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Documentation Index

Fetch the complete documentation index at: https://docs.coreflux.org/llms.txt

Use this file to discover all available pages before exploring further.

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Overview

Most MQTT payloads in IoT systems are JSON — from devices, HTTP webhooks, industrial gateways, and partner APIs. In Actions you read fields with GET JSON and write structured output with PUBLISH MODEL. Use raw PAYLOAD AS DOUBLE only when the message is a single value with no structure. For when to choose scalar topics, JSON ingress, or Models at the architecture level, see Choosing data formats on the data layer page.
Like fishing one specific item out of a packed grocery bag. You don’t unpack the whole bag — you reach in, grab the field by name, and leave the rest where it is.
For the full GET JSON syntax table and edge cases in the operations reference, see Operations — GET JSON.

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When to Use This

Any time an incoming MQTT message is a JSON object. Typical sources:
  • Sensors and PLCs publishing JSON over MQTT
  • REST or webhook Routes that forward JSON bodies to topics
  • Cloud or SCADA integrations bridged into the broker

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Reading JSON Fields

GET JSON takes a field name (or a dotted path), the source (almost always PAYLOAD), and a target type. It works inside any expression that needs a value: 
DEFINE ACTION ProcessReading
ON TOPIC "sensors/reading" DO
    SET "device_id" WITH (GET JSON "id" IN PAYLOAD AS STRING)
    SET "temperature" WITH (GET JSON "temperature" IN PAYLOAD AS DOUBLE)
    SET "active" WITH (GET JSON "enabled" IN PAYLOAD AS BOOL)

    PUBLISH TOPIC "processed/" + {device_id} + "/temperature" WITH {temperature}
Supported target types are STRING, DOUBLE, INT, and BOOL. The conversion happens at extraction time, so by the time the value is in your variable it is already typed correctly for comparisons and math.

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Nested Paths

For nested objects, use dot notation. For arrays, use bracket notation. Mix them freely: 
DEFINE ACTION ProcessNested
ON TOPIC "sensors/payload" DO
    SET "temp" WITH (GET JSON "data.sensors.temperature" IN PAYLOAD AS DOUBLE)
    SET "first_reading" WITH (GET JSON "readings[0].value" IN PAYLOAD AS DOUBLE)
    SET "second_id" WITH (GET JSON "readings[1].sensor_id" IN PAYLOAD AS STRING)
For a payload like: 
{
  "data": { "sensors": { "temperature": 23.5 } },
  "readings": [
    { "sensor_id": "S1", "value": 100 },
    { "sensor_id": "S2", "value": 200 }
  ]
}
{temp} becomes 23.5, {first_reading} becomes 100, {second_id} becomes "S2".

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Handling Missing Fields

If a field is absent, GET JSON returns a type-appropriate default: empty string for STRING, 0 for numeric types, FALSE for BOOL. For optional fields, check explicitly with EMPTY: 
SET "battery" WITH (GET JSON "battery_level" IN PAYLOAD AS DOUBLE)
IF {battery} == EMPTY THEN
    PUBLISH TOPIC "alerts/missing_field" WITH "battery_level not reported"
ELSE
    PUBLISH TOPIC "processed/battery" WITH {battery}
This pattern matters when you consume data from third parties — webhooks and partner APIs often omit fields rather than send nulls.

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End-to-End: Extract JSON and Publish a Model

To publish structured JSON without hand-building strings, define a COLLAPSED Model with typed fields and use PUBLISH MODEL ... TO ... WITH from an Action. The broker assembles the JSON for you. 
DEFINE MODEL SensorDataRecord COLLAPSED
    ADD STRING "sensor_id"
    ADD STRING "sensor_type"
    ADD DOUBLE "temperature"
    ADD DOUBLE "pressure"
    ADD STRING "timestamp"
    ADD STRING "quality_status"

DEFINE ACTION ProcessSensorJSON
ON TOPIC "sensors/+/json_data" DO
    SET "sensor_id" WITH TOPIC POSITION 2

    SET "temperature_val" WITH (GET JSON "temperature" IN PAYLOAD AS DOUBLE)
    SET "pressure_val" WITH (GET JSON "pressure" IN PAYLOAD AS DOUBLE)
    SET "sensor_type" WITH (GET JSON "type" IN PAYLOAD AS STRING)
    SET "quality" WITH (GET JSON "quality_status" IN PAYLOAD AS STRING)

    PUBLISH MODEL SensorDataRecord TO "sensors/processed/" + {sensor_id} WITH
        sensor_id = {sensor_id}
        sensor_type = {sensor_type}
        temperature = {temperature_val}
        pressure = {pressure_val}
        timestamp = TIMESTAMP "UTC"
        quality_status = {quality}
StepWhat Happens
1Action triggers when any sensor publishes JSON to sensors/+/json_data
2GET JSON extracts each field with the correct type
3PUBLISH MODEL builds a clean JSON object and publishes it to the processed topic
See Publish Model for more on COLLAPSED models and field mapping.

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Next Steps

Designing your data layer

Topic trees, scalar vs JSON payloads, and when to use Models.

Operations — GET JSON

Full syntax reference for GET JSON and related operations.
Last modified on May 20, 2026