Async Rust

“Async” is a concurrency model where multiple tasks are executed concurrently by executing each task until it would block, then switching to another task that is ready to make progress. The model allows running a larger number of tasks on a limited number of threads. This is because the per-task overhead is typically very low and operating systems provide primitives for efficiently identifying I/O that is able to proceed.

Rust’s asynchronous operation is based on “futures”, which represent work that may be completed in the future. Futures are “polled” until they signal that they are complete.

Futures are polled by an async runtime, and several different runtimes are available.


  • Python has a similar model in its asyncio. However, its Future type is callback-based, and not polled. Async Python programs require a “loop”, similar to a runtime in Rust.

  • JavaScript’s Promise is similar, but again callback-based. The language runtime implements the event loop, so many of the details of Promise resolution are hidden.