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!

[dejans]

Hi,

I will look into your questions and come back to you possibly in the first half of next week.

Best regards,
Dejan

[dejans]

Hi,

C2D topic is best used as a return path for data requested by the device on a D2C channel rather than asynchronous commands for the device. If device is online, it will not receive messages on C2D or any other MQTT topic. For issuing commands by any means you wish, shadow would be appropriate to use. 

We have an example using "config" section in multi-service sample. Using a section for commands would be the same process. You could create "cmd" section in your shadow. You can use built-in AWS shadow delta mechanism and/or newly added transform MQTT topic, with nrf_shadow_transform_request().

Best regards,
Dejan

[ragruslan]

Thank you!