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Showing content from http://docs.aws.amazon.com/iot/latest/developerguide/device-shadow-comms-device.html below:

Using shadows in devices - AWS IoT Core

This section describes device communications with shadows using MQTT messages, the preferred method for devices to communicate with the AWS IoT Device Shadow service.

Shadow communications emulate a request/response model using the publish/subscribe communication model of MQTT. Every shadow action consists of a request topic, a successful response topic (accepted), and an error response topic (rejected).

If you want apps and services to be able to determine whether a device is connected, see Detecting a device is connected.

Because MQTT uses a publish/subscribe communication model, you should subscribe to the response topics before you publish a request topic. If you don't, you might not receive the response to the request that you publish.

If you use an AWS IoT Device SDK to call the Device Shadow service APIs, this is handled for you.

The examples in this section use an abbreviated form of the topic where the ShadowTopicPrefix can refer to either a named or an unnamed shadow, as described in this table.

Shadows can be named or unnamed (classic). The topics used by each differ only in the topic prefix. This table shows the topic prefix used by each shadow type.

Make sure that your app's or service's use of the shadows is consistent and supported by the corresponding implementations in your devices. For example, consider how shadows are created, updated, and deleted. Also consider how updates are handled in the device and the apps or services that access the device through a shadow. Your design should be clear about how the device's state is updated and reported and how your apps and services interact with the device and its shadows.

To create a complete topic, select the ShadowTopicPrefix for the type of shadow to which you want to refer, replace thingName, and shadowName if applicable, with their corresponding values, and then append that with the topic stub as shown in the following table. Remember that topics are case sensitive.

See Shadow topics for more information about the reserved topics for shadows.

After a device registers with AWS IoT, it should subscribe to these MQTT messages for the shadows that it supports.

ShadowTopicPrefix/delete/accepted

The delete request was accepted and AWS IoT deleted the shadow.

The actions necessary to accommodate the deleted shadow, such as stop publishing updates.

ShadowTopicPrefix/delete/rejected

The delete request was rejected by AWS IoT and the shadow was not deleted. The message body contains the error information.

Respond to the error message in the message body.

ShadowTopicPrefix/get/accepted

The get request was accepted by AWS IoT, and the message body contains the current shadow document.

The actions necessary to process the state document in the message body.

ShadowTopicPrefix/get/rejected

The get request was rejected by AWS IoT, and the message body contains the error information.

Respond to the error message in the message body.

ShadowTopicPrefix/update/accepted

The update request was accepted by AWS IoT, and the message body contains the current shadow document.

Confirm the updated data in the message body matches the device state.

ShadowTopicPrefix/update/rejected

The update request was rejected by AWS IoT, and the message body contains the error information.

Respond to the error message in the message body.

ShadowTopicPrefix/update/delta

The shadow document was updated by a request to AWS IoT, and the message body contains the changes requested.

Update the device's state to match the desired state in the message body.

ShadowTopicPrefix/update/documents

An update to the shadow was recently completed, and the message body contains the current shadow document.

Confirm the updated state in the message body matches the device's state.

After subscribing to the messages in the preceding table for each shadow, the device should test to see if the shadows that it supports have already been created by publishing a /get topic to each shadow. If a /get/accepted message is received, the message body contains the shadow document, which the device can use to initialize its state. If a /get/rejected message is received, the shadow should be created by publishing an /update message with the current device state.

For example, suppose you have a thing My_IoT_Thing which doesn't have any classic or named shadows. If you now publish a /get request on the reserved topic $aws/things/My_IoT_Thing/shadow/get, it returns an error on the $aws/things/My_IoT_Thing/shadow/get/rejected topic because the thing doesn't have any shadows. To resolve this error, first publish an /update message by using the $aws/things/My_IoT_Thing/shadow/update topic with the current device state such as the following payload.

{
	"state": {
		"reported": {
			"welcome": "aws-iot",
			"color": "yellow"
		}
	}
}

A classic shadow is now created for the thing and the message is published to the $aws/things/My_IoT_Thing/shadow/update/accepted topic. If you publish to the topic $aws/things/My_IoT_Thing/shadow/get, it returns a response to the $aws/things/My_IoT_Thing/shadow/get/accepted topic with the device state.

For named shadows, you must first create the named shadow or publish an update with the shadow name before using the get request. For example, to create a named shadow namedShadow1, first publish the device state information to the topic $aws/things/My_IoT_Thing/shadow/name/namedShadow1/update. To retrieve the state information, use the /get request for the named shadow, $aws/things/My_IoT_Thing/shadow/name/namedShadow1/get.

While a device is connected to AWS IoT, it can receive /update/delta messages and should keep the device state matched to the changes in its shadows by:

1. Reading all /update/delta messages received and synchronizing the device state to match.

2. Publishing an /update message with a reported message body that has the device’s current state, whenever the device's state changes.

While a device is connected, it should publish these messages when indicated.

The device's state has changed.

ShadowTopicPrefix/update

A shadow document with the reported property.

ShadowTopicPrefix/get

An action on the device indicates that a shadow will no longer be supported by the device, such as when the device is being removed or replaced.

ShadowTopicPrefix/delete

When a device with one or more shadows connects to AWS IoT, it should synchronize its state with that of all the shadows that it supports by:

1. Reading all /update/delta messages received and synchronizing the device state to match.

2. Publishing an /update message with a reported message body that has the device’s current state.

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