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!

[teba99]

Hi Dejan,

I tried this, it doesn't seem to work as expected ... or maybe we are misunderstanding.

From the cloud, we use the API devices/${deviceid}/state to send
{
  "desired": {
    "cmd": "command"
  }
}

If the device is online then it receives the delta right away, as we expect:

TOPIC: shadow/update/delta
MESSAGE: {
  "state": {
    "nrfcloud_mqtt_topic_prefix": "prod/<tenant>/",
    "pairing": {
      "state": "paired",
      "topics": {
        "d2c": "prod/<tenant>/m/d/<deviceid>/d2c",
      }
    },
    "cmd": "command"
  }
}

If the device is offline then our understanding is the device would need to explicitly get the shadow via topic shadow/get/accepted when reconnecting . However, this is what the device gets (note that it does not contain the desired { "cmd": "command"} ):

TOPIC: shadow/get/accepted
MESSAGE: {
  "desired": {
    "pairing": {
      "state": "paired",
      "topics": {
        "d2c": "prod/<tenant>/m/d/<deviceid>/d2c",
      }
    },
    "nrfcloud_mqtt_topic_prefix": "prod/<tenant>/",
  },
  "reported": {
    "control": {
      "alertsEn": true,
       "logLvl": 3
     }
  },
  "config": {
    "activeMode": true,
    ...
  }
}

We also tried a shadow update with section
{
  "desired": {
    "config": {
      "cmd": "command"
    }
  }
}

but get the same result: the delta contains the update, but the full shadow does not. 

Could you please clarify where we would add the command and how the device gets that shadow change after it comes online?

Thanks!

[dejans]

Hi teba99 ,

nRF Cloud library does not actually request the full shadow because it can be too large for modem to receive it. Topic ${deviceId}/shadow/get/accepted receives only trimmed shadow which contains only essential shadow data. You can find more information about shadow topics in the documentation. The reason that you do not receive your "cmd" section is due to trimmed shadow being received. 

teba99 said:

We also tried a shadow update with section
{
  "desired": {
    "config": {
      "cmd": "command"
    }
  }
}

This is fine. 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.

Best regards,
Dejan

[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