# JT808 Daily Mileage Streaming

## Scope

The analytics app can calculate daily mileage from JT808 telemetry only. It consumes `vehicle.event.jt808.v1` and saves the current daily result into the common `vehicle-stat` metric repository.

Only supported message backbone: Kafka.

## Storage

Runtime state: none outside `vehicle_stat_metric`. There is no separate JT808 daily-mileage table. The derived value is stored as one `daily_mileage_km` metric row in the common JDBC/MySQL table `vehicle_stat_metric`; the local-day minimum and maximum GPS total mileage values are kept on that same row as calculation source columns.

The JT808 daily-mileage value is calculated from the GPS total mileage reported in location additional information:

```text
daily_mileage_km = max_total_mileage_km - min_total_mileage_km
calculation_method = JT808_TOTAL_MILEAGE_DIFF
```

The first valid JT808 location point for a vehicle and local day stores both calculation endpoints on the metric row and writes `daily_mileage_km=0.0`. Later ordered or replayed points update the lower endpoint when the reported GPS total mileage is smaller, update the upper endpoint when it is larger, and rewrite the derived `daily_mileage_km` on the same row.

## Runtime Settings

Set these in Portainer or Nacos, without committing secrets:

```text
KAFKA_TOPIC_JT808_EVENT=vehicle.event.jt808.v1
VEHICLE_STAT_JT808_MILEAGE_ENABLED=true
MYSQL_JDBC_URL=<jdbc-url>
MYSQL_USERNAME=<user>
MYSQL_PASSWORD=<password>
```

Algorithm defaults use the Asia/Shanghai daily boundary. Restart recovery reads the same `daily_mileage_km` metric row.