MQTT Node
The gateway runs its own HiveMQ broker on the same machine (default port 1883, TLS 1884). This is the standard pattern for connecting to external SCADA or cloud IoT.
Box Broker Connection Info (2026.05+)
- Broker URL:
tcp://127.0.0.1:1883(plain) /ssl://127.0.0.1:1884(TLS) - Username / Password:
app.properties'smqtt.server.user/mqtt.server.password— Mass-produced boxes have random credentials (verified in/etc/kopens/credentials.txt). The entrypoint automatically syncs HiveMQ auth.properties. - MQTT 5 Support (HiveMQ 2025.4): Full features for retained / will / session expiry / shared subscriptions.
- Shared broker with Sparkplug B — Monitor by subscribing to
spBv1.0/#simultaneously.
1. Node Types
| Node | Use Case |
|---|---|
mqtt in | Subscribe to topics — triggers the flow whenever a message arrives |
mqtt out | Publish messages |
2. Broker (Server) Configuration — Shared across all nodes once created
Double-click mqtt in or mqtt out node → Pencil icon on the right of the Server field → New broker configuration.
| Tab | Item | Recommended Value |
|---|---|---|
| Connection | Server | 127.0.0.1 (Gateway's own HiveMQ) or external broker IP |
| Port | 1883 (or 8883 for TLS) | |
| Client ID | Auto-generated if left blank. Fixed ID recommended for production (e.g., edge-<gateway-id>-flow1) | |
| Keep alive | 60 seconds recommended | |
| Use TLS | ON for external cloud connections | |
| Security | Username/Password | Used for broker authentication |
| Messages | LWT (Last Will) | Topic edge/<id>/status + payload offline — Automatic notification if gateway fails suddenly |
| Messages | Birth | Retain publish online to edge/<id>/status upon connection |
3. Publishing (mqtt out) — Scenario: Sending all tag values to the cloud
inject (1s) ──▶ 태그값 읽기 ──▶ change (topic 만들기) ──▶ mqtt out
change node:
| Action | Property | To |
|---|---|---|
| Set | msg.topic | plant/${msg.payload.tag_id}/value (J-Expression ${...} or mustache) |
| Set | msg.payload | { "ts": $millis(), "v": msg.payload.value, "q": msg.payload.value_read_status } (JSONata) |
mqtt out node settings:
| Item | Value |
|---|---|
| Topic | (Leave blank → Use msg.topic) |
| QoS | 1 (Guaranteed delivery at least once) — 0 is acceptable for standard telemetry |
| Retain | false (Real-time values) — true for state-based data |
4. Subscribing (mqtt in) — Scenario: Writing to a tag via external command
mqtt in (plant/+/cmd) ──▶ change (tag_id/value 추출) ──▶ 태그값 쓰기
mqtt in:
| Item | Value |
|---|---|
| Topic | plant/+/cmd (Wildcard + = single level) |
| QoS | 1 |
| Output | parsed JSON object — Payload is automatically objectified |
change:
| Action | Property | To |
|---|---|---|
| Set | msg.tag_id | msg.topic.split('/')[1] (JSONata) |
| Move | msg.payload.value → msg.payload | (Pass directly to write node) |
With this setup, if an external SCADA publishes {"value":"1"} to the plant/TAG_VALVE_01/cmd topic, the gateway writes that value to the PLC.
5. Common Pitfalls
| Symptom | Cause / Solution |
|---|---|
| No messages received | Topic wildcard (#/+) typo. Verify topic tree with separate tools like MQTT Explorer |
| Client frequently disconnects | Client ID conflict (another client connected with the same ID) — Assign unique IDs |
| Retain messages accumulating infinitely | Retain true publish on every message to the same topic. Use retain only for state-based data (online/offline, last alarm) |
| Incorrect message order | Limitation of QoS 0. Use QoS 1+ if order is critical |
6. Next Steps
- OPC-UA Node Details — Communication with external OPC-UA servers
- Sparkplug B Node Details — Automatic Sparkplug specification handling
- IIoT Example Collection