Publish messages to MQTT topics. Supports dynamic topics, QoS levels, retained messages, and MQTT 5.0 features.
Overview
The mqtt-out node publishes messages to an MQTT broker. Use it to:
- Send sensor data - Publish readings to cloud platforms
- Control devices - Send commands to other IoT devices
- Bridge systems - Forward data between different services
- State updates - Publish device status with retained messages
The node shares broker configuration with mqtt-in nodes, so you only need to configure connection details once.
Properties
| Property | Type | Required | Default | Description |
|---|---|---|---|---|
name | string | - | - | Display name for this node in the editor. |
broker | config | - | MQTT broker configuration node containing connection details. | |
topic | string | - | - | MQTT topic to publish to. Can be overridden by msg.topic. |
qos | number | - | 0 | Quality of Service level: 0 (at most once), 1 (at least once), 2 (exactly once). |
retain | boolean | - | false | If true, the message will be retained by the broker. |
respTopic | string | - | - | MQTT 5.0 response topic for request/response patterns. |
contentType | string | - | - | MQTT 5.0 content type (e.g., "application/json"). |
userProps | object | - | - | MQTT 5.0 user properties as key-value pairs. |
expiry | number | - | - | MQTT 5.0 message expiry interval in seconds. |
Inputs
The mqtt-out node accepts messages with the following properties:
| Property | Type | Description |
|---|---|---|
| msg.payload | any | The message content to publish (auto-converted to string/buffer) |
| msg.topic | string | Overrides the configured topic |
| msg.qos | number | Overrides the configured QoS (0, 1, or 2) |
| msg.retain | boolean | Overrides the configured retain flag |
Objects are automatically JSON-stringified. Numbers and booleans are converted to strings.
Buffers are sent as-is (binary). To send raw JSON, use JSON.stringify() first.
Dynamic Topics
You can set the topic dynamically using msg.topic. This is useful for:
- Publishing to different topics based on sensor ID
- Routing messages to device-specific channels
- Creating hierarchical topic structures
Topic Naming Conventions
# Good topic structure
sensors/living-room/temperature
devices/raspberry-pi-01/status
home/floor1/room2/light/state
commands/device-id/action
# Avoid
my topic # No spaces
sensors//temp # No double slashes
/sensors/temp # Don't start with /Retained Messages
When a message is published with the retain flag set to true:
- The broker stores the last message for that topic
- New subscribers immediately receive the retained message
- Useful for status updates, configurations, and last-known values
To clear a retained message, publish an empty message (empty string or null) with retain=true to the same topic.
Examples
Publish Sensor Data
Read temperature from a sensor and publish to MQTT.
Temperature Publisher
Reads DHT22 sensor and publishes temperature to MQTT.
View Flow JSON
[
{
"id": "sensor1",
"type": "dht22",
"name": "DHT22",
"pin": 4,
"wires": [["function1"]]
},
{
"id": "function1",
"type": "function",
"name": "Format",
"func": "msg.payload = msg.payload.temperature;\nmsg.topic = 'sensors/living-room/temperature';\nreturn msg;",
"wires": [["mqtt1"]]
},
{
"id": "mqtt1",
"type": "mqtt-out",
"name": "Publish",
"broker": "broker1",
"qos": "1",
"retain": false
}
]JSON Payload
Publish structured JSON data with multiple fields.
JSON Sensor Data
Publishes a JSON object with temperature, humidity, and timestamp.
View Flow JSON
[
{
"id": "sensor2",
"type": "dht22",
"name": "DHT22",
"pin": 4,
"wires": [["function2"]]
},
{
"id": "function2",
"type": "function",
"name": "Build JSON",
"func": "msg.payload = {\n temperature: msg.payload.temperature,\n humidity: msg.payload.humidity,\n timestamp: new Date().toISOString(),\n device: 'rpi-01'\n};\nmsg.topic = 'sensors/dht22/data';\nreturn msg;",
"wires": [["mqtt2"]]
},
{
"id": "mqtt2",
"type": "mqtt-out",
"name": "Publish JSON",
"broker": "broker1",
"qos": "1"
}
]Dynamic Topics per Device
Route messages to different topics based on device ID.
Multi-Device Publisher
Receives data from multiple sensors and publishes to device-specific topics.
View Flow JSON
[
{
"id": "input1",
"type": "mqtt-in",
"name": "All Sensors",
"topic": "raw/sensors/#",
"wires": [["function3"]]
},
{
"id": "function3",
"type": "function",
"name": "Route by Device",
"func": "// Extract device ID from topic\nconst parts = msg.topic.split('/');\nconst deviceId = parts[2];\n\n// Build new topic\nmsg.topic = 'processed/' + deviceId + '/data';\n\n// Add metadata\nmsg.payload = {\n value: msg.payload,\n device: deviceId,\n processed: Date.now()\n};\n\nreturn msg;",
"wires": [["mqtt3"]]
},
{
"id": "mqtt3",
"type": "mqtt-out",
"name": "Publish Processed",
"broker": "broker1",
"qos": "1"
}
]Troubleshooting
Messages not being published
- Check the broker connection status (node shows connected/disconnected)
- Verify the topic is set (either in config or msg.topic)
- Use an MQTT client to subscribe and verify messages arrive
- Check EdgeFlow logs for connection errors
Topic shows as undefined
Either set the topic in the node configuration or ensure your flow sets msg.topic before the mqtt-out node. You can use a change node to set msg.topic:
Set msg.topic to "sensors/temperature"Payload shows as [object Object]
Objects are automatically JSON-stringified when published. If you're seeing [object Object], make sure you're not accidentally converting to string elsewhere. Debug before mqtt-out to verify the payload structure.
Retained messages not working
- Verify retain is enabled in the node or set msg.retain = true
- Some brokers disable retained messages - check broker configuration
- Use a new subscriber to test (existing ones may have received it already)
High QoS messages slow down flow
QoS 1 and 2 require acknowledgments which add latency. For high-frequency sensor data where occasional loss is acceptable, use QoS 0. Reserve QoS 1/2 for critical commands and alerts.