FastAPI with MQTT client deployed to GAE not receive any message after running for a while - google-app-engine

Question

I built a FastAPI app to forward MQTT message using paho-mqtt lib from broker to another REST api service.
After deploying the app to google app engine, everything works fine (can normally send/receive message) at the beginning.
But about hours later, my app can still publish messages but not receiving any message.
app.yaml
runtime: python310
entrypoint: gunicorn -b :$PORT -w 1 -k uvicorn.workers.UvicornWorker main:app
...
instance_class: F1
inbound_services:
- warmup
automatic_scaling:
min_instances: 1
max_instances: 1
min_idle_instances: 1
mqtt client wrapper
class MqttClient:
_instance = None
_lock = threading.Lock()
_on_msg_callback: Set[Callable] = set()
_subscribe_topics: Set[str] = set()
_mqtt_client = None
def __new__(cls, *args, **kwargs):
if cls._instance is None:
with cls._lock:
if not cls._instance:
cls._instance = super().__new__(cls)
return cls._instance
def init(self, config: MqttConfig):
broker_host = config.broker_host
random_id = ''.join(random.choices(string.digits, k=6))
client_id = f'mqtt-forwarder-{random_id}'
port = config.port
keepalive = config.keepalive
client = mqtt.Client(client_id=client_id, userdata=self)
client.username_pw_set(
username=config.username,
password=config.password
)
context = ssl.SSLContext(protocol=ssl.PROTOCOL_TLSv1_2)
client.tls_set_context(context)
# set callbacks
client.on_log = self._on_log
client.on_connect = self._on_connect
client.on_disconnect = self._on_disconnect
client.on_publish = self._on_publish
client.on_subscribe = self._on_subscribe
client.on_message = self._on_message
client.connect(broker_host, port=port, keepalive=keepalive)
client.loop_start()
self._mqtt_client = client
main.py
import...
app = FastAPI()
mqtt_client = MqttClient()
mqtt_client.init(
MqttConfig(
broker_host='...',
port=8883,
keepalive=60,
username=...,
password=...
))
mqtt_client.subscribe('...')
mqtt_client.subscribe('...')
mqtt_client.register_on_message_callback(mqtt_cb.mqtt_on_msg_callback)
#app.get('/_ah/warmup')
def warmup():
return Response('', status_code=status.HTTP_200_OK)
After checking the GAE logging, I found a process is terminated (pid1, 16:23:44.397).
I am not sure if this caused MQTT client receiving no message, and curious about why the process was terminated.
GAE log
My main Question:
How can I fix this issue and keep receiving MQTT messages?
Other Question:
Is it a good idea to deploy a GAE FastAPI to forward MQTT messages to my other services? Any suggestion is appreciated.
Thank you.