Hi @liborjelinek_twitter !
Responder methods, such as
on_post etc, may be called in parallel (as part of handling parallel requests) in different processes, threads and/or greenlets/eventlets/coroutines etc if your WSGI server of choice supports that. Gunicorn can be configured to run multi-threaded.
So in that sense yes, if your responder methods are accessing shared resources, care needs to be taken they are thread-safe. The same applies to hooks, middleware methods, router converters, custom error handlers etc.