The tedge-mapper

The tedge-mapper is a key concept to support multiple cloud providers. The purpose is to translate messages written using the cloud-agnostic Thin Edge JSON format, into cloud-specific messages.

The tedge-mapper is composed of multiple cloud-specific mappers, such as Cumulocity mapper and Azure mapper. Each mapper is responsible for its dedicated cloud. These specific mappers are launched by the respective tedge connect command. For instance, tedge connect c8y establishes a bridge to Cumulocity and launches a Cumulocity mapper that translates the messages in the background.

A mapper subscribes to the reserved MQTT topic tedge/measurements with the QoS level 1 (at least once). The messages that arrive in the mapper should be formed in the Thin Edge JSON format. The mapper verifies whether the arrived messages are correctly formatted, in case the verification fails, the mapper publishes a corresponded error message on the topic tedge/errors with the QoS level 1 (at least once).

When the mapper receives a correctly formatted message, the message will be translated into a cloud-specific format.

Cumulocity mapper

The Cumulocity mapper translates Thin Edge JSON into Cumulocity's JSON via MQTT. The translated messages are published on the topic c8y/measurement/measurements/create from where they are forwarded to Cumulocity. This mapper is launched by the tedge connect c8y command, and stopped by the tedge disconnect c8y command.

Example in Thin Edge JSON:

  "temperature": 23

Translated into JSON via MQTT by the Cumulocity mapper:

  "type": "ThinEdgeMeasurement",
  "time": "2021-04-22T17:05:26.958340390+00:00",
  "temperature": {
    "temperature": {
      "value": 23

You can see the Cumulocity mapper added the three things which are not defined before translation.

  1. type is added.
  2. time is added.
  3. Another hierarchy level is added, as required by the cumulocity data model. String temperature is used as fragment and series.

(1) The type is a mandatory field in the Cumulocity's JSON via MQTT manner, therefore, the Cumulocity mapper always adds ThinEdgeMeasurement as a type. This value is not configurable by users.

(2) time will be added by the mapper only when it is not specified in a received Thin Edge JSON message. In this case, the mapper uses the device's local timezone. If you want another timezone, specify the time filed in Thin Edge JSON.

(3) The mapper uses a measurement name ("temperature" in this example) as both a fragment type and a fragment series in Cumulocity's measurements.

After the mapper publishes a message on the topic c8y/measurement/measurements/create, the message will be transferred to the topic measurement/measurements/create by the MQTT bridge.

For child devices

The Cumulocity mapper collects measurements not only from the main device but also from child devices. These measurements are collected under the tedge/measurements/<child-id> topics and forwarded to Cumulocity to corresponding child devices created under the parent device. (<child-id> is your desired child device ID.)

The mapper works in the following steps.

  1. When the mapper receives a Thin Edge JSON message on the tedge/measurements/<child-id> topic, the mapper sends a request to create a child device under the parent device. The child device is named after the <child-id> topic name, and the type is
  2. Publish corresponded Cumulocity JSON measurements messages over MQTT.
  3. The child device is created on receipt of the very first measurement for that child device.

If the incoming Thin Edge JSON message (published on tedge/measurements/child1) is as follows,

  "temperature": 23

it gets translated into JSON via MQTT by the Cumulocity mapper.


Azure IoT Hub mapper

Note: Child device measurements are not supported yet on Azure IoT Hub.

The Azure IoT Hub mapper takes messages formatted in the Thin Edge JSON as input. It validates if the incoming message is correctly formatted Thin Edge JSON, then outputs the message. The validated messages are published on the topic az/messages/events/ from where they are forwarded to Azure IoT Hub. This mapper is launched by the tedge connect az command, and stopped by the tedge disconnect az command.

The Azure IoT Hub Mapper processes a message in the following ways.

  1. Validates if it is a correct Thin Edge JSON message or not.
  2. Validates the incoming message size is below 255 KB. The size of all device-to-cloud messages must be up to 256 KB. The mapper keeps 1 KB as a buffer for the strings added by Azure.
  3. (default) Adds a current timestamp if a timestamp is not included in an incoming message. To stop this behavior, please refer to the following instruction.

So, if the input is below,

  "temperature": 23

the output of the mapper is

  "temperature": 23,
  "time": "2021-06-01T17:24:48.709803664+02:00"

Configure whether adding a timestamp or not

However, if you don't want to add a timestamp in the output of Azure IoT Hub Mapper, you can change the behavior by running this:

sudo tedge config set az.mapper.timestamp false 

After changing the configuration, you need to restart the mapper service by

sudo systemctl restart tedge-mapper-az.service

Error cases

When some error occurs in a mapper process, the mapper publishes a corresponded error message on the topic tedge/errors with the QoS level 1 (at least once).

Here is an example if you publish invalid Thin Edge JSON messages on tedge/measurements:

$ tedge mqtt pub tedge/measurements '{"temperature": 23,"pressure": 220'
$ tedge mqtt pub tedge/measurements '{"temperature": 23,"time": 220}'

Then, you'll receive error messages from the mapper on the topic tedge/errors:

$ tedge mqtt sub tedge/errors
[tedge/errors] Invalid JSON: Unexpected end of JSON: {"temperature":23,"pressure":220
[tedge/errors] Not a timestamp: the time value must be an ISO8601 timestamp string in the YYYY-MM-DDThh:mm:ss.sss.±hh:mm format, not a number.

Topics used by tedge-mapper

  • Incoming topics

    • tedge/measurements
    • tedge/measurements/<child-id> (for Cumulocity)
  • Outgoing topics

    • tedge/errors (for errors)
    • c8y/measurement/measurements/create (for Cumulocity)
    • az/messages/events/ (for Azure IoT Hub)