Send Thin Edge JSON data

Once your Thin Edge device is configured and connected to an IoT cloud provider, you can start sending measurements. Refer to Connecting to Cumulocity or tutorials for other cloud providers to learn how to connect your Thin Edge device to an IoT cloud provider.

In this tutorial, we'll see how different kinds of measurements are represented in Thin Edge JSON format and how they can be sent to the connected cloud provider. For a more detailed specification of this data format, refer to Thin Edge JSON Specification.

Sending measurements

A simple single-valued measurement like a temperature measurement, can be represented in Thin Edge JSON as follows:

{ "temperature": 25 }

with the key-value pair representing the measurement type and the numeric value of the measurement.

This measurement can be sent from the Thin Edge device to the cloud by publishing this message to the tedge/measurements MQTT topic. Processes running on the Thin Edge device can publish messages to the local MQTT broker using any MQTT client or library. In this tutorial, we'll be using the tedge mqtt pub command line utility for demonstration purposes.

The temperature measurement described above can be sent using the tedge mqtt pub command as follows:

$ tedge mqtt pub tedge/measurements '{ "temperature": 25 }'

The first argument to the tedge mqtt pub command is the topic to which the measurements must be published to. The second argument is the Thin Edge JSON representation of the measurement itself.

When connected to a cloud provider, a message mapper component for that cloud provider would be running as a daemon, listening to any measurements published to tedge/measurements. The mapper, on receipt of these Thin Edge JSON measurements, will map those measurements to their equivalent cloud provider native representation and send it to that cloud.

For example, when the device is connected to Cumulocity, the Cumulocity mapper component will be performing these actions. To check if these measurements have reached Cumulocity, login to your Cumulocity dashboard and navigate to Device Management => Devices => All devices => => Measurements and see if your temperature measurement is appearing in the dashboard.

Complex measurements

You can represent measurements that are far more complex than the single-valued ones described above using the Thin Edge JSON format.

A multi-valued measurement like three_phase_current that consists of L1, L2 and L3 values, representing the current on each phase can be represented as follows:

  "three_phase_current": {
    "L1": 9.5,
    "L2": 10.3,
    "L3": 8.8

Here is another complex message consisting of single-valued measurements: temperature and pressure along with a multi-valued coordinate measurement, all sharing a single timestamp captured as time.

  "time": "2020-10-15T05:30:47+00:00",
  "temperature": 25,
  "current": {
    "L1": 9.5,
    "L2": 10.3,
    "L3": 8.8
  "pressure": 98

The time field is not a regular measurement like temperature or pressure but a special reserved field. Refer to Thin Edge JSON Specification for more details on the kinds of telemetry data that can be represented in Thin Edge JSON format and the reserved fields like time used in the above example.

Sending measurements to child devices

Note: Currently this feature supports Cumulocity IoT only.

If valid Thin Edge JSON measurements are published to the tedge/measurements/<child-id> topic, the measurements are recorded under a child device of your device.

Given your desired child device ID is child1, publish a Thin Edge JSON message to the tedge/measurements/child1 topic:

$ tedge mqtt pub tedge/measurements/child1 '{ "temperature": 25 }'

Then, you will see a child device with the name child1 is created in your Cumulocity IoT tenant, and the measurement is recorded in Measurements of the child1 device.

Error detection

If the data published to the tedge/measurements topic are not valid Thin Edge JSON measurements, those won't be sent to the cloud but instead you'll get a feedback on the tedge/errors topic, if you subscribe to it. The error messages published to this topic will be highly verbose and may change in the future. So, use it only for debugging purposes during the development phase and it should not be used for any automation.

You can use the tedge mqtt sub command to subscribe to the error topic as follows:

$ tedge mqtt sub tedge/errors