AWS Greengrass
Developer Guide

Configure the Lambda Function for AWS Greengrass

You are now ready to configure your Lambda function for AWS Greengrass.

  1. In the AWS IoT console, under Greengrass, choose Groups, and then choose the group that you created in Module 2.

  2. On the group configuration page, choose Lambdas, and then choose Add Lambda.

    
                            The group page with Lambdas and Add Lambda highlighted.
  3. Choose Use existing Lambda.

    
                            Screenshot of Add a Lambda to your Greengrass Group with the Use
                                existing Lambda button highlighted.
  4. Search for the name of the Lambda you created in the previous step (Greengrass_HelloWorld, not the alias name), select it, and then choose Next:

    
                            Screenshot of Use existing Lambda with Greengrass_HelloWorld and
                                the Next button highlighted.
  5. For the version, choose Alias: GG_HelloWorld, and then choose Finish. You should see the Greengrass_HelloWorld Lambda function in your group, using the GG_HelloWorld alias.

  6. Choose the ellipsis (…) for the Lambda function, then choose Edit Configuration:

    
                            Screenshot of MyFirstGroup with the ellipsis and Edit
                                Configuration highlighted.
  7. On the Group-specific Lambda configuration page, make the following changes:

    • For Timeout - set to 25 seconds. This Lambda function sleeps for 20 seconds before each invocation.

    • For Lambda lifecycle - select Make this function long-lived and keep it running indefinitely.

     

    
                            Screenshot of the configuration page with 25 (seconds) and the
                                Make this function long-lived and keep it running indefinitely radio
                                button selected.

    A long-lived Lambda function starts automatically after AWS Greengrass starts and keeps running in its own container (or sandbox). This is in contrast to an on-demand Lambda function which starts when invoked and stops when there are no tasks left to execute. For more information, see Lifecycle Configuration for Greengrass Lambda Functions.

    Note

    The Lambda function in this tutorial accepts JSON input payloads, but Greengrass Lambda functions also support Binary input payloads. Binary support is especially useful when interacting with device data, because the restricted hardware capabilities of devices often make it difficult or impossible to construct a JSON data type.

  8. Choose Update to save your changes to the Lambda function configuration.

  9. An AWS Greengrass Lambda function can subscribe or publish messages (using the MQTT protocol):

    • To and from other devices (or device shadows) within the AWS Greengrass core. Information about device shadows is provided in Module 5.

    • To other Lambda functions.

    • To the AWS IoT cloud.

    The AWS Greengrass group controls the way in which these components interact by using subscriptions that enable greater security and to provide predictable interactions.

    A subscription consists of a source, target, and topic. The source is the originator of the message. The target is the destination of the message. The topic allows you to filter the data that is sent from the source to the target. The source or target can be an AWS Greengrass device, a Lambda function, a device shadow, or the AWS IoT cloud. A subscription is directed in the sense that messages flow in a specific direction. For an AWS Greengrass device to send messages to and receive messages from a Lambda function, you must set up two subscriptions: one from the device to the Lambda and another from the Lambda function to the device. The Greengrass_HelloWorld Lambda function sends messages only to the hello/world topic in the AWS IoT cloud, as shown in the following greengrassHelloWorld.py code snippet:

    def greengrass_hello_world_run(): if not my_platform: client.publish(topic='hello/world', payload='Hello world! Sent from Greengrass Core.') else: client.publish(topic='hello/world', payload='Hello world! Sent from Greengrass Core running on platform: {}'.format(my_platform)) # Asynchronously schedule this function to be run again in 5 seconds Timer(5, greengrass_hello_world_run).start() # Execute the function above: greengrass_hello_world_run()

    Because the Greengrass_HelloWorld Lambda function sends messages only to the hello/world topic in the AWS IoT cloud, you only need to create one subscription from the Lambda function to the AWS IoT cloud, as shown next.

  10. On the group configuration page, choose Subscriptions, and then choose Add your first Subscription.

    
                            Screenshot of the Group configuration page, with Subscription
                                and Add your first Subscription highlighted.
  11. In the Select a source field, choose Select. Then, on the Lambdas tab, choose Greengrass_HelloWorld as the source.

    
                            Screenshot of the Select a source page with Lambdas and
                                Greengrass_HelloWorld highlighted.
  12. For the Select a target field, choose Select. Then, on the Service tab, choose IoT Cloud, and then choose Next.

    
                            The Select a target with the Services tab, IoT Cloud and the
                                Next button highlighted.
  13. In the Optional topic filter field, type hello/world, then choose Next.

    
                            Screenshot with hello/world highlighted under Optional topic
                                filter.
  14. Finally, choose Finish.