Flask-sqlalchemy / uwsgi: DB connection problem when more than on process is used - flask

Question

I have a Flask app running on Heroku with uwsgi server in which each user connects to his own database. I have implemented the solution reported here for a very similar situation. In particular, I have implemented the connection registry as follows:
class DBSessionRegistry():
_registry = {}
def get(self, URI, **kwargs):
if URI not in self._registry:
current_app.logger.info(f'INFO - CREATING A NEW CONNECTION')
try:
engine = create_engine(URI,
echo=False,
pool_size=5,
max_overflow=5)
session_factory = sessionmaker(bind=engine)
Session = scoped_session(session_factory)
a_session = Session()
self._registry[URI] = a_session
except ArgumentError:
raise Exception('Error')
current_app.logger.info(f'SESSION ID: {id(self._registry[URI])}')
current_app.logger.info(f'REGISTRY ID: {id(self._registry)}')
current_app.logger.info(f'REGISTRY SIZE: {len(self._registry.keys())}')
current_app.logger.info(f'APP ID: {id(current_app)}')
return self._registry[URI]
In my create_app() I assign a registry to the app:
app.DBregistry = DBSessionRegistry()
and whenever I need to talk to the DB I call:
current_app.DBregistry.get(URI)
where the URI is dependent on the user. This works nicely if I use uwsgi with one single process. With more processes,
[uwsgi]
processes = 4
threads = 1
sometimes it gets stuck on some requests, returning a 503 error code. I have found that the problem appears when the requests are handled by different processes in uwsgi. This is an excerpt of the log, which I commented to illustrate the issue:
# ... EVERYTHING OK UP TO HERE.
# ALL PREVIOUS REQUESTS HANDLED BY PROCESS pid = 12
INFO in utils: SESSION ID: 139860361716304
INFO in utils: REGISTRY ID: 139860484608480
INFO in utils: REGISTRY SIZE: 1
INFO in utils: APP ID: 139860526857584
# NOTE THE pid IN THE NEXT LINE...
[pid: 12|app: 0|req: 1/1] POST /manager/_save_task =>
generated 154 bytes in 3457 msecs (HTTP/1.1 200) 4 headers in 601
bytes (1 switches on core 0)
# PREVIOUS REQUEST WAS MANAGED BY PROCESS pid = 12
# THE NEXT REQUEST IS FROM THE SAME USER AND TO THE SAME URL.
# SO THERE IS NO NEED FOR CREATING A NEW CONNECTION, BUT INSTEAD...
INFO - CREATING A NEW CONNECTION
# TO THIS POINT, I DON'T UNDERSTAND WHY IT CREATED A NEW CONNECTION.
# THE SESSION ID CHANGES, AS IT IS A NEW SESSION
INFO in utils: SESSION ID: 139860363793168 # <<--- CHANGED
INFO in utils: REGISTRY ID: 139860484608480
INFO in utils: REGISTRY SIZE: 1
# THE APP AND THE REGISTRY ARE UNIQUE
INFO in utils: APP ID: 139860526857584
# uwsgi GIVES UP...
*** HARAKIRI ON WORKER 4 (pid: 11, try: 1) ***
# THE FAILED REQUEST WAS MANAGED BY PROCESS pid = 11
# I ASSUME THIS IS WHY IT CREATED A NEW CONNECTION
HARAKIRI: -- syscall> 7 0x7fff4290c6d8 0x1 0xffffffff 0x4000 0x0 0x0
0x7fff4290c6b8 0x7f33d6e3cbc4
HARAKIRI: -- wchan> poll_schedule_timeout
HARAKIRI !!! worker 4 status !!!
HARAKIRI [core 0] - POST /manager/_save_task since 1587660997
HARAKIRI !!! end of worker 4 status !!!
heroku[router]: at=error code=H13 desc="Connection closed without
response" method=POST path="/manager/_save_task"
DAMN ! worker 4 (pid: 11) died, killed by signal 9 :( trying respawn ...
Respawned uWSGI worker 4 (new pid: 14)
# FROM HERE ON, NOTHINGS WORKS ANYMORE
This behavior is consistent over several attempts: when the pid changes, the request fails. Even with a pool_size = 1 in the create_engine function the issue persists. No issue instead is uwsgi is used with one process.
I am pretty sure it is my fault, there is something I don't know or I don't understand about how uwsgi and/or sqlalchemy work. Could you please help me?
Thanks

Answer 1

What is hapeening is that you are trying to share memory between processes.
There are some exaplanations in these posts.
(is it possible to share memory between uwsgi processes running flask app?).
(https://stackoverflow.com/a/45383617/11542053)
You can use an extra layer to store your sessions outsite of the app.
For that, you can use uWsgi's SharedArea(https://uwsgi-docs.readthedocs.io/en/latest/SharedArea.html) which is very low level or you can user other approaches like uWsgi's caching(https://uwsgi-docs.readthedocs.io/en/latest/Caching.html)
hope it helps.