Execute lambda function on local device using greengrass

Question

I am trying to learn AWS greengrass and so I was following this tutorial https://docs.aws.amazon.com/greengrass/latest/developerguide/gg-gs.html which explains step by step on setting up with greengrass on raspberry pi and publishing some messages using a lambda function.

A simple lambda function is as following :

import greengrasssdk
import platform
from threading import Timer
import time


# Creating a greengrass core sdk client
client = greengrasssdk.client('iot-data')

# Retrieving platform information to send from Greengrass Core
my_platform = platform.platform()


def greengrass_hello_world_run():
    if not my_platform:
        client.publish(topic='hello/world', payload='hello Sent from Greengrass Core.')
    else:
        client.publish(topic='hello/world', payload='hello 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()


# This is a dummy handler and will not be invoked
# Instead the code above will be executed in an infinite loop for our example
def function_handler(event, context):
    return

Here this works but I am trying to understand it better by having a lambda function to do some extra work for example opening a file and writing to it.

I modified the greengrass_hello_world_run() function as following

def greengrass_hello_world_run():
    if not my_platform:
        client.publish(topic='hello/world', payload='hello Sent from Greengrass Core.')
    else:
        stdout = "hello from greengrass\n"
        with open('/home/pi/log', 'w') as file:
            for line in stdout:
                file.write(line)
        client.publish(topic='hello/world', payload='hello Sent from Greengrass Core running on platform: {}'.format(my_platform))

I expect upon deploying, the daemon running on my local pi should create that file in the given directory coz I believe greengrass core tries to run this lambda function on local device. However it doesnt create any file nor it publish anything coz I believe this code might be breaking. Not sure how though, I tried looking into cloudwatch but I dont see any events or errors being reported.

Any help on this would be really appreciated, cheers !

score 3 · Answer 1 · answered May 10 '18 at 14:35

A few thoughts on this...

If you turn on the local logging in your GG group set up it will start to write logs locally on your PI. The settings are:

The logs are located in: /greengrass/ggc/var/log/system

If you tail the python_runtime.log you can see any errors from the lambda execution.

If you want to access local resources you will need to create a resource in your GG group definition. You can then give this access to a volume which is where you can write your file.

You do need to deploy your group once this has been done for the changes to take effect.

score 2 · Accepted Answer · answered Jan 26 '18 at 19:10

I think I found the answer, we have to create the resource in lambda environment and also make sure to give read and write access to lambda for accessing that resource. By default lambda can only access /tmp folder.

Here is the link to the documentation

https://docs.aws.amazon.com/greengrass/latest/developerguide/access-local-resources.html

Execute lambda function on local device using greengrass

2 Answers2