The appropriate value of max_packets for MQTT loop

问题:

I have this code that should run indefinitely, however, it doesn't. It keeps on stopping every few hours from the client's side (stop publishing, the loop keeps on running, but nothing is received at the broker), and the only thing that can be done is to rerun it again.

I was advised here to increase the number of max_packets for the loop function, but it's not working and the client stops publishing randomly without continuing. What should be done? I tried the values of 1, 3, 5, 50 and a 1000 but no use.

Code:

client = mqtt.Client()
client.connect(address, 1883, 60)

while True:
    data = getdata()
    client.publish("$ahmed/",data,0)
    client.loop(timeout=1.0, max_packets = 1) # what should be the parameters here so it doesn't stop publishing?
    time.sleep(0.2)

回答1:

In addition to applications messages which are published/subscribed, MQTT also have internal keepalive to avoid problem of half open TCP connections(1). And it is the responsibility of client to make sure keepalives are sent. As per specification, the broker will disconnect clients which doesn't send keepalives in one and half times of keepalive time interval( in absence of other messages).

In addition to sending messages, the loop()* functions also maintains this keepalive traffic flow between broker and client.

A random try: Try using loop_start() once instead of calling loop() in while loop. E.g.

client = mqtt.Client()
client.connect(address)
#runs a thread in background to call loop function internally. 
#In addition, this also reconnects to broker on a lost connection. 
client.loop_start()

while True:
    data = getdata()
    client.publish("$ahmed",data)
client.loop_stop()

回答2:

Just a random guess... has the client disconnected?

In your code you are not handling any callback like on_disconnect(client, userdata, rc) which is called when the client disconnects from the broker.

def on_disconnect_handler(client, userdata, rc):
    if rc != 0:
        print("Unexpected disconnection.")

client.on_disconnect = on_disconnect_handler

You are also not checking loop() return value:
Returns MQTT_ERR_SUCCESS on success.
Returns >0 on error.

You should do something like

while True:
    rc = client.loop(timeout=1.0)
    if rc:
        # handle loop error here

回答3:

Just make the client connect every time the loops is through. I have tested it and connecting to the brokers doesn't any significant extra latency on the flow. Since I have to rerun the program to make work it again, I may as well reconnect the client in the loop, so I don't have to do it myself. This is the rawest idea I could come up with that seems to be working without any problems.

client = mqtt.Client()
client.connect(address, 1883, 60)

while True:
    client.connect(address, 1883, 60) # just let it reconnect every time it loops ;)! 
    data = getdata()
    client.publish("$ahmed/",data,0)
    client.loop(timeout=1.0, max_packets = 1) 
    time.sleep(0.2)