Setting Up MQTT Client Based on ESP-IDF

The component used in ESP-IDF to implement MQTT client is ESP-MQTT, which has the following features:

• Support for MQTT, MQTT over TLS, MQTT over WebSocket, and MQTT over WebSocket, and TLS
• Easy to set up with URI
• Multiple clients in one application
• Support for subscribing, publishing, authentication, last will messages, keep-alive pings, and QoS messages

The following code is based on ESP-IDF and creates a connection to a local MQTT broker. For complete code, please go to ESP-IDF project on GitHub and navigate to the directory esp-idf/examples/protocols/mqtt/tcp.

//MQTT event handler function
static void mqtt_event_handler(void *handler_args, esp_event_base_t base,
                               int32_t event_id, void *event_data)
{
    esp_mqtt_event_handle_t event = event_data;
    esp_mqtt_client_handle_t client = event->client;
    int msg_id;
    switch ((esp_mqtt_event_id_t)event_id) {
        case MQTT_EVENT_CONNECTED:
        ESP_LOGI(TAG, "MQTT_EVENT_CONNECTED");
        //Subscribe to topic /topic/test
        msg_id = esp_mqtt_client_subscribe(client, "/topic/test", 0);
        ESP_LOGI(TAG, "sent subscribe successful, msg_id=%d", msg_id);
        break;
        case MQTT_EVENT_DISCONNECTED:
        ESP_LOGI(TAG, "MQTT_EVENT_DISCONNECTED");
        break;
        case MQTT_EVENT_SUBSCRIBED:
        ESP_LOGI(TAG, "MQTT_EVENT_SUBSCRIBED, msg_id=%d", event->msg_id);
        break;
        case MQTT_EVENT_UNSUBSCRIBED:
        ESP_LOGI(TAG, "MQTT_EVENT_UNSUBSCRIBED, msg_id=%d", event->msg_id);
        break;
        case MQTT_EVENT_PUBLISHED:
        ESP_LOGI(TAG, "MQTT_EVENT_PUBLISHED, msg_id=%d", event->msg_id);
        break;
        case MQTT_EVENT_DATA:
        ESP_LOGI(TAG, "MQTT_EVENT_DATA");
        ESP_LOGI(TAG, "TOPIC=%.*s\r\n", event->topic_len, event->topic);
        ESP_LOGI(TAG, "DATA=%.*s\r\n", event->data_len, event->data);
        break;
        case MQTT_EVENT_ERROR:
        ESP_LOGI(TAG, "MQTT_EVENT_ERROR");
        break;
        default:
        ESP_LOGI(TAG, "Other event id:%d", event->event_id);
        break;
    }
}

#define CONFIG_BROKER_URL "mqtt://192.168.3.4/"

static void esp_mqtt_start(void)
{
    //Configure MQTT URI
    esp_mqtt_client_config_t mqtt_cfg = {
        .uri = CONFIG_BROKER_URL,
    };

    //Initialise MQTT client
    esp_mqtt_client_handle_t client = esp_mqtt_client_init(&mqtt_cfg);

    //Register event handler function
    esp_mqtt_client_register_event(client, ESP_EVENT_ANY_ID, mqtt_event_handler, NULL);

    //Start MQTT client
    esp_mqtt_client_start(client);
}

The client device connects to the MQTT broker and subscribes to the topic /topic/test. After another MQTT client publishes the message hello world to the topic /topic/test, the following log will show up in the device:

I (2598) wifi station: MQTT_EVENT_CONNECTED
I (2598) wifi station: sent subscribe successful, msg_id=25677
I (2648) wifi station: MQTT_EVENT_SUBSCRIBED, msg_id=25677
I (314258) wifi station: MQTT_EVENT_DATA
I (314258) wifi station: TOPIC=/topic/test
I (314258) wifi station: DATA=hello world