[−][src]Crate tokio
A runtime for writing reliable network applications without compromising speed.
Tokio is an event-driven, non-blocking I/O platform for writing asynchronous applications with the Rust programming language. At a high level, it provides a few major components:
- Tools for working with asynchronous tasks, including synchronization primitives and channels and timeouts, sleeps, and intervals.
- APIs for performing asynchronous I/O, including TCP and UDP sockets, filesystem operations, and process and signal management.
- A runtime for executing asynchronous code, including a task scheduler, an I/O driver backed by the operating system's event queue (epoll, kqueue, IOCP, etc...), and a high performance timer.
Guide level documentation is found on the website.
A Tour of Tokio
Tokio consists of a number of modules that provide a range of functionality essential for implementing asynchronous applications in Rust. In this section, we will take a brief tour of Tokio, summarizing the major APIs and their uses.
The easiest way to get started is to enable all features. Do this by
enabling the full
feature flag:
tokio = { version = "1", features = ["full"] }
Authoring applications
Tokio is great for writing applications and most users in this case shouldn't
worry too much about what features they should pick. If you're unsure, we suggest
going with full
to ensure that you don't run into any road blocks while you're
building your application.
Example
This example shows the quickest way to get started with Tokio.
tokio = { version = "1", features = ["full"] }
Authoring libraries
As a library author your goal should be to provide the lighest weight crate that is based on Tokio. To achieve this you should ensure that you only enable the features you need. This allows users to pick up your crate without having to enable unnecessary features.
Example
This example shows how you may want to import features for a library that just
needs to tokio::spawn
and use a TcpStream
.
tokio = { version = "1", features = ["rt", "net"] }
Working With Tasks
Asynchronous programs in Rust are based around lightweight, non-blocking
units of execution called tasks. The tokio::task
module provides
important tools for working with tasks:
- The
spawn
function andJoinHandle
type, for scheduling a new task on the Tokio runtime and awaiting the output of a spawned task, respectively, - Functions for running blocking operations in an asynchronous task context.
The tokio::task
module is present only when the "rt" feature flag
is enabled.
The tokio::sync
module contains synchronization primitives to use when
needing to communicate or share data. These include:
- channels (
oneshot
,mpsc
, andwatch
), for sending values between tasks, - a non-blocking
Mutex
, for controlling access to a shared, mutable value, - an asynchronous
Barrier
type, for multiple tasks to synchronize before beginning a computation.
The tokio::sync
module is present only when the "sync" feature flag is
enabled.
The tokio::time
module provides utilities for tracking time and
scheduling work. This includes functions for setting timeouts for
tasks, sleeping work to run in the future, or repeating an operation at an
interval.
In order to use tokio::time
, the "time" feature flag must be enabled.
Finally, Tokio provides a runtime for executing asynchronous tasks. Most
applications can use the #[tokio::main]
macro to run their code on the
Tokio runtime. However, this macro provides only basic configuration options. As
an alternative, the tokio::runtime
module provides more powerful APIs for configuring
and managing runtimes. You should use that module if the #[tokio::main]
macro doesn't
provide the functionality you need.
Using the runtime requires the "rt" or "rt-multi-thread" feature flags, to
enable the basic single-threaded scheduler and the thread-pool
scheduler, respectively. See the runtime
module
documentation for details. In addition, the "macros" feature
flag enables the #[tokio::main]
and #[tokio::test]
attributes.
CPU-bound tasks and blocking code
Tokio is able to concurrently run many tasks on a few threads by repeatedly
swapping the currently running task on each thread. However, this kind of
swapping can only happen at .await
points, so code that spends a long time
without reaching an .await
will prevent other tasks from running. To
combat this, Tokio provides two kinds of threads: Core threads and blocking
threads. The core threads are where all asynchronous code runs, and Tokio
will by default spawn one for each CPU core. The blocking threads are
spawned on demand, can be used to run blocking code that would otherwise
block other tasks from running and are kept alive when not used for a certain
amount of time which can be configured with thread_keep_alive
.
Since it is not possible for Tokio to swap out blocking tasks, like it
can do with asynchronous code, the upper limit on the number of blocking
threads is very large. These limits can be configured on the Builder
.
To spawn a blocking task, you should use the spawn_blocking
function.
#[tokio::main] async fn main() { // This is running on a core thread. let blocking_task = tokio::task::spawn_blocking(|| { // This is running on a blocking thread. // Blocking here is ok. }); // We can wait for the blocking task like this: // If the blocking task panics, the unwrap below will propagate the // panic. blocking_task.await.unwrap(); }
If your code is CPU-bound and you wish to limit the number of threads used
to run it, you should run it on another thread pool such as rayon. You
can use an oneshot
channel to send the result back to Tokio when the
rayon task finishes.
Asynchronous IO
As well as scheduling and running tasks, Tokio provides everything you need to perform input and output asynchronously.
The tokio::io
module provides Tokio's asynchronous core I/O primitives,
the AsyncRead
, AsyncWrite
, and AsyncBufRead
traits. In addition,
when the "io-util" feature flag is enabled, it also provides combinators and
functions for working with these traits, forming as an asynchronous
counterpart to std::io
.
Tokio also includes APIs for performing various kinds of I/O and interacting with the operating system asynchronously. These include:
tokio::net
, which contains non-blocking versions of TCP, UDP, and Unix Domain Sockets (enabled by the "net" feature flag),tokio::fs
, similar tostd::fs
but for performing filesystem I/O asynchronously (enabled by the "fs" feature flag),tokio::signal
, for asynchronously handling Unix and Windows OS signals (enabled by the "signal" feature flag),tokio::process
, for spawning and managing child processes (enabled by the "process" feature flag).
Examples
A simple TCP echo server:
use tokio::net::TcpListener; use tokio::io::{AsyncReadExt, AsyncWriteExt}; #[tokio::main] async fn main() -> Result<(), Box<dyn std::error::Error>> { let listener = TcpListener::bind("127.0.0.1:8080").await?; loop { let (mut socket, _) = listener.accept().await?; tokio::spawn(async move { let mut buf = [0; 1024]; // In a loop, read data from the socket and write the data back. loop { let n = match socket.read(&mut buf).await { // socket closed Ok(n) if n == 0 => return, Ok(n) => n, Err(e) => { eprintln!("failed to read from socket; err = {:?}", e); return; } }; // Write the data back if let Err(e) = socket.write_all(&buf[0..n]).await { eprintln!("failed to write to socket; err = {:?}", e); return; } } }); } }
Feature flags
Tokio uses a set of feature flags to reduce the amount of compiled code. It
is possible to just enable certain features over others. By default, Tokio
does not enable any features but allows one to enable a subset for their use
case. Below is a list of the available feature flags. You may also notice
above each function, struct and trait there is listed one or more feature flags
that are required for that item to be used. If you are new to Tokio it is
recommended that you use the full
feature flag which will enable all public APIs.
Beware though that this will pull in many extra dependencies that you may not
need.
full
: Enables all Tokio public API features listed below.rt
: Enablestokio::spawn
, the basic (current thread) scheduler, and non-scheduler utilities.rt-multi-thread
: Enables the heavier, multi-threaded, work-stealing scheduler.io-util
: Enables the IO basedExt
traits.io-std
: EnableStdout
,Stdin
andStderr
types.net
: Enablestokio::net
types such asTcpStream
,UnixStream
andUdpSocket
, as well as (on Unix-like systems)AsyncFd
time
: Enablestokio::time
types and allows the schedulers to enable the built in timer.process
: Enablestokio::process
types.macros
: Enables#[tokio::main]
and#[tokio::test]
macros.sync
: Enables alltokio::sync
types.signal
: Enables alltokio::signal
types.fs
: Enablestokio::fs
types.test-util
: Enables testing based infrastructure for the Tokio runtime.
Note: AsyncRead
and AsyncWrite
traits do not require any features and are
always available.
Internal features
These features do not expose any new API, but influence internal implementation aspects of Tokio, and can pull in additional dependencies.
parking_lot
: As a potential optimization, use the parking_lot crate's synchronization primitives internally. MSRV may increase according to the parking_lot release in use.
Unstable features
These feature flags enable unstable features. The public API may break in 1.x
releases. To enable these features, the --cfg tokio_unstable
must be passed to
rustc
when compiling. This is easiest done using the RUSTFLAGS
env variable:
RUSTFLAGS="--cfg tokio_unstable"
.
tracing
: Enables tracing events.
Modules
fs | fs Asynchronous file and standard stream adaptation. |
io | Traits, helpers, and type definitions for asynchronous I/O functionality. |
net | TCP/UDP/Unix bindings for |
process | process An implementation of asynchronous process management for Tokio. |
runtime | rt The Tokio runtime. |
signal | signal Asynchronous signal handling for Tokio |
stream | Due to the |
sync | sync Synchronization primitives for use in asynchronous contexts. |
task | Asynchronous green-threads. |
time | time Utilities for tracking time. |
Macros
join | macros Wait on multiple concurrent branches, returning when all branches complete. |
pin | Pins a value on the stack. |
select | macros Wait on multiple concurrent branches, returning when the first branch completes, cancelling the remaining branches. |
task_local | rt Declares a new task-local key of type |
try_join | macros Wait on multiple concurrent branches, returning when all branches
complete with |
Functions
spawn | rt Spawns a new asynchronous task, returning a
|
Attribute Macros
main | rt and macros Marks async function to be executed by the selected runtime. This macro
helps set up a |
test | rt and macros Marks async function to be executed by runtime, suitable to test environment |