Arduino Azure IoT library contains messages with base64 encoding

Question

I'm using the Azure IoT library for Arduino. Based on the example in the GitHub repository for ESP32, please see: https://github.com/Azure/azure-sdk-for-c-arduino/tree/main/examples/Azure_IoT_Hub_ESP32

I'm sending messages to the Azure IoT Hub based on sensor data, and from there, I'm transferring them to Cosmos DB. However, I noticed that the messages are base64 encoded. My code for sending messages in my Arduino file looks like this:

static void generateTelemetryPayload(const char* activity) {
    time_t now = time(NULL);
    struct tm* timeinfo;
    char timeStr[64];

    timeinfo = localtime(&now);
    strftime(timeStr, sizeof(timeStr), "%Y-%m-%dT%H:%M:%S", timeinfo);

    DynamicJsonDocument json(1024);

    json["category"] = "human_presence";

    if (activity != "") {
        json["activity"] = activity;
    }

    json["zone"] = zone;
    json["timestamp"] = getFormattedTime();

    char mqtt_message[128];
    serializeJson(json, mqtt_message);
    telemetry_payload = mqtt_message;
}

static void sendTelemetry(const char* message) {
    Logger.Info("Sending telemetry ...");

    if (az_result_failed(az_iot_hub_client_telemetry_get_publish_topic(
        &client, NULL, telemetry_topic, sizeof(telemetry_topic), NULL))) {
        Logger.Error("Failed az_iot_hub_client_telemetry_get_publish_topic");
        return;
    }

    generateTelemetryPayload(message);

    if (esp_mqtt_client_publish(
        mqtt_client,
        telemetry_topic,
        (const char*)telemetry_payload.c_str(),
        telemetry_payload.length(),
        MQTT_QOS1,
        DO_NOT_RETAIN_MSG) == 0) {
        Logger.Error("Failed publishing");
    } else {
        Logger.Info("Message published successfully");
    }
}

I found a possible solution that might help. To send your messages as JSON instead of a base64 string, adding UTF-8 and application/json encoding to the message. I don't know how to implement this with the Arduino library. Is there a possibility to send my messages as JSON instead of a base64 string?

Answer 1

Last week, I delved into this issue and discovered a solution. References suggesting that the solution involved adding UTF-8 and application/json encoding to the message. I couldn't find any solutions specific to my issue, especially in combination with the Azure IoT Library for Arduino. After several attempts, I found a solution: adding the string $.ct=application%2Fjson%3Bcharset%3Dutf-8 to the topic resolves the issue. If anyone else is facing the same problem, this is the solution:

static void send_telemetry(const char* message, int time = -1)
{
    az_span telemetry = AZ_SPAN_FROM_BUFFER(telemetry_payload);

    Logger.Info("Sending telemetry ...");

    // The topic could be obtained just once during setup,
    // however if properties are used the topic need to be generated again to
    // reflect the current values of the properties.
    if (az_result_failed(az_iot_hub_client_telemetry_get_publish_topic(
            &hub_client, NULL, telemetry_topic, sizeof(telemetry_topic), NULL)))
    {
        Logger.Error("Failed az_iot_hub_client_telemetry_get_publish_topic");
        return;
    }

    get_telemetry_payload(telemetry, message, time, &telemetry);

    if (esp_mqtt_client_publish(
        mqtt_client,
        (std::string(telemetry_topic) + "$.ct=application%2Fjson%3Bcharset%3Dutf-8").c_str(),
        (const char*)az_span_ptr(telemetry),
        az_span_size(telemetry),
        MQTT_QOS1,
        DO_NOT_RETAIN_MSG) < 0)
    {
        Logger.Error("Failed publishing");
    }
    else
    {
        Logger.Info("Message published successfully");
    }
}