How to keep connectivity status between node.js and python?

Question

I have web application based on Node.js and python based hardware. I want to keep status of connectivity between web server and hardware. If Hardware gets disconnected from web application then web application should get event or notification so based on that i can send notification to user. I have used mqtt for data communication, but to keep connection status I can't use MQTT coz it is connected with broker. I don't want to increase more load on server.

Which tools/technology/protocol/method should i use to keep connection status that device is offline or online?. That I also want use when user tries to send data to hardware using web application and if device is not connected with server then user should get notification that device is offline based on connection status.

Answer 1

The following code demonstrates the process I was hinting at in the comments.

The LWT feature tells the broker to publish a message marking the client as offline when it fails to respond in 1.5 times the keepalive period. If the client cleanly disconnects then it needs to mark it's self as offline. The client marks it's self as online when it connects to the broker.

All the status messages have the retained bit set so they will always be delivered when a client subscribes to the status topic.

import paho.mqtt.client as mqtt

# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, rc):
    print("Connected with result code "+str(rc))
    # Subscribing in on_connect() means that if we lose the connection and
    # reconnect then subscriptions will be renewed.
    client.subscribe("some/application/topic")
    # set status message to online
    client.publish("status/client1", payload="online", retain=True)

# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))

    if str(msg.payload) == "shutdown":
        # update status to offline as this will be a clean dissconect
        client.publish("status/client1", payload="offline", retain=True)
        client.disconnect()

client = mqtt.Client(client_id="client1")
client.on_connect = on_connect
client.on_message = on_message
client.will_set("status/client1", payload="offline", retain=True)

client.connect("mqtt.eclipse.org", 1883, 60)

# Blocking call that processes network traffic, dispatches callbacks and
# handles reconnecting.
# Other loop*() functions are available that give a threaded interface and a
# manual interface.
client.loop_forever()

It will be up to the OP to implement the notification about sending messages to offline clients requested at the end of the question.