Tokio is an asynchronous runtime for the Rust programming language. It provides the building blocks needed for writing networking applications. It gives the flexibility to target a wide range of systems, from large servers with dozens of cores to small embedded devices.
At a high level, Tokio provides a few major components:
- A multi-threaded runtime for executing asynchronous code.
- An asynchronous version of the standard library.
- A large ecosystem of libraries.
Tokio's role in your project
When you write your application in an asynchronous manner, you enable it to scale much better by reducing the cost of doing many things at the same time. However, asynchronous Rust code does not run on its own, so you must choose a runtime to execute it. The Tokio library is the most widely used runtime, surpassing all other runtimes in usage combined.
Additionally, Tokio provides many useful utilities. When writing asynchronous code, you cannot use the ordinary blocking APIs provided by the Rust standard library, and must instead use asynchronous versions of them. These alternate versions are provided by Tokio, mirroring the API of the Rust standard library where it makes sense.
Advantages of Tokio
This section will outline some advantages of Tokio.
Tokio is fast, built on top of the Rust programming language, which itself is fast. This is done in the spirit of Rust with the goal that you should not be able to improve the performance by writing equivalent code by hand.
Tokio is scalable, built on top of the async/await language feature, which itself is scalable. When dealing with networking, there's a limit to how fast you can handle a connection due to latency, so the only way to scale is to handle many connections at once. With the async/await language feature, increasing the number of concurrent operations becomes incredibly cheap, allowing you to scale to a large number of concurrent tasks.
Tokio is built using Rust, which is a language that empowers everyone to build reliable and efficient software. A number of studies have found that roughly ~70% of high severity security bugs are the result of memory unsafety. Using Rust eliminates this entire class of bugs in your applications.
Tokio also focuses heavily on providing consistent behaviour with no surprises. Tokio's major goal is to allow users to deploy predictable software that will perform the same day in and day out with reliable response times and no unpredictable latency spikes.
With Rust's async/await feature, the complexity of writing asynchronous applications has been lowered substantially. Paired with Tokio's utilities and vibrant ecosystem, writing applications is a breeze.
Tokio follows the standard library's naming convention when it makes sense. This allows easily converting code written with only the standard library to code written with Tokio. With the strong type system of Rust, the ability to deliver correct code easily is unparalleled.
Tokio provides multiple variations of the runtime. Everything from a multi-threaded, work-stealing runtime to a light-weight, single-threaded runtime. Each of these runtimes come with many knobs to allow users to tune them to their needs.
When not to use Tokio
Although Tokio is useful for many projects that need to do a lot of things simultaneously, there are also some use-cases where Tokio is not a good fit.
- Speeding up CPU-bound computations by running them in parallel on several threads. Tokio is designed for IO-bound applications where each individual task spends most of its time waiting for IO. If the only thing your application does is run computations in parallel, you should be using rayon. That said, it is still possible to "mix & match" if you need to do both. See this blog post for a practical example.
- Reading a lot of files. Although it seems like Tokio would be useful for projects that simply need to read a lot of files, Tokio provides no advantage here compared to an ordinary threadpool. This is because operating systems generally do not provide asynchronous file APIs.
- Sending a single web request. The place where Tokio gives you an advantage is when you need to do many things at the same time. If you need to use a library intended for asynchronous Rust such as reqwest, but you don't need to do a lot of things at once, you should prefer the blocking version of that library, as it will make your project simpler. Using Tokio will still work, of course, but provides no real advantage over the blocking API. If the library doesn't provide a blocking API, see the chapter on bridging with sync code.