Futures and Tokio

This article was originally published at https://gist.github.com/joepie91/bc2d29fab43b63d16f59e1bd20fd7b6e. It may be out of date.

Event loops

If you're not familiar with the concept of an 'event loop' yet, watch this video first. While this video is about the event loop in JavaScript, most of the concepts apply to event loops in general, and watching it will help you understand Tokio and Futures better as well.

Concepts

• Futures: Think of a Future like an asynchronous Result; it represents some sort of result (a value or an error) that will eventually exist, but doesn't yet. It has many of the same combinators as a Result does, the difference being that they are executed at a later point in time, not immediately. Aside from representing a future result, a Future also contains the logic that is necessary to obtain it. A Future will 'complete' (either successfully or with an error) precisely once.
• Streams: Think of a Stream like an asynchronous Iterator; like a Future, it represents some sort of data that will be obtained at a later point in time, but unlike a Future it can produce multiple results over time. It has many of the same combinators as an Iterator does. Essentially, a Stream is the "multiple results over time instead of one" counterpart to a Future.
• Executor: An Executor is a thing that, when you pass a Future or Stream into it, is responsible for 1) turning the logic stored in the Future/Stream into an internal task, 2) scheduling the work for that task, and 3) wiring up the Future's state to any underlying resources. You don't usually implement these yourself, but use a pre-made Executor from some third-party library. The exact scheduling is left up to the implementation of the Executor.
• .wait(): A method on a Future that will block the current thread until the Future has completed, and that then returns the result of that Future. This is an example of an Executor, although not a particularly useful one; it won't allow you to do work concurrently.
• Tokio reactor core: This is also an Executor, provided by the Tokio library. It's probably what you'll be using when you use Tokio. The frontpage of the Tokio website provides an example on how to use it.
• futures_cpupool: Yet another Executor; this one schedules the work across a pool of threads.