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!

[Tuomas P]

Hello teba99 

You are right, the nRF Cloud trims the config from the shadow. 

There is now a BETA feature in production for you to try out. Unfortunately I have not been able to test this yet, so only "copy and pasting" instructions for you.

There are now new topics:

subscribe "{deviceId}/shadow/get/accepted/tf" to receive the transformed subset of the device’s full shadow (response to `.../get/tf`). See (3)

Publish to "{deviceId}/shadow/get/tf" to request a subset of the device’s full shadow, defined by a transform.

Transformation example

The required 't' property defines the transform string or JSON object.

The optional 'l' property defines the payload size limit (500 bytes in this example):

```
{'t':'state.desired.config', 'l': 500}
```

The transformation is used in REST side and you can check https://docs.nordicsemi.com/bundle/nrf-cloud/page/APIs/REST/Tutorials/Transforms.html 

This should enable you to get only the parts you need with the get/tf. 

[teba99]

Tuomas P Thanks, will look into this