[zzzeek]

yes, parent was referring to "each thread with a blocking connection", but you still can (and probably should) use a thread pool for that. In the naive approach, new connections beyond the limit of your threadpool either have to wait, or you have to dynamically expand your threadpool. mod_wsgi's daemon mode has the option to use a thread pool of a fixed size to handle requests.

you can also use non-blocking handles with a fixed /dynamic threadpool and use epoll or similar to find those handles with data ready, and send those into your pool, thereby servicing an arbitrary number of connections with a controlled level of concurrency among them. MariaDB has an option to do that here: https://mariadb.com/kb/en/thread-pool-in-mariadb/ . this is not as trivial as spinning up asyncio tasks but that's because there's (AFAIK) no friendly library giving you an easy way of doing it. But it's Python and if you're writing a server to handle MariaDB server loads using a thread pool with direct use of epoll(), you're likely in the wrong language.