AWS Greengrass
Developer Guide

Testing Long-Lived Lambda Functions

A long-lived Lambda function starts automatically when the AWS Greengrass core starts (and runs in a single container or sandbox). Any variables or preprocessing that are defined outside of the function handler are retained for every invocation of the function handler. Multiple invocations of the function handler are queued until earlier invocations have been executed. The Lambda function is similar to the function except there is a variable, my_counter, that is outside of the function_handler(event, context) method (code comments were removed for brevity):

import greengrasssdk import platform import time import json client = greengrasssdk.client('iot-data') my_platform = platform.platform() my_counter = 0 def function_handler(event, context): global my_counter my_counter = my_counter + 1 if not my_platform: client.publish( topic='hello/world/counter', payload=json.dumps({'message': 'Hello world! Sent from Greengrass Core. Invocation Count: {}'.format(my_counter)}) ) else: client.publish( topic='hello/world/counter', payload=json.dumps({'message': 'Hello world! Sent from Greengrass Core running on platform: {}. Invocation Count: {}' .format(my_platform, my_counter)}) ) time.sleep(20) return
  1. On the GREENGRASS GROUP page, choose Subscription, then Add Subscription. Under Select a source, choose the Lambdas tab, then choose Greengrass_HelloWorld_Counter. Next, under Select a target, select the Services tab, choose IoT Cloud, and then choose Next.

                            Select your source and target screenshot with
                                Greengrass_HelloWorld_Counter, IoT Cloud, and the Next button

    For Optional topic filter, type hello/world/counter. Choose Next and then choose Finish.

                            Screenshot with hello/world/counter and the Next button
  2. This single subscription goes in one direction only: from the Greengrass_HelloWorld_Counter Lambda function to the AWS IoT cloud. To trigger this Lambda function from the cloud, you need to create a subscription in the opposite direction. Add another subscription with IoT Cloud as the source and Greengrass_HelloWorld_Counter as the target. Use the hello/world/counter/trigger topic:

                            Screenshot of the Confirm and save your Subscription page with
                                IoT Cloud, hello/world/counter/trigger,
                                Greengrass_HelloWorld_Counter, and Finish highlighted.

    Note the /trigger extension above – because you have created two subscriptions, you do not want them to interfere with each other.

  3. Make sure that your AWS Greengrass core device is connected to the internet and that the AWS Greengrass daemon is running. Note that with the daemon running, the prior Lambda function will continue to send messages to the hello/world topic (in the AWS IoT cloud). This does not, however, interfer with the messages sent from the Lambda function to the AWS IoT cloud, since they're directed to a different topic, namely hello/world/counter.

    Next, under the Actions menu, choose Deploy to deploy your new configurations to the AWS Greengrass device.

                            Deployments and Deploy (under the Actions" menu) are
  4. After your deployment is complete, in the AWS IoT console, choose Test. In Subscription topic, type hello/world/counter. For Quality of Service, select 0. For MQTT payload display, select Display payloads as strings, and then choose Subscribe to topic.

                            Subscriptions screenshot with hello/world/counter, 0, Display
                                payloads as string, and the Subscribe to topic button all

    Unlike Part 1 of this module, you should not be able to see any messages after you subscribe to hello/world/counter. This is because the code to publish to the topic hello/world/counter is inside the function_handler(event, context) function, and function_handler(event, context) is triggered only when it receives an MQTT message on the hello/world/counter/trigger topic. To help further explain this, consider the greengrass_HelloWorld_Counter related subscriptions:

                            Subscriptions webpage showing greengrass_HelloWorld_Counter
                                related subscriptions.

    In the second row, we see that the greengrass_HelloWorld_Counter Lambda function can send messages to the IoT Cloud on the hello/world/counter topic. In the third row, we see that the IoT Cloud will can send messages to the greengrass_HelloWorld_Counter Lambda function when that message is sent to the hello/world/counter/trigger topic (note that there is nothing special about the word trigger). The greengrass_HelloWorld_Counter Lambda function ignores these sent messages and merely runs the code within function_handler(event, context), which sends a message back to the hello/world/counter topic in the AWS IoT cloud (see the prior code listing).

    So, to trigger the function_handler(event, context) handler, publish any message (the default message is fine) to the hello/world/counter/trigger topic, as shown next.

                            Default Hello from AWS IoT console message sent to
                                hello/world/counter/trigger with the Publish to topic button

    Every time a message is published to the hello/world/counter/trigger topic, the my_counter variable is incremented (see Invocation Count in the following). Because the function handler in the Lambda function includes a 20-second sleep cycle (i.e., time.sleep(20)), repeatedly triggering the handler queues up responses from the AWS Greengrass core.

                            Screenshot showing the incrementing of Invocation Count from 1,
                                2, and 3.