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MQTT Publish

This block publishes a message to an MQTT topic. Use it to send data to MQTT brokers for communication with other devices and cloud services.

MQTT Publish Block

Overview​

The MQTT Publish block sends messages to specified topics on your configured MQTT broker, enabling device-to-device communication and cloud integration.

Configuration​

  • Topic: MQTT topic to publish to (e.g., "sensors/temperature")
  • Message: Payload to send (text, JSON)
  • QoS: Quality of Service level (0, 1, or 2)
  • Retain: Whether broker should retain message for new subscribers

Topic Structure​

Design topic hierarchy:

device/sensor/temperature
device/sensor/humidity
device/status/online
alerts/critical/temperature

Use forward slashes (/) to organize topics hierarchically.

Quality of Service​

Choose appropriate QoS level:

  • QoS 0: At most once delivery (fire and forget)

    • Fastest, lowest overhead
    • No delivery guarantee
    • Use for frequent, non-critical data

  • QoS 1: At least once delivery (acknowledged)

    • Guaranteed delivery
    • Possible duplicates
    • Use for important data

  • QoS 2: Exactly once delivery (assured)

    • Highest reliability
    • Highest overhead
    • Use for critical data

Message Format​

Send various message types:

Plain Text​

Temperature: 22.5

JSON​

{"temperature": 22.5, "unit": "C"}

Retained Messages​

Enable retain flag for:

  • Latest status updates
  • Configuration information
  • State that should persist

New subscribers immediately receive last retained message on topic.

Dynamic Topics​

Build topics dynamically:

  • Include device ID: devices/${deviceId}/status
  • Include location: building1/floor2/room3/temp
  • Include timestamp: logs/2023/10/09/events

Use variables to construct topic names.

Use Cases​

Common MQTT publish scenarios:

  • Send sensor readings to cloud
  • Publish device status updates
  • Broadcast alerts and alarms
  • Share data between devices
  • Log events to central server

Message Payload​

Prepare message payload from:

  • Variable values
  • Sensor readings
  • Formatted strings
  • JSON objects
  • Calculated values

Error Handling​

Handle publish failures:

  • Connection lost: Message queued for retry
  • Invalid topic: Check topic format
  • Authentication error: Verify broker credentials
  • QoS timeout: Check network reliability

Performance Considerations​

Optimize MQTT publishing:

  • Use QoS 0 for high-frequency data
  • Batch data when possible
  • Minimize payload size
  • Avoid publishing in tight loops

Security​

Secure MQTT publishing:

  • Use TLS/SSL encryption
  • Implement authentication
  • Restrict topic permissions
  • Validate message content

Troubleshooting​

Common issues:

  • Message not published: Check MQTT connection status
  • No subscribers receiving: Verify topic name matches subscriptions
  • High latency: Reduce QoS level or check network
  • Authentication failed: Verify broker credentials

See Also​