Concurrently is a concurrency framework for Ruby and mruby. With it, concurrent code can be written sequentially similar to async/await. The concurrency primitive of Concurrently is the concurrent proc. It is very similar to a regular proc. Calling a concurrent proc creates a concurrent evaluation which is kind of a lightweight thread: It can wait for stuff without blocking other concurrent evaluations. Under the hood, concurrent procs are evaluated inside fibers. They can wait for readiness of I/O or a period of time (or the result of other concurrent evaluations).