Serving Requests in Parallel

Our example so far has largely presented concurrency (using async code) as an alternative to parallelism (using threads). However, async code and threads are not mutually exclusive. Async executors can be either single-threaded or multithreaded. For example, the async-executor crate used by async-std has both a single-threaded LocalExecutor and a multi-threaded Executor.

Tasks can either be run on the thread that created them or on a separate thread. Async runtimes often provide functionality for spawning tasks onto separate threads. Even if tasks are executed on separate threads, they should still be non-blocking.

Some runtimes provide functions for spawning blocking tasks onto dedicated threads, which is useful for running synchronous code from other libraries. Tasks are usually required to be Send, so they can be moved to separate threads. Some runtimes also provide functions for spawning non-Send tasks onto a thread-local executor.

In our example, for_each_concurrent processes each connection concurrently on the same thread as the main function. Here, handle_connection is both Send and non-blocking, so we could have instead spawned new tasks to run handle_connection. We can use async_std::task::spawn for this purpose:

use async_std::task::spawn;

async fn main() {
    let listener = TcpListener::bind("").await.unwrap();
        .for_each_concurrent(/* limit */ None, |stream| async move {
            let stream = stream.unwrap();

Now we are using both concurrency and parallelism to handle multiple requests at the same time.