FIWARE: IotAgent-json over MQTT - json

Question

I'm working on connecting sensors to a FIWARE system.
The sensor can report the monitoring data with MQTT in json payload.
With a public mqtt broker(hivemq), I can get the monitoring data payload as below:
{
"src":"shellyplus1pm-7c87ce64d540",
"dst":"shellyplus1pm-7c87ce64d540/events",
"method":"NotifyStatus",
"params":{
"ts":1659606613.35,
"switch:0":{
"id":0,
"apower":38.04
}
}
}.
my questions are:
(1) the sensor can only publish data on topic DeviceId/events/rpc, but if I use FIWARE IotAgent-json, the IotAgent-json expects subscribing data on topic /json/{{api-key}}/{{device-id}}/attrs (according https://github.com/yanpengwuIoT/tutorials.IoT-over-MQTT), how can I let the IotAgent-json subscribe data on topic DeviceId/events/rpc, which is sensor firmware defined and can't be changed.
(2) the sensor publishs monitoring payload as a nested multi-level json object as showed above, but the IotAgent-json can only support a single level json object (like '{"h": 70, "t": 15}' as described in https://github.com/telefonicaid/iotagent-json/blob/master/docs/usermanual.md), how can I parse the multi-level json object in IotAgent-json?
Any comment, sample or documentation for this are very appreciated! Thank you very much.

Answer 1

You can create a small relay middleware to read from one topic and write to another:
const mqtt = require('mqtt');
const MQTT_BROKER_URL = process.env.MQTT_BROKER_URL || 'mqtt://mosquitto';
const MQTT_TOPIC_PROTOCOL = process.env.MQTT_TOPIC_PROTOCOL || 'json';
global.MQTT_CLIENT = mqtt.connect(MQTT_BROKER_URL);
MQTT_CLIENT.on('connect', () => {
MQTT_CLIENT.subscribe('/+/events/rpc');
});
MQTT_CLIENT.on('message', measureReceived);
function measureReceived(topic, message) {
const parts = topic.toString().split('/');
// Extract the deviceId
const deviceId = parts[2];
// Muck around with the payload and use the right topic
const apiKey = 'XXX'
process.nextTick(() => {relay(apiKey, deviceId, newPayload, newTopic)});
}
function relay(apiKey, deviceId, state, topic) {
let mqttTopic = '/' + apiKey + '/' + deviceId + '/' + topic;
MQTT_CLIENT.publish(mqttTopic, state);
}
You could hard-code the logic to unpack the incoming complex measure and a create a simpler payload, or you could investigate the use of the IoT Agent's expression library and provision the device so that it cherry picks the correct parts of the payload. Doing this is more sustainable in the long run, but would require knowledge of the JEXL expression language