MQTT Sub Broker Documentation

Version: 3.1.0

Retrieved: 2025-10-09 15:16:05

MQTT Subscriber

The module is used to subscribe to MQTT messages that are sent to the Crosser server from connected MQTT clients. This module will subscribe to messages by using the topic in the Settings. You can also send message to this module by using the MQTT publisher module from another flow.

This module is designed to be located at the start of a flow.

Settings

Name	Requirements	Purpose	Default
Topic	Length: 1-256	Match Topic against the topic of incoming MQTT messages	""
Target Property	Length: 1-64	The property to write the payload into	"data"
Output Format	Raw (no formatting) or JSON	Will convert the data property on the input to an object from the specified format	Json

The Topicsetting accepts wildcards:

+ is single-level

# is multi-level

Settings Example:

Setting.Topic	MQTT.Topic	Match
foo/bar	foo/bar	true
foo/+/baz	foo/bar/baz	true
foo/+/bar/baz	foo/bar/baz	false
foo/bar/#	foo	false
foo/bar/#	foo/bar/baz/boo	true

Input

The input is a MQTT message described in the table below. The only mandatory properties are topic and data, the rest could in theory be omitted, and others could in theory be added.

Name	Required	Type	Description
topic	yes	string	The topic of the MQTT message
data	yes	byte[]	The actual payload of the MQTT message.
qos	no	int	The quality of service level for the message
retain	no	bool	The flag for storing the message (aka store the last known good value)
dupFlag	no	bool	The flag for knowing if the message was sent more than once.

Output

The output from the MQTT Sub Broker is always the same as the input but data will be placed on the Target Property default ('data'). So the output is listed in the table above. Note that Output Format in the settings will affect the data property.

Example output

{
  "crosser": {
    "success": true
  },
  "data": [
    {
      "pressure": 420,
      "temp": 32
    },
    {
      "pressure": 487,
      "temp": 78
    },
    {
      "pressure": 317,
      "temp": 79
    }
  ],
  "dupFlag": false,
  "qos": 0,
  "retain": false,
  "topic": "array"
}

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