Quality RTOS & Embedded Software

LIBRARIES
WHAT'S NEW
FreeRTOS:
AWS Reference Integrations:
FreeRTOS SMP:
Embedded World track keynote:

AWS IoT Device Shadow Operations Demo

 

Introduction

This demo shows how to use the AWS IoT Device Shadow library to connect to the AWS Device Shadow Service. It uses the coreMQTT library to establish an MQTT connection with TLS (Mutual Authentication) to the AWS IoT MQTT Broker and the coreJSON parser to parse shadow documents it receives from the AWS Shadow service. The demo showcases some basic shadow operations, such as how to update a shadow document and how to delete a shadow document. The demo also shows how to register a callback function with the MQTT library to handle messages like the shadow /update and /update/delta messages that are sent from the AWS Device Shadow service.

This demo is intended only as a learning exercise because the request to update the shadow document (state) and the update response are done by the same application. In a realistic production scenario, an external application (e.g. an application running on a user's phone) would request an update of the state of the device remotely, even if the device is not currently connected. The device will acknowledge the update request when it is connected.

This demo project uses the FreeRTOS Windows port, so you can build and evaluate it with the free Community version of Visual Studio on Windows without the need for any MCU hardware.

 

Source Code Organization

The demo project is called shadow_device_operations_demo.sln and can be found on Github in the following directory:

FreeRTOS-Plus\Demo\AWS\Device_Shadow_Windows_Simulator\Device_Shadow_Demo

 

Configure the Demo Project

The demo uses the FreeRTOS+TCP TCP/IP stack, so follow the instructions provided for the TCP/IP starter project to:

  1. Install the pre-requisite components (such as WinPCap).
  2. Optionally set a static or dynamic IP address, gateway address and netmask.
  3. Optionally set a MAC address.
  4. Select an Ethernet network interface on your host machine.
  5. (Important!) Test your network connection before you attempt to run the Shadow demo.

All of these settings should be set in the Shadow demo project.

 

Configure the AWS IoT MQTT Broker Connection

In this demo you use an MQTT connection to the AWS IoT MQTT broker. This connection is configured in the same way as the MQTT mutual authentication demo.

 

Build the Demo Project

The demo project uses the free community edition of Visual Studio. To build the demo:

  1. Open the Visual Studio solution file FreeRTOS-Plus\Demo\AWS\Device_Shadow_Windows_Simulator\Device_Shadow_Demo\shadow_main_demo.sln from within the Visual Studio IDE.
  2. Select 'build solution' from the IDE's 'build' menu.

 

Functionality

The demo creates a single application task that loops through a set of examples that demonstrate shadow /update and /update/delta callbacks to simulate toggling a remote IoT device's state. It sends a shadow update with the new desired state and waits for the IoT device to change its reported state in response to the new desired state. In addition, a shadow /update callback is used to print the changing shadow states. This demo also uses a secure MQTT connection to the AWS IoT MQTT Broker, and assumes there is a powerOn state in the device shadow. By default, the demo uses the classic unnamed shadow. Optionally democonfigSHADOW_NAME can be defined to select a named shadow.

The demo performs the following operations:

  1. Establish a MQTT connection by using the helper functions in shadow_demo_helpers.c.
  2. Assemble MQTT topic strings for IoT device shadow operations, using macros defined by the Device Shadow library.
  3. Publish to the MQTT topic used for deleting a device shadow to delete any existing device shadow.
  4. Subscribe to the MQTT topics for /update/delta, /update/accepted and /update/rejected using helper functions in shadow_demo_helpers.c.
  5. Publish a desired state of powerOn using helper functions in shadow_demo_helpers.c. This will cause an /update/delta message to be sent to the device.
  6. Handle incoming MQTT messages in prvEventCallback, and determine whether the message is related to the device shadow by using a function defined by the Device Shadow library (Shadow_MatchTopicString). If the message is a device shadow /update/delta message, then the main demo function will publish a second message to update the reported state to powerOn . If an /update/accepted message is received, verify that it has the same clientToken as previously published in the update message. That will mark the end of the demo.

The structure of the demo can be found on GitHub.

This screenshot shows the expected output when the demo executes correctly:

Click to enlarge

 

Connect to the AWS IoT MQTT Broker

To connect to the AWS IoT MQTT broker, we use the same method as MQTTConnect() in the MQTT mutual authentication demo.

 

Delete the Shadow Document

To delete the shadow document, call xPublishToTopic with an empty message, using macros defined by the Device Shadow library. This uses MQTT_Publish to publish to the /delete topic. An example showing how this is done in the prvShadowDemoTask can be found on GitHub.

 

Subscribe to Shadow Topics

Subscribe to the Device Shadow topics to receive notifications from the AWS IoT broker about shadow changes. The Device Shadow topics are assembled by macros defined in the Device Shadow library. An example showing how this is done in the prvShadowDemoTask function can be found on GitHub.

 

Send Shadow Updates

To send a shadow update, the demo calls xPublishToTopic with a message in JSON format, using macros defined by the Device Shadow library. This uses MQTT_Publish to publish to the /delete topic. An example showing how this is done in the prvShadowDemoTask function can be found on GitHub.

 

Handle Shadow Delta Messages and Shadow Update Messages

The user callback function, that was registered to the coreMQTT Client library using the function MQTT_Init(), will notify us about an incoming packet event. The code for an example callback function can be found on GitHub.

The callback function confirms the incoming packet is of type MQTT_PACKET_TYPE_PUBLISH, and uses the Device Shadow Library API Shadow_MatchTopic to confirm that the incoming message is a shadow message.

If the incoming message is a shadow message with type ShadowMessageTypeUpdateDelta, then we call prvUpdateDeltaHandler to handle this message. The handler prvUpdateDeltaHandler uses the coreJSON library to parse the message to get the delta value for the powerOn state and compares this against the current device state maintained locally. If these are different, the local device state is updated to reflect the new value of the powerOn state from the shadow document. The code for prvUpdateDeltaHandler can be found on GitHub.

If the incoming message is a shadow message with type ShadowMessageTypeUpdateAccepted, then we call prvUpdateAcceptedHandler to handle this message. The handler prvUpdateAcceptedHandler parses the message using the coreJSON library to get the clientToken from the message. This handler function checks that the client token from the JSON message matches the client token used by the application. If it doesn't match, the function logs a warning message. The code for prvUpdateAcceptedHandler can be found on GitHub.

Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.