A mental model of an async runtime

I find it easier to reason about how futures work by creating a high-level mental model we can use. To do that, I have to introduce the concept of a runtime that will drive our futures to completion.

Note

The mental model I create here is not the only way to drive futures to completion, and Rust’s futures do not impose any restrictions on how you actually accomplish this task.

A fully working async system in Rust can be divided into three parts:

• Reactor (responsible for notifying about I/O events)
• Executor (scheduler)
• Future (a task that can stop and resume at specific points)

So, how do these three parts work together?

Let’s take a look at a diagram that shows a simplified overview of an async runtime:

Figure 6.1 – Reactor, executor, and waker

In step 1 of the figure, an executor holds a list of futures. It will try to run the future by polling it (the poll phase...