PAHO MQTT Python客户端-缺少确认，保证为订阅者交付

Question

开发人员

我正在研究Paho MQTT客户端(用于Python)，我想我理解发布者的QoS设置，例如传感器或任何数据源，确实是有意义的——毕竟，您希望能够确保(代理/服务器)已经接收到消息。

我还认为，从订户的角度来看，请求QoS"2"以确保MQTT服务器确实向订户发送了每条消息是有意义的，但我很难解决这个问题：订户似乎没有一种方式来表示接收到的消息处理成功(或否)，换句话说，是否缺少某种方式的显式确认？

用例-成功处理消息

我想订阅一个主题并成功处理每个数据点，例如存储到数据库中。因此，我需要适应订阅者在"飞行中"失败的情况，即在收到来自代理的消息后，但在成功处理(存储到DB)之前。

假设

现在，如果订阅者(fixed client_id)在处理数据时失败，它将重新启动，然后重新连接到MQTT代理，broker通过id识别这个特定的客户端，并再次开始推送消息——从订阅者断开连接后的下一条消息开始——就broker而言，它确实成功地传递了最后一条消息(不知道订阅者崩溃了)。

潜在解决方案

若上面的假设是真的，那个么我最好不要使用固定的client_id，而是使用"clean_session"和随机client_id；通过这种方式，代理开始传递特定主题的所有消息。当然，这将把跟踪成功处理的消息的责任推给订户。

需要这样做吗？或者有没有一种方法可以显式地确认订阅者成功处理了消息，这样代理就可以重新传输了——我对Paho Python库特别感兴趣。

提前感谢！

编辑1:

相关代码：

def _handle_on_message(self, message):
matched = False
with self._callback_mutex:
try:
topic = message.topic
except UnicodeDecodeError:
topic = None
if topic is not None:
for callback in self._on_message_filtered.iter_match(message.topic):
with self._in_callback:
callback(self, self._userdata, message)
matched = True
if matched == False and self.on_message:
with self._in_callback:
self.on_message(self, self._userdata, message)

来源：https://github.com/eclipse/paho.mqtt.python/blob/v1.3.1/src/paho/mqtt/client.py#L2631

编辑2:

@hardlib确实是正确的-当回调函数内部失败时，客户端代码不会向代理确认

说明的一些代码

订阅者中的on_message回调

def on_message(client, userdata, msg):
print(msg.topic+" "+str(msg.payload)+ " mid:" + str(msg.mid))
num = ''.join(x for x in str(msg.payload) if x.isdigit())
if int(num) % 3 == 0:
print("Going to show myself out now (sys.exit(1))")
time.sleep(1)
sys.exit(1)

出版商

while True:
count += 1
logging.debug("At: " + str(count))
msg = "message: {counter}".format(counter=count)
mqttc.publish("paho/stacko", msg, qos=2, retain=False)

日志

出版商

root@14f00c2576b2:/usr/src/app# python publisher.py
DEBUG:root:Sending CONNECT (u0, p0, wr0, wq0, wf0, c1, k60) client_id=b''
DEBUG:root:At: 1
DEBUG:root:Sending PUBLISH (d0, q2, r0, m1), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received CONNACK (0, 0)
DEBUG:root:Received PUBREC (Mid: 1)
DEBUG:root:Sending PUBREL (Mid: 1)
DEBUG:root:Received PUBCOMP (Mid: 1)
DEBUG:root:At: 2
DEBUG:root:Sending PUBLISH (d0, q2, r0, m2), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 2)
DEBUG:root:Sending PUBREL (Mid: 2)
DEBUG:root:Received PUBCOMP (Mid: 2)
DEBUG:root:At: 3
DEBUG:root:Sending PUBLISH (d0, q2, r0, m3), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 3)
DEBUG:root:Sending PUBREL (Mid: 3)
DEBUG:root:Received PUBCOMP (Mid: 3)
DEBUG:root:At: 4
DEBUG:root:Sending PUBLISH (d0, q2, r0, m4), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 4)
DEBUG:root:Sending PUBREL (Mid: 4)
DEBUG:root:Received PUBCOMP (Mid: 4)
DEBUG:root:At: 5
DEBUG:root:Sending PUBLISH (d0, q2, r0, m5), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 5)
DEBUG:root:Sending PUBREL (Mid: 5)
DEBUG:root:Received PUBCOMP (Mid: 5)
DEBUG:root:At: 6
DEBUG:root:Sending PUBLISH (d0, q2, r0, m6), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 6)
DEBUG:root:Sending PUBREL (Mid: 6)
DEBUG:root:Received PUBCOMP (Mid: 6)
DEBUG:root:At: 7
DEBUG:root:Sending PUBLISH (d0, q2, r0, m7), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 7)
DEBUG:root:Sending PUBREL (Mid: 7)
DEBUG:root:Received PUBCOMP (Mid: 7)
DEBUG:root:At: 8
DEBUG:root:Sending PUBLISH (d0, q2, r0, m8), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 8)
DEBUG:root:Sending PUBREL (Mid: 8)
DEBUG:root:Received PUBCOMP (Mid: 8)
DEBUG:root:At: 9
DEBUG:root:Sending PUBLISH (d0, q2, r0, m9), 'b'paho/stacko'', ... (10 bytes)
DEBUG:root:Received PUBREC (Mid: 9)
DEBUG:root:Sending PUBREL (Mid: 9)
DEBUG:root:Received PUBCOMP (Mid: 9)
DEBUG:root:At: 10
DEBUG:root:Sending PUBLISH (d0, q2, r0, m10), 'b'paho/stacko'', ... (11 bytes)
DEBUG:root:Received PUBREC (Mid: 10)
DEBUG:root:Sending PUBREL (Mid: 10)
DEBUG:root:Received PUBCOMP (Mid: 10)
DEBUG:root:At: 11
DEBUG:root:Sending PUBLISH (d0, q2, r0, m11), 'b'paho/stacko'', ... (11 bytes)
DEBUG:root:Received PUBREC (Mid: 11)
DEBUG:root:Sending PUBREL (Mid: 11)
DEBUG:root:Received PUBCOMP (Mid: 11)

订阅者

root@ca7dcaaed68f:/usr/src/app# python subscriber.py
DEBUG:root:Sending CONNECT (u0, p0, wr0, wq0, wf0, c0, k60) client_id=b'client_02'
DEBUG:root:Received CONNACK (0, 0)
DEBUG:root:Connected
Connected with result code 0
DEBUG:root:Sending SUBSCRIBE (d0) [(b'#', 2)]
DEBUG:root:Received SUBACK
DEBUG:root:Received PUBLISH (d0, q2, r0, m1), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 1)
DEBUG:root:Received PUBREL (Mid: 1)
DEBUG:root:Message!!!! b'message: 2'
paho/stacko b'message: 2' mid:1
Num: 2
DEBUG:root:Sending PUBCOMP (Mid: 1)
DEBUG:root:Received PUBLISH (d0, q2, r0, m2), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 2)
DEBUG:root:Received PUBREL (Mid: 2)
DEBUG:root:Message!!!! b'message: 3'
paho/stacko b'message: 3' mid:2
Num: 3
Going to show myself out now (sys.exit(1))
root@ca7dcaaed68f:/usr/src/app# python subscriber.py
DEBUG:root:Sending CONNECT (u0, p0, wr0, wq0, wf0, c0, k60) client_id=b'client_02'
DEBUG:root:Received CONNACK (1, 0)
DEBUG:root:Connected
Connected with result code 0
DEBUG:root:Sending SUBSCRIBE (d0) [(b'#', 2)]
DEBUG:root:Received PUBREL (Mid: 2)
DEBUG:root:Received PUBLISH (d0, q2, r0, m3), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 3)
DEBUG:root:Received PUBLISH (d0, q2, r0, m4), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 4)
DEBUG:root:Received SUBACK
DEBUG:root:Received PUBREL (Mid: 3)
DEBUG:root:Message!!!! b'message: 4'
paho/stacko b'message: 4' mid:3
Num: 4
DEBUG:root:Sending PUBCOMP (Mid: 3)
DEBUG:root:Received PUBREL (Mid: 4)
DEBUG:root:Message!!!! b'message: 5'
paho/stacko b'message: 5' mid:4
Num: 5
DEBUG:root:Sending PUBCOMP (Mid: 4)
DEBUG:root:Received PUBLISH (d0, q2, r0, m5), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 5)
DEBUG:root:Received PUBREL (Mid: 5)
DEBUG:root:Message!!!! b'message: 6'
paho/stacko b'message: 6' mid:5
Num: 6
Going to show myself out now (sys.exit(1))
root@ca7dcaaed68f:/usr/src/app# python subscriber.py
DEBUG:root:Sending CONNECT (u0, p0, wr0, wq0, wf0, c0, k60) client_id=b'client_02'
DEBUG:root:Received CONNACK (1, 0)
DEBUG:root:Connected
Connected with result code 0
DEBUG:root:Sending SUBSCRIBE (d0) [(b'#', 2)]
DEBUG:root:Received PUBREL (Mid: 2)
DEBUG:root:Received PUBREL (Mid: 5)
DEBUG:root:Received SUBACK
DEBUG:root:Received PUBLISH (d0, q2, r0, m6), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 6)
DEBUG:root:Received PUBREL (Mid: 6)
DEBUG:root:Message!!!! b'message: 7'
paho/stacko b'message: 7' mid:6
Num: 7
DEBUG:root:Sending PUBCOMP (Mid: 6)
DEBUG:root:Received PUBREL (Mid: 2)
DEBUG:root:Received PUBLISH (d0, q2, r0, m7), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 7)
DEBUG:root:Received PUBREL (Mid: 7)
DEBUG:root:Message!!!! b'message: 8'
paho/stacko b'message: 8' mid:7
Num: 8
DEBUG:root:Sending PUBCOMP (Mid: 7)
DEBUG:root:Received PUBLISH (d0, q2, r0, m8), 'paho/stacko', ...  (10 bytes)
DEBUG:root:Sending PUBREC (Mid: 8)
DEBUG:root:Received PUBREL (Mid: 8)
DEBUG:root:Message!!!! b'message: 9'
paho/stacko b'message: 9' mid:8
Num: 9
Going to show myself out now (sys.exit(1))
root@ca7dcaaed68f:/usr/src/app# python subscriber.py
DEBUG:root:Sending CONNECT (u0, p0, wr0, wq0, wf0, c0, k60) client_id=b'client_02'
DEBUG:root:Received CONNACK (1, 0)
DEBUG:root:Connected
Connected with result code 0
DEBUG:root:Sending SUBSCRIBE (d0) [(b'#', 2)]
DEBUG:root:Received PUBREL (Mid: 2)
DEBUG:root:Received PUBREL (Mid: 5)
DEBUG:root:Received PUBREL (Mid: 8)
DEBUG:root:Received SUBACK
DEBUG:root:Received PUBLISH (d0, q2, r0, m9), 'paho/stacko', ...  (11 bytes)
DEBUG:root:Sending PUBREC (Mid: 9)
DEBUG:root:Received PUBREL (Mid: 9)
DEBUG:root:Message!!!! b'message: 10'
paho/stacko b'message: 10' mid:9
Num: 10
DEBUG:root:Sending PUBCOMP (Mid: 9)
DEBUG:root:Received PUBREL (Mid: 5)
DEBUG:root:Received PUBLISH (d0, q2, r0, m10), 'paho/stacko', ...  (11 bytes)
DEBUG:root:Sending PUBREC (Mid: 10)
DEBUG:root:Received PUBREL (Mid: 10)
DEBUG:root:Message!!!! b'message: 11'
paho/stacko b'message: 11' mid:10
Num: 11
DEBUG:root:Sending PUBCOMP (Mid: 10)
DEBUG:root:Received PUBREL (Mid: 2)

结论(目前)

MQTT(至少是Mosquitto)在持久性方面确实按预期工作：如果客户端重新连接，它可以"弥补"自上次连接以来丢失的消息。然而，即使为订阅者和发布者都设置了qos=2，崩溃前的最后一条消息也不会被重新处理

Answer 1

如果您的客户端在处理消息时有可能失败，那么在开始处理消息之前，您应该将消息存储在某个地方(可能在磁盘或数据库中)。

然后，您可以在客户端重新启动时检查此存储，并尝试再次处理它。如果在重新连接(使用固定的client_id)之前这样做，那么在处理失败的消息时就不必担心会传递新消息。

编辑：

还要更详细地查看代码，对于QOS2，传递确认的最后一段似乎只在on_message完成后发送，因此，如果在on_message中处理消息时出错，那么代理应该重新传递该消息。

https://github.com/eclipse/paho.mqtt.python/blob/e9914a759f9f5b8081d59fd65edfd18d229a399e/src/paho/mqtt/client.py#L2506