Problem with >10 parallel SSH connections

Hello there,

I am using AsyncSSH (which has been absolutely great thus far 🙏 ) in combination with FastAPI to build a performant, async-ready API. Now I have noticed that the opening of multiple parallel connections can cause a few SSH connections to be dropped, and I was wondering if this was expected behavior given my approach or if I misunderstood something about AsyncSSH/asyncio (which I assume to be the case).

My first assumption was that .run might reuse existing connections and open separate channels (which by default most SSH-servers only allow 10 of per connection), but from investigating the source code and the access log, it doesn’t look like that. Rather, a separate SSH-connection is opened for every command - perfect!

I’ve created a minimal reproducible build here. By tweaking the parallel_tasks variable to >10 I start noticing the issue…

The error I keep seeing is asyncssh.misc.ConnectionLost: Connection lost, the stacktrace looks as follows:

Error trying to connect to server via SSH...
Traceback (most recent call last):
  File "/home/user/Coding/asyncssh-test/main.py", line 8, in executeCommand
    async with asyncssh.connect(
  File "/home/user/.local/share/virtualenvs/asyncssh-test-LAQchyfJ/lib/python3.10/site-packages/asyncssh/misc.py", line 274, in __aenter__
    self._coro_result = await self._coro
  File "/home/user/.local/share/virtualenvs/asyncssh-test-LAQchyfJ/lib/python3.10/site-packages/asyncssh/connection.py", line 7646, in connect
    return await asyncio.wait_for(
  File "/usr/lib/python3.10/asyncio/tasks.py", line 408, in wait_for
    return await fut
  File "/home/user/.local/share/virtualenvs/asyncssh-test-LAQchyfJ/lib/python3.10/site-packages/asyncssh/connection.py", line 439, in _connect
    await options.waiter
asyncssh.misc.ConnectionLost: Connection lost

Is this an issue with AsyncSSH or am I simply doing something wrong here?

[ronf]

Happy to help - that sounds like a good approach. If you have any further questions, feel free to ask.

[seandlg]

Hi Ronald,

Thanks a million for your feedback and time! This sounds like a really promising approach and I will give it a shot 😃 I think I will start with using semaphores to prevent the MaxStartups-issue and later move towards keeping a connection- and session-pool as you suggested.

Best, Sean