Amazon FreeRTOS
User Guide

Device Shadow Demo Application

The device shadow example demonstrates how to programmatically update and respond to changes in a device shadow. The device in this scenario is a light bulb whose color can be set to red or green. The device shadow example app is located in the AmazonFreeRTOS/demos/common/shadow directory. This example creates three tasks:

  • A main demo task that calls prvShadowMainTask.

  • A device update task that calls prvUpdateTask.

  • A number of shadow update tasks that call prvShadowUpdateTasks.

prvShadowMainTask initializes the device shadow client and initiates an MQTT connection to AWS IoT. It then creates the device update task. Finally, it creates shadow update tasks and then terminates. The democonfigSHADOW_DEMO_NUM_TASKS constant defined in AmazonFreeRTOS/demos/common/shadow/aws_shadow_lightbulb_on_off.c controls the number of shadow update tasks created.

prvShadowUpdateTasks generates an initial thing shadow document and updates the device shadow with the document. It then goes into an infinite loop that periodically updates the thing shadow's desired state, requesting the light bulb change its color (from red to green to red).

prvUpdateTask responds to changes in the device shadow's desired state. When the desired state changes, this task updates the reported state of the device shadow to reflect the new desired state.

  1. Add the following policy to your device certificate:

    { "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": "iot:Connect", "Resource": "arn:aws:iot:us-west-2:123456789012:client/<yourClientId>" }, { "Effect": "Allow", "Action": "iot:Subscribe", "Resource": "arn:aws:iot:us-west-2:123456789012:topicfilter/$aws/things/thingName/shadow/*" }, { "Effect": "Allow", "Action": "iot:Receive", "Resource": "arn:aws:iot:us-west-2:123456789012:topic/$aws/things/thingName/shadow/*" }, { "Effect": "Allow", "Action": "iot:Publish", "Resource": "arn:aws:iot:us-west-2:123456789012:topic/$aws/things/thingName/shadow/*" } ] }
  2. Uncomment the declaration of and call to vStartShadowDemoTasks in aws_demo_runner.c. This function creates a task that runs the prvShadowMainTask function.

You can use the AWS IoT console to view your device's shadow and confirm that its desired and reported states change periodically.

  1. In the AWS IoT console, from the left navigation pane, choose Manage.

  2. Under Manage, choose Things, and then choose the thing whose shadow you want to view.

  3. On the thing detail page, from the left navigation pane, choose Shadow to display the thing shadow.

For more information about how devices and shadows interact, see Device Shadow Data Flow.