mqtt客户端在线程和rest-api的情况下不接收消息

Question

I have a python script, which based on flask and mqtt. The use case is to receive a request via rest-api then to create a new thread which publishes some messages on mosquitto mqtt and expects a response (see subscribe). My problem is that I don't receive any messages. I think it has something to do with the thread, because without the thread it's working very fine.. Do you know what can be the problem?

Thank you in anticipation!

here the code:

from flask import Flask, Response
import paho.mqtt.client as mqtt
from threading import Thread
import threading

app = Flask(__name__)
lock = threading.Lock()

def on_connect(client, userdata, flags, rc):  # The callback for when the client connects to the broker
    print("Connected with result code {0}".format(str(rc)))  # Print result of connection attempt
    client.subscribe("/mytopic")


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


client = mqtt.Client(client_id=client_name, clean_session=True)
client.on_connect = on_connect  # Define callback function for successful connection
client.on_message = on_message  # Define callback function for receipt of a message
client.username_pw_set(mqtt_user, mqtt_password)
client.loop_start()
client.connect(mqtt_host)    


def test(param1, param2):
   lock.acquire()
   try:

      ret = client.publish("/mytopic", "")
      while True:
            check the response from mqtt => but i don't get the response anymore
            ....
            break
    finally:
        lock.release()
    return result


@app.route('/test/check', methods=['POST'])
def check():
    global sessionId
    sessionId = sessionId + 1
    t = Thread(target=test, args=(sessionId,None))
    t.start()
    return {'id': sessionId, 'eta': 0}


if __name__ == '__main__':
    app.run(debug=True)

Answer 1

There are a couple of problems with this.

1. Both the client.connect() and the client.subscribe() calls need iterations of the client network loop to run in order to complete properly.
2. The network loop needs to run at least once every keep alive period the time after the connection has been made in order to stop the broker disconnecting the client as dead. This means if there is a delay between starting the code and the first REST request then the client will be disconnected.

Better to use the client.start_loop() function to run MQTT client network loop continuously in the background on it's own.

You should also remove the call to client.subscribe() that is outside the on_connect() callback.

EDIT: As hashed out in the comments/chat the following works. It looks like running the flask app in debug mode does some strange things and creates multiple MQTT clients over and over again with the same client id. This leads to the broker constantly kicking the old ones off so messages never get delivered.

from flask import Flask, Response
import paho.mqtt.client as mqtt
import time
from threading import Thread
import threading

app = Flask(__name__)
lock = threading.Lock()
sessionId=0
cont=True

def on_connect(client, userdata, flags, rc): # The callback for when the client connects to the broker
print("Connected with result code {0}".format(str(rc))) # Print result of connection attempt
client.subscribe("mytopic")


def on_message(client, userdata, msg): # The callback for when a PUBLISH message is received from the server.
global cont
print(msg.topic)
cont=False


client = mqtt.Client(client_id="foo", clean_session=True)
client.on_connect = on_connect # Define callback function for successful connection
client.on_message = on_message # Define callback function for receipt of a message
#client.username_pw_set(mqtt_user, mqtt_password)
client.connect("localhost", port=1884)
client.loop_start()

def test(param1, param2):
lock.acquire()
try:
ret = client.publish("mytopic", "foo")
while cont:
time.sleep(5)
print("loop")
finally:
lock.release()

result = "foo"

return result


@app.route('/test/check', methods=['POST'])
def check():
global sessionId
sessionId = sessionId + 1
t = Thread(target=test, args=(sessionId,None))
t.start()
return {'id': sessionId, 'eta': 0}


if __name__ == '__main__':
print("started")
app.run()