thin-edge Data Model
The data model identifies all data send or received from/to thin-edge and its components, to interact with those. For all data it defines format and explains behaviour.
Use of MQTT​
thin-edge expects the MQTT broker mosquitto to be available on the device. thin-edge uses mosquitto to consume and provide telemetry data. All telemetry data are reflected with specific MQTT topics and payload in JSON format.
thin-edge assumes mosquitto is configured in a secure manner, to avoid any inappropriate access to thin-edge topics and payload. Any malicious access to the broker can hazard thin-edge and all connected devices. Mosquitto provides a wide range of authentication and access control options. For more details see Authentication and ACL (Access Control List) in mosquitto documentation.
Telemetry Data on MQTT​
All telemetry data (Measurements, Events, Alarms) are reflected with MQTT topics, where each has its specific subtopic (e.g. tedge/measurements, tedge/events, tedge/alarms etc.).
- each provider of a measurement, event or alarm sends the occurring data to thin-edge's MQTT broker
- a provider can be the domain application1, other SW components / 3rd parties
- all processes (e.g. the domain application1, other SW components / 3rd parties) on the main-device and all child-devices can consume those telemetry data from the MQTT broker
- the cloud mapper on the main-device picks-up all telemetry data from the MQTT broker and transfers those to the cloud
The communication diagram below illustrates that behaviour.
Telemetry Data for Child-Devices​
All telemetry data provided to the MQTT bus are associated by thin-edge and all consumers with the thin-edge main-device or some child-device (see more details about child-devices in the domain model).
Therefore the child-id of the child-device is can be appended to the MQTT topic, if the message is meant for a child-device;
or no child-id is appended, if the message is meant for the main-device.
MQTT topics for the main-device:
tedge/measurements
tedge/events/<event-type>
tedge/alarms/<severity>/<alarm-type>
MQTT topics for a child-device, including the child-device's specific child-id:
tedge/measurements/<child-id>
tedge/events/<event-type>/<child-id>
tedge/alarms/<severity>/<alarm-type>/<child-id>
Telemetry Data​
Telemetry Data consists of measurements, events and alarms. Each is defined by a set of data-elements, each with specific behaviour.
Measurements​
Measurements carry values from physical Sensors1 or a device's Domain Application1; e.g. voltage and current of an electricity meter, or current state of the manufacturing control process
MQTT topics for measurements​
tedge/measurements
tedge/measurements/<child-id>
MQTT payload for measurements​
A measurement can carry a single value, or multiple values all taken at a single point in time.
One MQTT message can contain a mixture of more than one single-value and multi-value measurements.
Example for a single-value measurement payload​
{
"temperature": // 'name' of that measurement
25.3, // 'value' of that measurement
"time": "2020-10-15T05:30:47+00:00", // optional 'timestamp' of that measurement
}
Example for a multi-value measurement payload​
{
"current": { // 'name' of that measurement
"L1": 9.5, // the 1st 'value' of that measurement, named as "L1"
"L2": 1.3 // the 2nd 'value' of that measurement, named as "L2"
// ...even more values can occur
},
"time": "2020-10-15T05:30:47+00:00", // optional 'timestamp' of that measurement
}
| Reference | Description |
|---|---|
name | a string that identifies the measurement uniquely in context of the device |
value | the value that was sampled; can be named (especially in context of a multi-value measurement) or unnamed; must be an integer or floating point number |
timestamp | optional time that indicates when values were sampled; when not provided, thin-edge.io uses the current system time as the time of the sample; when provided must be conform to ISO 8601 |
Behaviour of measurements​
- thin-edge does not store any historical sampled values for measurements
- there is no initialization value for measurements; i.e. a measurement is not visible on thin-edge before the 1st sample was sent to thin-edge
- a measurement should never be published as MQTT retain message; That is as a single retained measurement might be consumed and processed more than once by a consuming software component (e.g. when that software component restarts and subscribes again).
Events​
Events are notifications that something happened on a device's environment or software system; e.g. a sensor1 detected something like a door has been closed, or a system notification that e.g. a user has started an ssh session
MQTT topics for events​
tedge/events/<event-type>
tedge/events/<event-type>/<child-id>
MQTT payload for events​
{
// example of an event
"text": "A user just logged in", // 'text' message of that event
"time": "2021-01-01T05:30:45+00:00", // optional 'timestamp' of that event
"someOtherCustomFragment": { // optional 'custom fragments'
"nested": {
"value": "extra info"
}
}
}
| Reference | Description |
|---|---|
event-type | a string part of the MQTT topic, that identifies the event uniquely in context of the device |
text | carries a human readable event-text; must be UTF-8 encoded |
timestamp | optional time that indicates when the event has occurred; when not provided, thin-edge.io uses the current system time as the time of the event; when provided must be conform to ISO 8601 |
custom fragments | additional fields are handled as custom specific information; if the connected cloud supports custom fragments its mapper transfers those accordingly to the cloud |
Behaviour of events​
- thin-edge does not store any historical occurrences for events
- an event should never be published as MQTT retain message; That is as a single retained event might be consumed and processed more than once by a consuming software component (e.g. when that software component restarts and subscribes again).
Alarms​
Alarms are notifications about some critical behaviour of the device's environment or software system; e.g. when a temperature sensor detects a temperature went out of its valid range
MQTT topics for alarms​
tedge/alarms/<severity>/<alarm-type>
tedge/alarms/<severity>/<alarm-type>/<child-id>
MQTT payload for alarms​
{
// example for an alarm
"text": "Temperature is very high", // 'text' message of that alarm
"time": "2021-01-01T05:30:45+00:00", // optional 'timestamp' of that alarm
"someOtherCustomFragment": { // optional 'custom fragments'
"nested": {
"value": "extra info"
}
}
}
| Reference | Description |
|---|---|
alarm-type | a string part of the MQTT topic, that identifies the alarm uniquely in context of the device |
severity | a string part of the MQTT topic, that indicates the severity of the alarm; must be critical, major, minor or warning |
text | carries a human readable alarm-text; must be UTF-8 encoded |
timestamp | optional time that indicates when the alarm has occurred; when not provided, thin-edge.io uses the current system time as the time of the alarm; when provided must be conform to ISO 8601 |
custom fragments | additional fields are handled as custom specific information; if the connected cloud supports custom fragments its mapper transfers those accordingly to the cloud |
Behaviour of alarms​
- thin-edge does not store any historical occurrences for alarms
- alarms are stateful; i.e. once raised, an alarm is active until it was explicitly cleared by the device's software or the cloud
- all alarms shall be published as MQTT retain message to reflect the alarm's stateful behaviour in the broker; The retain messages is kept in the MQTT broker as long as the alarm is raised. When a raised alarm is gone again, an empty retain message shall be published to clear the alarm message in the broker.