Designing Custom Async Executors

In this lesson, you will learn how to orchestrate the lifecycle of Future objects by building the engine that drives them to completion.

The Anatomy of an Executor

At its core, a Rust Future is just a state machine that does nothing until you call poll. An executor is the component responsible for calling poll and deciding what to do when a task isn't ready yet. To build one, you need three primary ingredients: a Task Queue, a Waker, and a Loop.

1. The Task Queue: This is a thread-safe collection (often a std::sync::mpsc channel) that holds tasks waiting to be polled.
2. The Waker: When a Future returns Poll::Pending, it must be "woken up" when it’s finally ready. You provide the Future with a Waker, which, when triggered, pushes the task back onto your executor’s queue.
3. The Loop: A simple loop that pulls tasks off the queue and calls poll on them.

A Minimal Example

While production executors like Tokio are complex, the pattern follows this fundamental structure:

use std::sync::mpsc::{channel, Receiver, Sender};
use std::task::{Context, Poll};
use std::future::Future;

struct Executor {
    ready_queue: Receiver<Task>,
}

impl Executor {
    fn run(&self) {
        while let Ok(task) = self.ready_queue.recv() {
            let mut future = task.future;
            let waker = /* create a waker that pushes to our sender */;
            let mut cx = Context::from_waker(&waker);
            
            if let Poll::Pending = future.as_mut().poll(&mut cx) {
                // Future isn't ready; it will notify the waker later.
            }
        }
    }
}

Why this matters

The executor abstracts away the complexity of the state machine. By implementing a custom executor, you gain total control over scheduling priorities, thread affinity, and power consumption—key requirements for specialized environments like embedded systems or high-performance networking where standard runtimes might be too heavy or ill-suited.

Remember, the executor doesn't "run" the code in the sense of executing instructions; it acts as a traffic controller that ensures poll is only invoked when progress is likely to be made.

Key Takeaway: An executor is a scheduler that connects the non-blocking state transitions of a Future to an event-driven loop via the Waker mechanism.