AWS IoT Device SDK C  202108.00
SDK for connecting to AWS IoT from a device using embedded C.
MQTT

How to migrate an MQTT application from v3 to 202009.00 and subsequent releases.

The MQTT library has been refactored in 202009.00 and subsequent releases, to use only a single thread, and therefore no longer has any thread synchronization dependencies. New features such as persistent session support and QoS 2 publish delivery have also been added.

The features of the MQTT libraries in v3 and 202009.00 and subsequent releases are defined by the common MQTT 3.1.1 spec; therefore, the two versions are similar.

Removed features

The following features were present in v3, but removed in 202009.00 and subsequent releases:

  • Auto-reconnect
    Version 3 has an auto-reconnect feature triggered through a call to aws_iot_mqtt_yield. In version 202009.00 and subsequent releases, the auto-reconnect feature has been removed.
    Workaround: When a version 202009.00 and subsequent releases API call does not return MQTTSuccess, the application should re-establish the underlying transport connection and then call MQTT_Connect. We recommend that the application employ an exponential backoff strategy when re-establishing connections as explained in the AWS blog for Exponential Back off and Jitter.
  • Thread support
    Version 3 has an _ENABLE_THREAD_SUPPORT_ macro for multi-threading use cases. Version 202009.00 and subsequent releases does not spawn any threads and therefore, the macro _ENABLE_THREAD_SUPPORT_ has been removed.

Data Types

The following table lists equivalent data types in v3 and 202009.00 and subsequent releases.

Version 3 Version 202009.00 and subsequent releases Notes
enum QoS MQTTQoS_t MQTT Quality of service.
AWS_IoT_Client MQTTContext_t MQTT connection handle.
IoT_Publish_Message_Params MQTTPublishInfo_t Parameters of an MQTT publish.
IoT_MQTT_Will_Options MQTTPublishInfo_t Optional Will parameter of an MQTT connect.
IoT_Client_Connect_Params MQTTConnectInfo_t Parameters of an MQTT connect.
IoT_Client_Init_Params None The members of this struct in v3 handled setup of the network connection.
This is handled outside of the MQTT library in 202009.00 and subsequent releases.
pApplicationHandler_t MQTTEventCallback_t Function pointer of application callback to receive incoming data.

Functions

The following table lists equivalent API functions in v3 and 202009.00 and subsequent releases. These functions are the API functions declared in:

Version 3 Version 202009.00 and subsequent releases Notes
aws_iot_mqtt_init MQTT_Init
aws_iot_mqtt_free None 202009.00 and subsequent releases does not allocate any memory and therefore, has nothing to free.
aws_iot_mqtt_connect MQTT_Connect
aws_iot_mqtt_publish MQTT_Publish
aws_iot_mqtt_subscribe MQTT_Subscribe
aws_iot_mqtt_resubscribe None Function removed because auto-reconnect was removed.
aws_iot_mqtt_unsubscribe MQTT_Unsubscribe
aws_iot_mqtt_disconnect MQTT_Disconnect
aws_iot_mqtt_yield MQTT_ProcessLoop MQTT_ReceiveLoop is the equivalent function without any keep-alive mechanism.
aws_iot_mqtt_attempt_reconnect None Function removed because auto-reconnect was removed.