Complex sentence: It was very common for a handler to be state-machine oriented so that requests it was fielding from a client were all done asynchronously, this allowed for a single handler to field requests from multiple clients simultaneously without them blocking each other unnecessarily.
Split into simpler sentences: It was very common for a handler to be state-machine oriented so that requests it was fielding from a client were all done asynchronously. This allowed for a single handler to field requests from multiple clients simultaneously without them blocking each other unnecessarily.