Communicate using custom MQTT topics-IoT Platform (Deprecated)(IoT)-阿里云帮助中心

The software development kit (SDK) provides MQTT API operations for direct communication with Alibaba Cloud IoT Platform. You can publish messages to custom topics to send data to the platform and subscribe to custom topics to receive data from the platform.

Communication flowchart

The following flowchart shows how to use an MQTT topic to communicate with IoT Platform.

Call the IOT_MQTT_Construct() function to establish a persistent connection between the device and the cloud platform.
- This function sends a connection request to the cloud platform. The required signature information is obtained from the IOT_Sign_MQTT() function in the dev_sign feature.
- If the connection is successful, the function returns a handle parameter. This handle can be used as an input parameter for all subsequent MQTT network API operations.
Call the IOT_MQTT_Subscribe() function to specify the topics from which the device will receive messages.
Enter the main loop.
- Call IOT_MQTT_Publish() or IOT_MQTT_Publish_Simple() to publish messages to the platform.
- The IOT_MQTT_Yield() function receives messages from the platform and calls the callback function specified in IOT_MQTT_Subscribe() to process the data.

Feature API operations

Function name	Description
IOT_MQTT_CheckStateNormal	After an MQTT connection is established, call this function to check if the persistent connection is normal.
IOT_MQTT_Construct	The constructor for an MQTT instance. The input parameter is the `iotx_mqtt_param_t` struct. This function connects to the MQTT server and returns the handle of the created instance.
IOT_MQTT_Destroy	The function to destroy an MQTT instance. The input parameter is the handle created by `IOT_MQTT_Construct`.
IOT_MQTT_Publish	During an MQTT session, this function constructs a complete `MQTT Publish` message and sends the message to the server.
IOT_MQTT_Publish_Simple	During an MQTT session, this function constructs a complete `MQTT Publish` message and sends the message to the server. The parameters do not include complex data types such as structs.
IOT_MQTT_Subscribe	During an MQTT session, this function constructs a complete `MQTT Subscribe` message and sends a subscription request to the server.
IOT_MQTT_Subscribe_Sync	During an MQTT session, this function constructs a complete `MQTT Subscribe` message, sends a subscription request to the server, and waits for an acknowledgement.
IOT_MQTT_Unsubscribe	During an MQTT session, this function constructs a complete `MQTT Unsubscribe` message and sends an unsubscribe request to the server.
IOT_MQTT_Yield	During an MQTT session, this is the main MQTT loop function. It maintains the heartbeat and receives downlink messages from the server.

Required HAL functions

These functions are optional. You must implement them if you want the SDK to provide the MQTT channel feature.

Function name	Description
HAL_UptimeMs	Returns the number of milliseconds that have elapsed since the device was powered on.
HAL_SleepMs	Puts the device to sleep for the number of milliseconds specified by the input parameter.
HAL_SSL_Destroy	Destroys a TLS connection. This is used for the MQTT feature with an encrypted connection.
HAL_SSL_Establish	Establishes a TLS connection. This is used for the MQTT feature with an encrypted connection.
HAL_SSL_Read	Reads data from a TLS connection. This is used for the MQTT feature with an encrypted connection.
HAL_SSL_Write	Writes data to a TLS connection. This is used for the MQTT feature with an encrypted connection.
HAL_TCP_Destroy	Terminates the TLS connection for the MQTT feature.
HAL_TCP_Establish	Establishes a TCP connection. This includes domain name resolution and TCP connection establishment.
HAL_TCP_Read	Reads stream data from a TCP connection within a specified time and returns the number of bytes read.
HAL_TCP_Write	Sends stream data to a TCP connection within a specified time and returns the number of bytes sent.

Note

The user API operations provided by the SDK are listed in src/mqtt/mqtt_api.h. The HAL functions that you may need to implement are listed in src/mqtt/mqtt_wrapper.h. For more information, see the code.