Get thing name associated with MQTT subscribed message

[elliotmakuh]

I am creating a Python script that will listen for shadow change events for multiple IoT things, decide if the changes are actionable and if so modify the thing shadow.
My script is subscribing to the following topics for one or more IoT thing names:

• update accepted/rejected
• delta updated events
• get accepted/rejected

The issue I am running into is when my subscription callback is called I do not have any indication which thing triggered the shadow event. I am testing using two IoT things in the backend (e.g. thing-1 and thing-2). When I modify the shadow for thing-1 I receive a callback for the delta but the ShadowDeltaUpdatedEvent does not contain the thing name that triggered the callback.

Here is a short snippet of how I am subscribing:

    for thing in things:
        print("Subscribing to Update responses...")
        update_accepted_subscribed_future, _ = shadow.subscribe_to_update_shadow_accepted(
            request=iotshadow.UpdateShadowSubscriptionRequest(thing_name=thing),
            qos=mqtt.QoS.AT_LEAST_ONCE,
            callback=on_update_shadow_accepted)

        update_rejected_subscribed_future, _ = shadow.subscribe_to_update_shadow_rejected(
            request=iotshadow.UpdateShadowSubscriptionRequest(thing_name=thing),
            qos=mqtt.QoS.AT_LEAST_ONCE,
            callback=on_update_shadow_rejected)

        # Wait for subscriptions to succeed
        update_accepted_subscribed_future.result()

I am getting the following payload in my on_update_shadow_accepted callback:

Update accepted: awsiot.iotshadow.UpdateShadowResponse(client_token=None, metadata=awsiot.iotshadow.ShadowMetadata(desired={'welcome': {'timestamp': 1660602853}, 'turd': {'timestamp': 1660602853}}, reported={'welcome': {'timestamp': 1660602853}}), state=awsiot.iotshadow.ShadowState(desired={'welcome': 'aws-iot', 'turd': True}, desired_is_nullable=False, reported={'welcome': 'aws-iot'}, reported_is_nullable=False), timestamp=datetime.datetime(2022, 8, 15, 16, 34, 13), version=7)

Notice there is no reference to the thing name that triggered the update message. I dug into the SDK a bit and didn't see any place where the thing name is associated with a given callback. I believe the thing name can be inferred based on the topic that the message was received on but it looks like the topic is not passed to the final callback for a subscription.

Is there a different way to subscribe so that I can associate a thing name and callback? I assume I am taking the wrong approach here but am not sure how to fix it.

Thanks for any thoughts/feedback.

[jmklix]

You could always use the something similar to the callback from the pubsub sample and use a wildcard to get all shadow topics. You can look here to find info on the reserved topics.

    shadow_topic = "$aws/things/#/shadow"
    print("Subscribing to topic '{}'...".format(shadow_topic))
    subscribe_future, packet_id = mqtt_connection.subscribe(
        topic=shadow_topic,
        qos=mqtt.QoS.AT_LEAST_ONCE,
        callback=on_message_received)

This would return the topics that you are looking for.

[elliotmakuh]

Great idea although I ended up using the suggestion below. This would work fine as well though. Thanks!

[bretambrose]

You could make your message handlers lambdas that capture the associated thing name internally.

[elliotmakuh]

I ended up making a class to store the thing name and associated callbacks but it was inspired by your suggestion. Thanks!

[github-actions[bot]]

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