CODEWITHSUNDEEP
pythonmqtt
R Xue
R Xue

Reputation: 35

paho-MQTT python: How to let the loop_forever support subscribe message?

I have tested the example program on paho-mqtt, and I know that the function loop_forever() can handles reconnecting. But my question is that, although loop_forever() can reconnect, it cannot re-subscribe. It should be a problem when the server suddenly crashes, in this case, the client is still listening, but when the server is restarted, the client can reconnect, but cannot subscribe message any more. I think maybe I should re-write the loop_forever() function, but I am not sure if I were right, and how to do it.

import sys
try:
    import paho.mqtt.client as mqtt
except ImportError:
    # This part is only required to run the example from within the examples
    # directory when the module itself is not installed.
    #
    # If you have the module installed, just use "import paho.mqtt.client"
    import os
    import inspect
    cmd_subfolder = os.path.realpath(os.path.abspath(os.path.join(os.path.split(inspect.getfile( inspect.currentframe() ))[0],"../src")))
    if cmd_subfolder not in sys.path:
        sys.path.insert(0, cmd_subfolder)
    import paho.mqtt.client as mqtt

def on_connect(mqttc, obj, flags, rc):
    print("rc: "+str(rc))

def on_message(mqttc, obj, msg):
    print(msg.topic+" "+str(msg.qos)+" "+str(msg.payload))

def on_publish(mqttc, obj, mid):
    print("mid: "+str(mid))

def on_subscribe(mqttc, obj, mid, granted_qos):
    print("Subscribed: "+str(mid)+" "+str(granted_qos))

def on_log(mqttc, obj, level, string):
    print(string)

# If you want to use a specific client id, use
# mqttc = mqtt.Client("client-id")
# but note that the client id must be unique on the broker. Leaving the client
# id parameter empty will generate a random id for you.
mqttc = mqtt.Client()
mqttc.on_message = on_message
mqttc.on_connect = on_connect
mqttc.on_publish = on_publish
mqttc.on_subscribe = on_subscribe
# Uncomment to enable debug messages
#mqttc.on_log = on_log
mqttc.connect("m2m.eclipse.org", 1883, 60)
mqttc.subscribe("$SYS/#", 0)


mqttc.loop_forever()

Upvotes: 1

Views: 18744

Answers (2)

C.Priem
C.Priem

Reputation: 21

While instantiating your mqtt client you can set the "clean session" flag to false.

mqttc = mqtt.Client(clean_session=False)

Citation from the mosquitto manual:

Clean session / Durable connections

On connection, a client sets the "clean session" flag, which is sometimes also known as the "clean start" flag. If clean session is set to false, then the connection is treated as durable. This means that when the client disconnects, any subscriptions it has will remain and any subsequent QoS 1 or 2 messages will be stored until it connects again in the future. If clean session is true, then all subscriptions will be removed for the client when it disconnects.

Upvotes: 0

ralight
ralight

Reputation: 11608

The easy way to deal with this is to do your subscribing in the on_connect callback, then when you reconnect all the subscriptions will be restored as well.

Upvotes: 3

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