Provide digital twin capabilities

[ctron]

Drogue Cloud should provide some digital twin capabilities out of the box. This should be optional on two layers: for the overall infrastructure, but also for the application.

[JulianFeinauer]

I like the feature and the idea. What are you exactly considering? My initial idea would be to have some kind of "last message seen" API for each message (probably with some kind of TTL, ...).

So that if you receive (in whatever order) messages {"a":1}, {"b":2}, {"c":3} one could get the message

{
  "a": 1,
  "b": 2,
  "c": 3
}

and so on ...

[ctron]

Yes, that would be one aspect of the twin I guess. In addition to that, I would like to be able to receive aggregated (full state) updates. From your example, when the next change of {"c": 4} comes in, I want to get the following event:

{
  "a": 1,
  "b": 2,
  "c": 4
}

In order to make it easier for clients to work with the full state of a device.

The come a set of additional features:

• Search/streaming query: Similar to the above, but limiting the output by a filter
• state reconciliation: I set an "desired" state of a property, and expect the system to reconcile this, by sending out commands to the device until the state has been achieved
• RBAC rules, which allow different access to the state

I think all of this requires some kind of payload normalization. Like defining "features" of a device, which has "reported properties" and "desired properties". So that the different functionalities of the system can operate on the level of .<device>.<feature>.(reported|desired).<property>.

Like:

{
  "temperature": {
    "properties": {
      "value": 1.23
    }
  },
  {
    "accelerometer": {
      "properties": {
        "x": 123,
        "y": 123,
        "z": 123
      }
    }
  },
  {
    "led_1": {
      "properties": {
        "state": true,
      }
      "desiredProperties": {
        "state": false,
      }
      "reconciliation": {
        "last": "2022-02-23T16:44:12Z"
      }
    }
  }
}

Where each property of a feature is treated atomically, and can be any JSON value.

[JulianFeinauer]

So you would (also) agree that json is the default message format and there is some kind of mapping from keys to "jsonPath" to specify the values, right?