Sending commands from nRF Cloud to LTE Device

[ragruslan]

Hi!

In our project, we need to send cloud-to-device commands. By "command" I mean a request to do some short action, not a request to change device's configuration/state. We see two ways of sending such requests:

1. C2D messages

2. Device shadow

What are the pros and cons of each way?

Here I see a recommendation to use Device shadow for commands sending. But if we talk about commands in the sense I described above, would this recommendation be still actual?

If yes, is there a recommended way (or some sample) of Device shadow usage for sending such commands? I mean, which fields Device shadow should contain and some algorithm of updating desired and reported section during sending command and its execution.

In case of C2D messages, the main problem seems to be command delivery when the device is offline. Right? Or is there some mechanism for this case in nRF Cloud?

Thank you!

[teba99]

Hi dejans  ,

Thanks for the quick reply. You wrote:

In this case, reported config section is sent on trimmed shadow topic. The delta for "cmd" change in desired will come in on the delta topic

That is not what I am seeing. As I wrote above, when the device reconnects and retrieves the (trimmed) shadow, it does not contain the delta for "cmd" in desired. 

Please compare the two screenshorts:

1. shadow in the cloud

2. device log  

Note that I am using the device-simulator-v2, not a real device. But the device-simulator is suscribed to the correct deviceId shadow topic, so it shouldn't matter.

What might be wrong?

Thanks,

-- Terrence

 

[teba99]

Hi dejans ,

Despite more experiments, I have not yet been able to retrieve the full shadow (including the cmd section) yet. Any advice is appreciated.

In the meantime, I am now trying to at least have the device handle the delta message it receives while it is connected. Upon receiving the delta message (as per above) the device now publishes to $aws/things/<devceid>/shadow/update the following:

{

  "reported": {

    "config": {

      "cmd": "command"

    }

  }

}

My expectation is that the "cmd" in "desired: config" in the shadow moves to "reported: config" or that "cmd" now is in both "desired" and "reported", indicating that the device has processed the shadow change. 

But that is not what I am seeing. Instead, when I retrieve the full shadow via the cloud API the "cmd" is still present in the desired section, now also in the "delta" section as well, but not in "reported". It is as if the cloud hasn't processed the update message from the device and it is still pending in "delta".

I have spent several hours on this topic now, and what I am seeing does not match the documentation or my understanding of AWS device shadow. https://docs.nordicsemi.com/bundle/nrf-cloud/page/Devices/Properties/Shadows.html#configuring-a-device provides a round-trip example, but  this is not what I am seeing.

I would be very grateful if you could help clear this up.

Thanks!

[Tuomas P]

This issue has been notified to simulator and backend teams. 

Do I understand correctly that you use the simulator V2 and have modified it.

The simulator does not seem to have the report back functionality even a dummy one. It inits shadow with semi hardcoded JSON and subscribes shadow topics. But it does not seem to do anything based on received events.

The device side would need to report the cmd being hendled. 

[teba99]

Hi Tuomas P ,

Thanks for the answer. Yes, I've added the necessary code to the device simulator. I subscribe to the appropriate topics to handle full and delta "desired" shadows, I handle the cmd, and I publish the device shadow "reported" update to the appropriate topic, as described above.

To remove the device simulator from the equation I will try and replicate the problem with a desktop- or web-based MQTT client.

Thanks,
-- Terrence

[justin]

In the meantime, I am now trying to at least have the device handle the delta message it receives while it is connected. Upon receiving the delta message (as per above) the device now publishes to $aws/things/<devceid>/shadow/update the following:

Try the following:

{
  "state": {
    "reported": {
      "config": {
        "cmd": "command"
      }
    }
  }
}