MQTT

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How to configure the server address so as to make it an autonomic cloud platform by using MQTT?

Please refer to MQTT Examples.

I’m using ESP8266 release/v3.3 version of SDK to test the example/protocols/esp-mqtt/tcp example. Then during Wi-Fi configuration, the connection fails after configuring SSID, password and connecting to the default server. The log is as follows, what is the reason?

W (4211) MQTT_CLIENT: Connection refused, not authorized
I (4217) MQTT_CLIENT: Error MQTT Connected
I (4222) MQTT_CLIENT: Reconnect after 10000 ms
I (4228) MQTT_EXAMPLE: MQTT_EVENT_DISCONNECTED
I (19361) MQTT_CLIENT: Sending MQTT CONNECT message, type: 1, id: 0000

When such error occurs, it indicates that the server rejected the connection because the client’s wrong MQTT username and password caused the server-side authentication to fail. Please check if you are using the correct MQTT username and password.

What is the default keepalive value of the MQTT component in ESP-IDF?

The default value is 120 s, which is defined by MQTT_KEEPALIVE_TICK in file mqtt_config.h.

Does MQTT support automatic reconnection?

  • The automatic reconnection of MQTT is controlled by the disable_auto_reconnect variable of struct esp_mqtt_client_config_t. The default value of disable_auto_reconnect is false, which means that automatic reconnection is enabled.

  • The reconnection timeout value can be set using reconnect_timeout_ms.

What are the supported MQTT versions of ESP-IDF?

ESP-IDF currently supports MQTT 3.1 and MQTT 3.1.1, and MQTT 5.0.

When a Wi-Fi connection is disconnected in ESP-IDF, will the memory previously requested by MQTT upper layer protocol be automatically released?

  • No, but you do not need to care about this memory. What you need to care about is the application layer that ESP encapsulates.

  • For MQTT application layer components, you get an MQTT handle when initializing MQTT. You only need to care about the memory in this handle. When not using MQTT, you can call stop or destroy to release the corresponding MQTT memory. When Wi-Fi is disconnected and connected, you do not need to release the MQTT memory or reapply for the handle, because there is an automatic reconnection mechanism in the MQTT component.

For ESP32-C3 MQTT, can I not set corrresponding client_id but configure it as an empty string by default?

  • Yes, you can achieve this by setting set_null_client_id to true in the application code.

When MQTT_EVENT_PUBLISHED is triggered after an ESP-IDF MQTT client has published data with QoS of 1 or 2, does it mean that a proper ack has been received from the other side to prove that the publish has completed? Or does it just mean that the data was successfully sent to the server once?

The MQTT_EVENT_PUBLISHED event triggered means that the broker has acknowledged receipt of the messages published by the client, proving that the publish has completed successfully.

How does an ESP MQTT client manually release MQTT resources after disconnection?

Calling the esp_mqtt_client_destroy API will do the trick.

How should I configure the MQTT keepalive time when ESP32 Wi-Fi and Bluetooth LE coexist? Is there any appropriate configuration time?

  • When using Wi-Fi and Bluetooth LE concurrently in ESP32, it is recommended to configure the MQTT keepalive time properly. Since both Wi-Fi and Bluetooth LE require system resources, setting the keepalive time too short may cause high system load, affecting system stability and performance.

  • Generally, it is advisable to set the MQTT keepalive time based on actual needs to ensure the device stays online while minimizing system resource consumption. In the case of Wi-Fi and Bluetooth LE coexistence, it is recommended to set the MQTT keepalive time to a longer duration, such as 30 seconds or 60 seconds, to reduce communication between the device and the MQTT broker, thereby reducing system load.

  • It is important to note that setting the keepalive time too long may cause a delay in detecting the device offline when it disconnects, which may affect real-time performance and reliability. Therefore, the MQTT keepalive time should be set based on actual needs and system performance.

When will the disconnect event message be triggered for ESP-MQTT clients?

The disconnect message only occurs in the follow cases:

  • A TCP connection error occurs while the MQTT connection is being established.

  • An MQTT connection error occurs while the MQTT connection is being established.

  • You actively call the disconnect function.

  • An exception is received or sent.

  • The MQTT PING RESPONSE is not received within the specified time.

  • The MQTT PING request failed to be sent.

  • Reconnection.

Does the ESP32 MQTT client automatically try to reconnect after disconnecting from the server?

The esp_mqtt_client_config_t structure in the ESP-MQTT client has the disable_auto_reconnect parameter, which can be configured as true or false to determine to reconnect or not. By default, it will reconnect.

How to check if the ESP32 is disconnected from the MQTT server?

To detect if the ESP32 has been disconnected from the server, you can use MQTT’s PING mechanism by configuring the keepalive parameters disable_keepalive and keepalive in the esp_mqtt_client_config_t structure in ESP-MQTT. For example, if you configure disable_keepalive to false (default setting) and keepalive to 120 s (default setting), the MQTT client will periodically send PING to check if the connection to the server is working.

How can I identify the specific error and troubleshoot the issue when encountering a connection failure to the MQTT server?

When MQTT connection fails, you can parse the data in the esp_mqtt_error_codes_t structure within the MQTT event MQTT_EVENT_ERROR. For more details, please refer to the MQTT_EVENT_ERROR event in the example.

How to adjust the sending time of MQTT’s will message?

The delay in sending will messages can be reduced by shortening the MQTT heartbeat time.

While sending data, the MQTT client encounters a timeout. How to determine which network layer the problem occurred in?

Issues can be identified by packet capture analysis to determine whether they occur at the transport layer, network layer, or data link layer. The specific failure point could be that the server did not return an ACK, the server returned an ACK but Wi-Fi did not receive it, or the data packet was not successfully sent.

Why does the MQTT client still report a write timeout when the network condition is good?

This may be due to the underlying LWIP buffer being full, resulting in an inability to write. This is usually because the packets in the buffer have not received an ACK from the other end. Specific reasons may include the server not sending an ACK, the server sending an ACK but Wi-Fi not receiving it, or the data packet not being successfully sent out.

Why does memory usage drop sharply when using MQTT communication?

The outbox in MQTT consumes memory. For messages with QoS greater than 0, the related memory will only be released after the ACK from the MQTT layer is sent by the peer. You can track the allocation and release of memory by configuring CONFIG_HEAP_USE_HOOKS.


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