× 10** at capture time (derived from the pitch encoder) |
| `time` | int | Capture timestamp, Unix epoch **milliseconds** (matches the `.jxl` filename) |

> The `time` value is the same Unix-ms timestamp used as the image filename, so a consumer can locate the file
> for a given event: `<RGB|ACR|NIR>/<time>.jxl`.

### Env payload

Built by `MqttControlChannel::publishEnv` from an `EnvEvent`, sourced from whichever `IEnvSensor` backend is
active (SHT41 on the LattePanda's own I2C bus by default; see [configuration.md](configuration.md)):

```json
{ "fwt":"ExampleTower", "temp_c":22.4, "humid_pct":46.1, "time":1719312345678 }
```

| Field | Type | Meaning |
|-------|------|---------|
| `fwt` | string | Tower name (`config.ini` `tower_name`) |
| `temp_c` | float | Ambient temperature, °C |
| `humid_pct` | float | Ambient relative humidity, % |
| `time` | int | Sample timestamp, Unix epoch **milliseconds** |

## Topic summary

```
subscribe:  GGS/FWT/<tower>/target_HDG      (int heading)
            GGS/FWT/<tower>/ControlCode     (0 = auto sweep, 1 = directed)

publish:    GGS/FWT/<tower>/StatusCode      (echoed control code)
            GGS/FWT/<tower>/CamEvent        (JSON: fwt, cam, hdg×10, pit×10, time-ms)
            GGS/FWT/<tower>/Env             (JSON: fwt, temp_c, humid_pct, time-ms)
```

## Local testing

Point `zkms_server_ip` at a local broker and exercise the topics with Mosquitto:

```bash
mosquitto_sub -t 'GGS/FWT/ExampleTower/#' -v          # watch everything for the tower
mosquitto_pub -t 'GGS/FWT/ExampleTower/ControlCode' -m '0'
mosquitto_pub -t 'GGS/FWT/ExampleTower/target_HDG'  -m '180'
```
