The "large applications or asynchronous servers" bit might be better understood in comparison to the monolithic versus micro kernel debate. Flask is well suited

The "large applications or asynchronous servers" bit might be better understood in comparison to the monolithic versus micro kernel debate. Flask is well suited for building complex applications that can be decomposed into simple moving parts with a clear separation of concerns. (My personal bias is that all complex applications can and should be developed this way.) The documented limitation you reference is based on the WSGI server being used. "If your server uses some kind of concurrency that is not based on threads or greenlets, Flask will no longer be able to support these global proxies."[0] As nginx+gunicorn is becoming more popular, this isn't really an issue at all. [0]http://flask.pocoo.org/docs/becomingbig/#scale-like-a-pro http://flask.pocoo.org/docs/becomingbig/#scale-like-a-pro