Google IoT - Right mode to receive notifications (subscribe working)

Question

I am following this tutorial and I already have my code publishing messages to /devices/sm1/events topic, in which sm1 is my device id.

I would like to know how to subscribe to this topic since the tutorial says to use /devices/sm1/config but I am getting empty messages. I already tried use the same "path" used in publishing (/devices/sm1/events), but it also did not work.

It is strange that the name I gave to the topic was sm1 and the topic associated to my device is on GoogleIoT console is exhibited as projects/myprojectname/topics/sm1. So, besides to discover how to subscribe to mentioned topic, I appreciate also any explanation related to the correct way of using pub/sub topics in GoogleIoT (the documentation is not so clear).

This is my subscribe.py:

mqtt_url = "mqtt.googleapis.com"
mqtt_port = 8883
topic = "/devices/sm1/config"

def on_connect(client, userdata, flags, response_code):
    print("Connected with status: {0}".format(response_code))
    client.subscribe(topic, 1)

def on_message(client, userdata, msg):
    print("Topic: {0}  --  Payload: {1}".format(msg.topic, msg.payload))

if __name__ == "__main__":    
    client = mqtt.Client("projects/{}/locations/{}/registries/{}/devices/{}".format(
                         project_id,
                         cloud_region,
                         registry_id,
                         device_id))

    client.username_pw_set(username='unused',
                           password=jwt_maker.create_jwt(project_id,
                                               private_key,
                                               algorithm="RS256"))

    client.tls_set(root_ca,
                   certfile = public_crt,
                   keyfile = private_key,
                   cert_reqs = ssl.CERT_REQUIRED,
                   tls_version = ssl.PROTOCOL_TLSv1_2,
                   ciphers = None)


    client.on_connect = on_connect
    client.on_message = on_message

    print "Connecting to Google IoT Broker..."
    client.connect(mqtt_url, mqtt_port, keepalive=60)
    client.loop_forever()

My output:

Connected with status: 0
Topic: /devices/sm1/config -- Payload:
Topic: /devices/sm1/config -- Payload:

Answer 1

I had to surrender to Google Pub/Sub library in order to receive notifications related to my specified topic.

My publish code (only the important parts):

mqtt_url = "mqtt.googleapis.com"
mqtt_port = 8883
mqtt_topic = "/devices/sm1/events"

client = mqtt.Client("projects/{}/locations/{}/registries/{}/devices/{}".format(
                     project_id,
                     cloud_region,
                     registry_id,
                     device_id), protocol=mqtt.MQTTv311)

client.username_pw_set(username='unused',
                       password=jwt_maker.create_jwt(project_id,
                                           private_key,
                                           algorithm="RS256"))

client.tls_set(root_ca, 
               certfile = public_crt, 
               keyfile = private_key, 
               cert_reqs = ssl.CERT_REQUIRED, 
               tls_version = ssl.PROTOCOL_TLSv1_2, 
               ciphers = None)

client.connect(mqtt_url, mqtt_port, keepalive=60)
res = client.publish(mqtt_topic, some_data, qos=1)

In the Google Cloud Platform portal, I had to create a subscription, in the Pub/Sub section, assigning it to my created topic, which was already my default topic (probably linked to /events topic). The created subscription has the following format:

projects/project_name/subscriptions/subscription_name

My subscribe code, using the Google Pub/Sub library, since it is not possible to use the MQTT protocol:

from google.cloud import pubsub

def callback(message):
    print(message.data)
    message.ack()

project_id = "project_name"
subscription_name = "sm1"

subscriber = pubsub.SubscriberClient()
subscription_name = 'projects/{}/subscriptions/{}'.format(project_id, subscription_name)

subscription = subscriber.subscribe(subscription_name)
future = subscription.open(callback)

try:
    future.result()
except Exception as ex:
    subscription.close()
    raise

I hope this can help anyone. More details can be found here(github).