PAHO MQTT Python 客户端 - 缺少确认，保证为订户交付
PAHO MQTT Python client - acknowledgement missing, guaranteed delivery for subscriber
各位开发者，
我正在查看 Paho MQTT 客户端（Python），我想我了解发布者的 QoS 设置，例如传感器或任何数据源确实有意义 - 毕竟您希望能够确保已收到消息（由 broker/server）。
我也认为从订阅者的角度来看，请求例如QoS“2”，以确保 MQTT 服务器确实将每条消息发送给订阅者，但我正在努力解决这个问题：订阅者似乎没有办法表示成功（或不）处理收到的消息消息，换句话说，似乎缺少某种明确确认的方式？
用例 - 成功处理消息
我想订阅一个主题并成功处理每个个数据点，例如通过存储到数据库。因此，我需要适应订阅者失败 "in-flight" 的情况，即在收到来自代理的消息之后但在成功处理（存储到数据库）之前。
假设
现在，如果订阅者（已修复 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，代理通过 ID 识别此特定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 - 就代理而言，它确实成功传递了最后一条消息（不知道订阅者崩溃了）。
可能的解决方案
如果上述假设成立，那么我最好不要使用固定的 client_id，而是使用 "clean_session" 和随机的 client_id；这样，代理开始传递所有消息，这些消息是为特定主题保留的。这当然会将跟踪成功处理的消息的责任放在订阅者身上。
需要这样做吗？或者有没有一种方法可以明确确认订户成功处理消息，因此代理可以重新传输应该需要 - 我对 Paho Python 库特别感兴趣。
提前致谢！
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
用于说明的一些代码
on_message 订阅者回调
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)
订户
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（暂时）
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 "catching up" 处理自上次连接以来丢失的那些消息。但是，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
如果您的客户端在处理消息时有可能失败，您可以在开始处理之前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。如果您在重新连接之前执行此操作（使用固定的 client_id），那么您不必担心在尝试处理失败的消息时传递新消息。
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仅在 on_message 完成后发送，因此如果您在处理 [=] 中的消息时崩溃10=] 那么代理应该重新传送该消息。
各位开发者，
我正在查看 Paho MQTT 客户端（Python），我想我了解发布者的 QoS 设置，例如传感器或任何数据源确实有意义 - 毕竟您希望能够确保已收到消息（由 broker/server）。
我也认为从订阅者的角度来看，请求例如QoS“2”，以确保 MQTT 服务器确实将每条消息发送给订阅者，但我正在努力解决这个问题：订阅者似乎没有办法表示成功（或不）处理收到的消息消息，换句话说，似乎缺少某种明确确认的方式？
用例 - 成功处理消息
我想订阅一个主题并成功处理每个个数据点，例如通过存储到数据库。因此，我需要适应订阅者失败 "in-flight" 的情况，即在收到来自代理的消息之后但在成功处理（存储到数据库）之前。
假设
现在，如果订阅者（已修复 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，代理通过 ID 识别此特定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 - 就代理而言，它确实成功传递了最后一条消息（不知道订阅者崩溃了）。
可能的解决方案
如果上述假设成立，那么我最好不要使用固定的 client_id，而是使用 "clean_session" 和随机的 client_id；这样，代理开始传递所有消息，这些消息是为特定主题保留的。这当然会将跟踪成功处理的消息的责任放在订阅者身上。
需要这样做吗？或者有没有一种方法可以明确确认订户成功处理消息，因此代理可以重新传输应该需要 - 我对 Paho Python 库特别感兴趣。
提前致谢！
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
用于说明的一些代码
on_message 订阅者回调
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)
订户
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（暂时）
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 "catching up" 处理自上次连接以来丢失的那些消息。但是，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
如果您的客户端在处理消息时有可能失败，您可以在开始处理之前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。如果您在重新连接之前执行此操作（使用固定的 client_id），那么您不必担心在尝试处理失败的消息时传递新消息。
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仅在 on_message 完成后发送，因此如果您在处理 [=] 中的消息时崩溃10=] 那么代理应该重新传送该消息。