Tasks

A task is a lightweight, non-blocking unit of execution, and its job is to drive one future to completion. Lightweight because it is a small heap allocation rather than a thread; non-blocking because it occupies a worker thread only for the length of a single poll. tokio::spawn creates one from a future:

#![allow(unused)]
fn main() {
let handle = tokio::spawn(async move {
    store.get(&bucket, &key).cloned()
});

let value = handle.await.unwrap();
}

Three things change the moment you spawn.

It runs whether or not you await it. A future sitting in a variable is inert. A spawned task is in the runtime’s queue, and it makes progress as soon as there is a thread free. Awaiting the JoinHandle waits for the result; dropping the handle does not stop the task.

It has to be Send + 'static. The task may be picked up by any worker thread, and it may outlive whatever spawned it, so it cannot borrow from the caller and cannot hold anything that is not Send across an .await. Rc, RefCell, and MutexGuard from std all fall foul of this, and the compiler’s error will point at the .await rather than at the value, which takes some getting used to.

It can fail on its own. handle.await returns a Result, and the error case means the task panicked or was aborted. A panicking task does not bring the process down; it quietly stops existing, and the only way anybody finds out is by looking at its handle. Chapter 7 is about what to do with that.

Tasks are not threads

A task is a heap allocation and a state machine. Spawning one costs a few hundred bytes and no system call, which is why a Tokio server can have a hundred thousand of them and would fall over with a hundred thousand threads.

The tradeoff is that tasks are cooperative. A task only yields at an .await, so a task that does not await does not give anything else a turn. On a multi-threaded runtime that means one worker thread is stuck; if enough tasks do it, the whole runtime is. The next two exercises are the two halves of that: how to get concurrency when you want it, and what to do with work that cannot yield.

Exercise

The exercise for this section is located in 02_tasks/00_intro