# Communicate with the Greengrass nucleus, other components, and AWS IoT Core

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This page was generated from content adapted from the [AWS Developer Guide](https://github.com/awsdocs/aws-iot-greengrass-v2-developer-guide.git)
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## Publish/subscribe local messages

* **Note**\
  You can't use this publish/subscribe IPC service to publish or subscribe to AWS IoT Core MQTT. For more information about how to exchange messages with AWS IoT Core MQTT, see [Publish/subscribe AWS IoT Core MQTT messages](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/ipc-iot-core-mqtt.md).

## Publish/subscribe AWS IoT Core MQTT messages

* **Note**\
  This MQTT messaging IPC service lets you exchange messages with AWS IoT Core. For more information about how to exchange messages between components, see [Publish/subscribe local messages](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/ipc-publish-subscribe.md).

## Interact with component lifecycle

* **Important**\
  Local deployments don't notify components before updates.
* **Tip**\
  You can follow a tutorial to learn how to develop a component that conditionally defers component updates. For more information, see [Tutorial: Develop a Greengrass component that defers component updates](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/defer-component-updates-tutorial.md).
* **Tip**\
  You can follow a tutorial to learn how to develop a component that conditionally defers component updates. For more information, see [Tutorial: Develop a Greengrass component that defers component updates](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/defer-component-updates-tutorial.md).
* **Note**\
  This operation can't pause containerized processes, such as Docker containers. To pause and resume a Docker container, you can use the [docker pause](https://docs.docker.com/engine/reference/commandline/pause/) and [docker resume](https://docs.docker.com/engine/reference/commandline/unpause/) commands.
* **Important**\
  To use this operation, you must define an authorization policy that grants permission to use this operation. For more information, see [Authorization](#ipc-component-lifecycle-authorization).
* **Important**\
  To use this operation, you must define an authorization policy that grants permission to do so. For more information, see [Authorization](#ipc-component-lifecycle-authorization).

## Interact with component configuration

* **Important**\
  Local deployments don't notify components of updates.

## Retrieve secret values

* **Important**\
  This example component prints the value of a secret, so use it only with secrets that store test data. Don't use this component to print the value of a secret that stores important information.
* **Note**We recommend that in a production environment, you reduce the scope of the authorization policy, so that the component retrieves only the secrets that it uses. You can change the `*` wildcard to a list of secret ARNs when you deploy the component.

## Interact with local shadows

* **Note**To enable a core device to interact with client device shadows, you must also configure and deploy the MQTT bridge component. For more information, see [Enable shadow manager to communicate with client devices](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/work-with-client-device-shadows.md).

## Manage local deployments and components

* **Note**\
  This feature is available for v2.6.0 and later of the [Greengrass nucleus component](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/greengrass-nucleus-component.md).
* **Note**\
  While you can restart any component, we recommend that you restart only [generic components](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/develop-greengrass-components.md#component-types).
* **Note**\
  While you can stop any component, we recommend that you stop only [generic components](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/develop-greengrass-components.md#component-types).

## Authenticate and authorize client devices

* **Note**\
  This feature is available for v2.6.0 and later of the [Greengrass nucleus component](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_iot_greengrass/greengrass-nucleus-component.md).


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