Description Jürn Brodersen 2023-11-16 09:41:24 CET

Global connection limit of ten between umc server and all running module processes

`tornado.httpclient.AsyncHTTPClient` returns a global singleton by default. That has `max_clients` set to ten by default. If you start more requests, they are just queued and not started.

https://www.tornadoweb.org/en/stable/httpclient.html#tornado.httpclient.AsyncHTTPClient

- Why can this be a problem?
As long as all modules answer requests reasonable fast and not many users are active, this is fine. But with more users and slower modules these ten connections can quickly cloak up which prevents users from connecting to their module processes.

We noticed this in the computerroom module. Multiple users had the "Watch" overview page open which creates a lot of slow requests for screenshots from student computers. That results in an unusable slow umc for all users because requests to other modules, like the password reset module, end up in the same queue.

- How to reproduce:
To make testing easier I artificially slowed down a request. In my case the rooms requests from the computerroom. But it should work with any module.

Add to /usr/lib/python3/dist-packages/univention/management/console/modules/computerroom/__init__.py::"def rooms"
```
import time
time.sleep(60)
```

I used the following snippet for testing:
```
from univention.lib.umc import Client
import concurrent.futures

def load_room():
     return c.umc_command("computerroom/rooms", options={"school": "school1"}).data

def pool_runner():
     with concurrent.futures.ThreadPoolExecutor(max_workers=10) as executor:
         futures = [executor.submit(load_room) for i in range(100)]
         for future in concurrent.futures.as_completed(futures):
             print(future.result())

c = Client()
c.authenticate("Administrator", "univention")
pool_runner()
```
If you now try to open a umc module you should notice that it got significantly slower.

- Possible fix:
I propose to use `force_instance=true` in https://git.knut.univention.de/univention/ucs/-/blob/5.0-5/management/univention-management-console/src/univention/management/console/resources.py?ref_type=heads#L120 so each module process has its own independent instance. That would imitate the old behavior.