MQTT-to-Kuksa gRPC Bridge

The MQTT-to-Kuksa gRPC Bridge (grpc-mqtt-bridge) is the ingest path that turns JSON messages published by the Arduino ECUs (joystick, RFID, brake) into VSS datapoints in the Kuksa Databroker. It runs as an Ankaios workload on the Raspberry Pi 5 next to the broker and databroker.

Source: devices/raspberry-pi5/grpc-mqtt-bridge/

Role in the system

The bridge is a small Python service that:

1. Connects to the local Mosquitto broker and subscribes to one or more topics (default: InVehicleTopics).
2. On every message, parses the JSON payload, extracts values via RFC 6901 JSON pointers, and coerces them to the target VSS scalar type.
3. Writes the resulting datapoints to the Kuksa Databroker via gRPC, automatically routing ACTUATOR entries to set_target_values and all other entry types to set_current_values.

Configuration

The runtime config lives in devices/raspberry-pi5/ankaios/grpc-mqtt.yaml and is mounted into the container by the Ankaios manifest.

Top-level layout

mqtt:
  broker: "tcp://192.168.88.100:1883"
  clientId: "kuksa-mqtt-bridge"
  subscriptions:
    - topic: "InVehicleTopics"
      qos: 0
grpc:
  target: "192.168.88.100:55555"
mappings:
  - name: "joystick-vss-update"
    mqtt:
      topic: "InVehicleTopics"
      jsonPointer: "/"
    grpc:
      updates:
        - path: "Vehicle.Body.Lights.DirectionIndicator.Left.IsSignaling"
          type: "bool"
          jsonPointer: "/Vehicle.Body.Lights.DirectionIndicator.Left.IsSignaling"

mqtt

Key	Required	Description
broker	yes	Broker URL — tcp://host:port or mqtt://host:port
clientId	no	MQTT client identifier (default: kuksa-mqtt-bridge)
subscriptions[].topic	yes	Topic filter to subscribe to
subscriptions[].qos	no	MQTT QoS level (default: 0)

grpc

Key	Required	Description
target	yes	Kuksa Databroker host:port (default: localhost:55555)

mappings[]

Each mapping connects one MQTT topic to one or more VSS datapoints.

Key	Required	Description
name	no	Human-readable label (logging only)
mqtt.topic	yes	Exact topic this mapping reacts to
mqtt.jsonPointer	no	JSON pointer scoping the payload before per-update extraction (default: /)
grpc.updates[].path	yes	Fully qualified VSS path to write (e.g. Vehicle.Speed)
grpc.updates[].jsonPointer	yes	JSON pointer into the scoped payload picking the value
grpc.updates[].type	no	Target scalar type: bool, int, float, string

VSS metadata (data type, value restriction, entry type) is fetched lazily on first use and cached, so string-typed booleans, enum-restricted strings, and numeric arrays are coerced to the exact representation the databroker expects.

Default signal mapping

The shipped grpc-mqtt.yaml forwards the following keys from InVehicleTopics into VSS:

MQTT JSON pointer	VSS path	Type	Origin
/Vehicle.Body.Lights.DirectionIndicator.Left.IsSignaling	Vehicle.Body.Lights.DirectionIndicator.Left.IsSignaling	bool	Joystick ECU
/Vehicle.Body.Lights.DirectionIndicator.Right.IsSignaling	Vehicle.Body.Lights.DirectionIndicator.Right.IsSignaling	bool	Joystick ECU
/Vehicle.Body.Lights.Brake.IsActive	Vehicle.Body.Lights.Brake.IsActive	string	Brake ECU
/Vehicle.Driver.Identifier.Subject	Vehicle.Driver.Identifier.Subject	string	RFID ECU

See Signal Mapping for the full chain ECU → MQTT → VSS → CAN/LED/LIVI.

Container

The bridge ships as a small python:3.11-slim image. The published image is available at:

ghcr.io/<owner>/e2e-vehicle-signals/grpc-mqtt-bridge:main

Built locally:

podman build -t grpc-mqtt-bridge:latest devices/raspberry-pi5/grpc-mqtt-bridge

Run standalone for debugging:

docker run --rm --net=host \
  -v $(pwd)/devices/raspberry-pi5/ankaios/grpc-mqtt.yaml:/config/grpc-mqtt.yaml:ro \
  grpc-mqtt-bridge:latest --config /config/grpc-mqtt.yaml

Troubleshooting

• "Skipping non-JSON MQTT payload" — A subscriber received a message that wasn't valid JSON; check the publishing ECU.
• No VSS update visible via kuksa-client — Confirm grpc.target is reachable from inside the bridge container and that the VSS path is registered with the databroker (see vehicle-signals.yaml).
• Type conversion errors — Use the explicit type: field on each update; the bridge will silently skip values it can't coerce.
• Broker drops — Paho's built-in reconnect logic (loop_forever()) recovers automatically; no extra configuration needed.

Related pages

• Architecture — full workload table including this bridge.
• Communication Workflow — end-to-end sequence including the MQTT → gRPC step.
• Signal Mapping — VSS path catalogue and downstream consumers.
• Kuksa-to-LIVI Telemetry Bridge — sister bridge in the opposite direction (gRPC → Socket.IO).
• IVI Head Unit (LIVI) — Pi 4 device that hosts the LIVI dashboard.