AWS GreenGrass check Topic in Lambda

Question

I am currently fiddeling around with AWS GreenGrass and I have gone through most of the basic examples. Now the problem I have understanding how I would perform different actions in my lambda function depending on the MQTT topic it was triggered by. I could not really find a documentation on how to differentiate which topic triggered the function handler in my python script. The thing I am trying to build is a small controller for my sonos system which uses the following very basic topics:

sonos/play

Which just tries playing a certain song I hardcoded.

sonos/pause

Which should be pausing the system.

I know I could create several lambdas for each command but that seems pretty finicky. Can someone please explain how one would tackle this sort of issue?

Answer 1

Based on this dev guide, in your lambda you can get the topic from the context:

def get_input_topic(context):
    try:
        topic = context.client_context.custom['subject']
    except Exception as e:
        logging.error('Topic could not be parsed. ' + repr(e))
    return topic

action = get_input_topic(context).split('/')[-1]

Answer 2

I don't actually think this is possible. If you look at both of the event and context objects passed into your lambda function, neither of them contain the topic address.

What I have had to do is to have a separate lambda function to respond for messages for each MQTT topic.

Happy to be corrected if this can be done.