Async Ruby adds new concurrency features to the language; you can think of it as “threads with none of the downsides”. It’s been in the making for a couple of years, and with Ruby 3.0, it’s finally ready for prime time. - Bruno Sutic / HN

Ruby 3.0 implements a “fiber scheduler” feature that enables “colorless Async”.

• Async Ruby is much more performant than threads. There are less context switches, enabled by the event reactor. The performance benefits are visible in simple scenarios like making a thousand HTTP requests.

• Async is more scalable, can handle millions concurrent tasks (like HTTP connections). There can only be a couple thousand threads at the same time.

I regret adding threads. —Matz

see also

• thread
• Concurrent Ruby