nRF Cloud - sending custom data to custom topic

[Bard]

Hi 

I'm currently trying to modify the nRF Cloud multi-service sample (SDK2.6) to send custom data to custom topics. But I can't find a way to do this. 

I want to send a JSON package like this to the custom topic "tdg":

{
"ts":"1726226122",
"loc":"1000",
"id":"123456789",
"at":"2055",
"ap":"999",
"wt":"1255",
"wp":"2121",
"gt":"1255",
"gp":"2222"
}

Other similar messages should be sent to other topics ("pH", "O2"). Is this possible? And do you have some sample code? 

[Mariano Goluboff]

I would start from send_sensor_sample in nRF Cloud multi-service sample. That sends a single numeric value to a sensor appId ("TEMP" and "COUNT" in this sample).

If I understand correctly, you want to send to appId "tdg", and send an object rather than a single value. Change `sensor` to "tdg" in create_timestampled_device_message call.

Where `send_sensor_sample` calls nrf_cloud_obj_num_add, you'll want to use nrf_cloud_obj_object_add, but before calling that you'll need to create the object that you'll pass to it.

There's a convenience macro to do that: NRF_CLOUD_OBJ_JSON_DEFINE, then call nrf_cloud_obj_init, and add the fields above. i.e. nrf_cloud_obj_str_add(obj, "gp", "2222", false).

Remember to call nrf_cloud_obj_free if there are errors, to avoid memory leaks.

Mariano

[Bard]

Thanks a lot, Mariano.

This will certainly work, but maybe I wasn't clear about my objective. I want to send the object to a custom topic. We have a cloud service that subscribe to topics like: {mqtt_config.topic_prefix}m/d/{mqtt_config.client_id}/d2c/tdg, and hence, I want to send the JSON data to the same topic. The solution you suggest will send the data to {mqtt_config.topic_prefix}m/d/{mqtt_config.client_id}/d2c since this is the default for nrf cloud library (?). Using "tdg" in create_timestampled_device_message call will result in 

appId:"TDG"

messageType:"DATA"

ts:1731675207965

in nrf cloud, but it will not change the topic sent to(?).

[Mariano Goluboff]

Ah - I misunderstood the original ask. I'm not aware of the nRF Cloud firmware library supporting sending to additional topics. You can do it with MQTT directly to additional topics:

https://docs.nordicsemi.com/bundle/nrf-cloud/page/APIs/MQTT/Topics.html#additional-topics-supported-by-nrf-cloud

Another possible solution would be to change the cloud service. Instead of using MQTT to subscribe to topics, you could use the Message Routing Service to send the data to a REST cloud service:

https://docs.nordicsemi.com/bundle/nrf-cloud/page/Devices/MessagesAndAlerts/MessageRoutingService/MessageRouting.html