/**
@page mqtt_migration MQTT
@brief 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.

@section migration_mqtt_removed Removed features
@brief The following features were present in v3, but removed in 202009.00 and subsequent releases:
- <u>Auto-reconnect</u> <br>
  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. <br>
  <b>Workaround:</b> When a version 202009.00 and subsequent releases API call does not return @ref MQTTSuccess, the application should re-establish
  the underlying transport connection and then call @ref 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](https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/).
- <u>Thread support</u> <br>
  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.

@section migration_mqtt_datatypes Data Types
@brief The following table lists equivalent data types in v3 and 202009.00 and subsequent releases.

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

@section migration_mqtt_functions Functions
@brief The following table lists equivalent API functions in v3 and 202009.00 and subsequent releases. These functions are the API functions declared in:
- In v3: <a href="https://github.com/aws/aws-iot-device-sdk-embedded-C/blob/0da87f06f1501fad7b03b5d89a1322cc8a8848b4/include/aws_iot_mqtt_client_interface.h">aws_iot_mqtt_client_interface.h</a>
- In 202009.00 and subsequent releases: [MQTT functions](@ref mqtt_functions)

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