umqtt.robust dies when MQTT broker gets restarted about micropython-lib HOT 15 OPEN

See #186 for a fix which will resubscribe to all existing topics if a reconnect is made.

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In the screenshot above, I have restarted the broker after two successful MQTT publish messages (two temperature values in this case)

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I use code to avoid such problem:

try: retries = 0 while (retries < 20): retries += 1; client.check_msg() time.sleep(1); except OSError: connect_and_subscribe()

def connect_and_subscribe():
global client
client = MQTTClient(CONFIG['client_id'], CONFIG['broker'], CONFIG['port'], "user", "pass", 120)
client.set_callback(callback)
client.connect(False)
print("Conn to {}".format(CONFIG['broker']))
client.subscribe(b"MBI/CURRENT_DATETIME")
time.sleep(1);
client.check_msg()
.....
So I always check if there any OsError and reconnect to MQTT.

You need to upgrade the robust by this PR: [https://github.com//pull/117]

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Is there a better way to go about handling this situation where the broker 'disappears' and then 're-appears' at some later time? The current implementation of umqtt.robust is not robust at all, even with the reconnect. check_msg never works even though it seems the client reconnected.

Should the umqtt.robust object include a list of subscribed topics to auto-resubscribe in the reconnect() function?

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That last message as a microPython newby worries me... I don't want to start coding in microPython on the basis of unreliable MQTT as we have reliable MQTT in C......

Can someone give an example of this "robust" code which WILL stay connected and which will resubscribe on reconnection - i.e. so that it just works in the background.

Is this possible?

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Is there a better way to go about handling this situation where the broker 'disappears' and then 're-appears' at some later time?

Yes, the current implementation of umqtt.robust does not handle the case when the broker is restarted and forgets all of its state (at least the state related to your client).

Should the umqtt.robust object include a list of subscribed topics to auto-resubscribe in the reconnect() function?

Prehaps. This is indeed how other libraries work (eg https://github.com/fusesource/mqtt-client). The robust MQTTClient class would need to override the "subscribe" method to record the topics (and qos), and then in the reconnect() method it would call subscribe() again after reconnecting.

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Damn, I was in the process of writing a fix for this. You win the day, sir!

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@craftyguy I'd be interested to see your solution. And also if you want to test my solution and give feedback that would be great.

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I literally started about less than an hour ago, but my approach was pretty much the same as yours. Your solution looks to be more elegant/robust. I'll give this a shot possibly as early as tomorrow, since I'm tired of hacking together a more robust robust mqtt 😄

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@dpgeorge

I tried your patch (#186), and it doesn't seem to work with this simple test program:

import machine
from umqtt.robust import MQTTClient
import utime

MQTT_SERVER = '1.1.1.1'
IN = 'in'
OUT = 'out'

# mqtt subscription callback
def sub_cb(topic, msg):
    t = topic.decode('ASCII')
    m = msg.decode('ASCII')
    print("received new topic/msg: %s / %s" % (t, m))
    if t == IN:
        print("IN: %s" % m)

umqtt_client = MQTTClient("test_client", MQTT_SERVER)
umqtt_client.DEBUG = True
umqtt_client.set_callback(sub_cb)
umqtt_client.connect(clean_session=False)
umqtt_client.subscribe(IN)
print("Connected to MQTT broker: %s" % MQTT_SERVER)


def main():
    global umqtt_client
    while True:
        utime.sleep(1)
        umqtt_client.check_msg()
        umqtt_client.publish(OUT, b'hi!')

I should note that I am invoking the main() function here from main.py.

When I restart the mqtt broker, I get an mqtt: OSError(-1,) printed to console. I can see the client reconnects to the broker since there's a message in the broker log about this, but the client doesn't respond to messages published to the IN topic, nor does it publish anything else to OUT topic.

If my test is an invalid use of umqtt, please let me know since I am relatively new to using mqtt!

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@craftyguy for your example to work I think you need to connect with clean_session=True, because you'll be explicitly resubscribing upon reconnection.

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@dpgeorge I see, thank you for pointing that out. I also see the PR was merged :)
I will give it another try!

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I also see the PR was merged

@craftyguy No it wasn't, so you'll need to pull the PR explicitly to test it.

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I tried the PR and it does recover from a broker restart but when the broker loses power and comes back, it doesn't recover. It is completely reproducible, happens every time I take the power out from my raspberry pi zero running the mosquitto broker. Does anyone else face this behaviour?
The problem seems to be that umqtt.robust does raise an error when killing my mqtt broker from the terminal but doesn't raise any error when broker dies from a power outage.

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