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Janus Live Streaming Guide

Live Janus queries use an MQTT-backed stream path. The local Compose setup provides Eclipse Mosquitto on localhost:1883.

Start the local services

docker-compose up -d mosquitto
cargo run --bin http_server -- --host 127.0.0.1 --port 8080 --storage-dir ./data/storage

Check the broker separately if a live query does not receive results:

docker exec -it janus-mosquitto mosquitto_sub -t sensors -v

Replay RDF events

The CLI reads N-Triples or N-Quads, writes events to storage, and optionally publishes them to MQTT:

cargo run --bin stream_bus_cli -- \
  --input data/sensors.nq \
  --broker mqtt \
  --topics sensors \
  --rate 64 \
  --mqtt-host localhost \
  --mqtt-port 1883

Use --broker none to write to storage without publishing. Use --rate 0 for unlimited publication and --loop-file to repeat the source file.

Register a live query

Use an ON STREAM declaration with a positive range and step:

PREFIX ex: <http://example.org/>
REGISTER RStream ex:output AS
SELECT ?sensor ?value
FROM NAMED WINDOW ex:live ON STREAM ex:stream [RANGE 60000 STEP 30000]
WHERE { WINDOW ex:live { ?sensor ex:hasValue ?value . } }

Register and start it through the HTTP API, then subscribe to WS /api/queries/:id/results before replaying data.

Troubleshooting

  • Check /health and /ops/status before diagnosing the query.
  • Verify the broker host, port, and topic used by replay match the live setup.
  • Use the WebSocket result endpoint, not an HTTP poll, to receive running-query results.
  • Stop the query and replay explicitly when repeating an experiment.