An async runtime schedules tasks while they wait for external events. With Tokio, a task waiting for file, network, or database I/O can yield so another task runs; when the event completes, the runtime can schedule the original task again. This is most useful for I/O-bound work, not necessarily CPU-bound computation. section 10.1 section 10.3
A Tokio task moves through a small execution cycle:
- An async function creates a future. Calling it alone does not run its body; the example’s simulated file reads produce no output until they are awaited. section 10.2
- The executor polls the future. Poll::Pending means it cannot return a value yet; Poll::Ready contains its result. section 10.4
- A Waker notifies the runtime when a pending task should be polled again. section 10.4
pub trait Future {
type Output;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}
The Future trait’s poll signature shows the interface the executor uses: a pinned future, a task context, and a Poll result. section 10.3 section 10.4
Spawn two file-read tasks
In src/main.rs, annotate async main with #[tokio::main], launch each operation with tokio::spawn, await each async file-read function inside its task, and use tokio::join! to wait for both handles. Omitting .await leaves the functions unexecuted. The sample reads are simulated with std::thread::sleep(), so they illustrate task structure rather than a fully nonblocking I/O implementation. section 10.2 section 10.5
To see why waking matters, implement a timer as a custom Future. Its poll method returns Poll::Pending before an expiration time, arranging for a Waker to notify the executor later; once the time has passed, it returns Poll::Ready. An always-pending future never finishes, while repeatedly waking it without progress causes repeated polling. section 10.4 section 10.5
For a web service, the runtime lets work continue while requests wait on I/O. Actix Web combines worker threads with async I/O and uses a Tokio-based runtime; this differs from assigning a separate operating-system thread to every connection. section 10.3 section 3.3
