Coroutines

In short: Coroutines are functions that can pause midway and later resume at the same spot.

In more detail: Unlike threads, it is not the operating system that switches but the code that hands over control voluntarily (cooperative multitasking). That makes them very lightweight: tens of thousands run in one thread.

In Depth

• Keywords: yield (Python generators), await (async/await), suspend (Kotlin), goroutines (Go, scheduled by the runtime).
• Strength: Lots of waiting on network or database without idle threads piling up.
• Weakness: A coroutine that computes instead of waiting blocks all others on the same thread. CPU-heavy work belongs in processes or threads.
• Flow: An event loop starts coroutines, remembers their state and resumes them once their result is ready.
• Under the hood: The compiler usually turns them into a state machine.

See also: async/await