Subscribe device shadows using mqtt bridge component

Question

Hi,

I want to subscribe to the device shadow changes on IoTCore. For example if someone changes the device shadow via AWS Greengrass console, I want to get the change on my local mqtt broker on the device. For this purpose I made a deployment to the device with this aws.greengrass.clientdevices.mqtt.Bridge configuration.

```
{
  "reset": [],
  "merge": {
    "mqttTopicMapping": {
      "HelloWorldIotCore": {
        "topic": "clients/{iot:thingName}/hello/world",
        "source": "LocalMqtt",
        "target": "IotCore"
      },
      "IotCoreToDevice": {
        "topic": "clients/{iot:thingName}/iotcore/to/device",
        "source": "IotCore",
        "target": "LocalMqtt"
      },
      "GetApplicationsShadowLocalMqttToIoTCore": {
        "topic": "$aws/things/+/shadow/name/applications-shadow/get",
        "source": "LocalMqtt",
        "target": "IotCore"
      },
      "PublishApplicationsShadowLocalMqttToIoTCore": {
        "topic": "$aws/things/+/shadow/name/applications-shadow/update",
        "source": "LocalMqtt",
        "target": "IotCore"
      },
      "GetApplicationsShadowAcceptedIotCoreToLocalMqtt": {
        "topic": "$aws/things/+/shadow/name/applications-shadow/get/accepted",
        "source": "IotCore",
        "target": "LocalMqtt"
      },
      "UpdateApplicationsShadowAcceptedIotCoreToLocalMqtt": {
        "topic": "$aws/things/+/shadow/name/applications-shadow/update/accepted",
        "source": "IotCore",
        "target": "LocalMqtt"
      },
    },
    "brokerUri": "tcp://localhost:1883"
  }
}
```

I can send a message using HelloWorldIoTCore topic from the local mqtt broker (localhost:1883) to cloud and receive a message from the cloud on local mqtt broker using IotCoreToDevice topic.
But it seems if I make an update on the named shadow, applications-shadow, I can not get the change on local mqtt broker using $aws/things/+/shadow/name/applications-shadow/update/accepted topic.
What am I missing here?

Thanks

Answer

Good afternoon **savcuoglu**,

Sorry for the issues you are going through.

My initial piece of advice will be to check your Greengrass Core Device policy, and make sure it allows subscribing, sending and/or receiving to/from shadow topics.
Policy should include something like:
```
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "iot:Subscribe"
      ],
      "Resource": [
        "arn:aws:iot:region:account:topicfilter/$aws/things/thingName/shadow/name/applications-shadow/update/accepted"
      ]
    },
    {
      "Effect": "Allow",
      "Action": [
        "iot:Receive"
      ],
      "Resource": [
        "arn:aws:iot:region:account:topic/$aws/things/thingName/shadow/name/applications-shadow/update/accepted"
      ]
    }
  ]
}
```
You can always refer to [documentation here](https://docs.aws.amazon.com/iot/latest/developerguide/device-shadow-mqtt.html), to tailor your policy to the corresponding shadow topic you are willing to track.

Subscribe device shadows using mqtt bridge component

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