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docs: write sync mod API docs (#2175)

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Fixes #2171
This commit is contained in:
Carl Lerche
2020-01-27 09:11:12 -08:00
committed by GitHub
parent 71c47fabf4
commit bcba4aaa54
2 changed files with 438 additions and 11 deletions
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tokio/src/sync/mod.rs
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@@ -1,19 +1,414 @@
#![cfg_attr(loom, allow(dead_code, unreachable_pub, unused_imports))]
//! Future-aware synchronization
//! Synchronization primitives for use in asynchronous contexts.
//!
//! This module is enabled with the **`sync`** feature flag.
//! Tokio programs tend to be organized as a set of [tasks] where each task
//! operates independently and may be executed on separate physical threads. The
//! synchronization primitives provided in this module permit these independent
//! tasks to communicate together.
//!
//! Tasks sometimes need to communicate with each other. This module contains
//! basic abstractions for doing so:
//! [tasks]: crate::task
//!
//! - [oneshot](oneshot/index.html), a way of sending a single value
//! from one task to another.
//! - [mpsc](mpsc/index.html), a multi-producer, single-consumer channel for
//! sending values between tasks.
//! - [`Mutex`](struct.Mutex.html), an asynchronous `Mutex`-like type.
//! - [watch](watch/index.html), a single-producer, multi-consumer channel that
//! only stores the **most recently** sent value.
//! # Message passing
//!
//! The most common form of synchronization in a Tokio program is message
//! passing. Two tasks operate independently and send messages to each other to
//! synchronize. Doing so has the advantage of avoiding shared state.
//!
//! Message passing is implemented using channels. A channel supports sending a
//! message from one producer task to one or more consumer tasks. There are a
//! few flavors of channels provided by Tokio. Each channel flavor supports
//! different message passing patterns. When a channel supports multiple
//! producers, many separate tasks may **send** messages. When a channel
//! supports muliple consumers, many different separate tasks may **receive**
//! messages.
//!
//! Tokio provides many different channel flavors as different message passing
//! patterns are best handled with different implementations.
//!
//! ## `oneshot` channel
//!
//! The [`oneshot` channel][oneshot] supports sending a **single** value from a
//! single producer to a single consumer. This channel is usually used to send
//! the result of a computation to a waiter.
//!
//! **Example:** using a `oneshot` channel to receive the result of a
//! computation.
//!
//! ```
//! use tokio::sync::oneshot;
//!
//! async fn some_computation() -> String {
//! "represents the result of the computation".to_string()
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let (tx, rx) = oneshot::channel();
//!
//! tokio::spawn(async move {
//! let res = some_computation().await;
//! tx.send(res).unwrap();
//! });
//!
//! // Do other work while the computation is happening in the background
//!
//! // Wait for the computation result
//! let res = rx.await.unwrap();
//! }
//! ```
//!
//! Note, if the task produces the the computation result as its final action
//! before terminating, the [`JoinHandle`] can be used to receive the
//! computation result instead of allocating resources for the `oneshot`
//! channel. Awaiting on [`JoinHandle`] returns `Result`. If the task panics,
//! the `Joinhandle` yields `Err` with the panic cause.
//!
//! **Example:**
//!
//! ```
//! async fn some_computation() -> String {
//! "the result of the computation".to_string()
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let join_handle = tokio::spawn(async move {
//! some_computation().await
//! });
//!
//! // Do other work while the computation is happening in the background
//!
//! // Wait for the computation result
//! let res = join_handle.await.unwrap();
//! }
//! ```
//!
//! [`JoinHandle`]: crate::task::JoinHandle
//!
//! ## `mpsc` channel
//!
//! The [`mpsc` channel][mpsc] supports sending **many** values from **many**
//! producers to a single consumer. This channel is often used to send work to a
//! task or to receive the result of many computations.
//!
//! **Example:** using an mpsc to incrementally stream the results of a series
//! of computations.
//!
//! ```
//! use tokio::sync::mpsc;
//!
//! async fn some_computation(input: u32) -> String {
//! format!("the result of computation {}", input)
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let (mut tx, mut rx) = mpsc::channel(100);
//!
//! tokio::spawn(async move {
//! for i in 0..10 {
//! let res = some_computation(i).await;
//! tx.send(res).await.unwrap();
//! }
//! });
//!
//! while let Some(res) = rx.recv().await {
//! println!("got = {}", res);
//! }
//! }
//! ```
//!
//! The argument to `mpsc::channel` is the channel capacity. This is the maximum
//! number of values that can be stored in the channel pending receipt at any
//! given time. Properly setting this value is key in implementing robust
//! programs as the channel capacity plays a critical part in handling back
//! pressure.
//!
//! A common concurrency pattern for resource management is to spawn a task
//! dedicated to managing that resource and using message passing betwen other
//! tasks to interact with the resource. The resource may be anything that may
//! not be concurrently used. Some examples include a socket and program state.
//! For example, if multiple tasks need to send data over a single socket, spawn
//! a task to manage the socket and use a channel to synchronize.
//!
//! **Example:** sending data from many tasks over a single socket using message
//! passing.
//!
//! ```no_run
//! use tokio::io::{self, AsyncWriteExt};
//! use tokio::net::TcpStream;
//! use tokio::sync::mpsc;
//!
//! #[tokio::main]
//! async fn main() -> io::Result<()> {
//! let mut socket = TcpStream::connect("www.example.com:1234").await?;
//! let (tx, mut rx) = mpsc::channel(100);
//!
//! for _ in 0..10 {
//! // Each task needs its own `tx` handle. This is done by cloning the
//! // original handle.
//! let mut tx = tx.clone();
//!
//! tokio::spawn(async move {
//! tx.send(&b"data to write"[..]).await.unwrap();
//! });
//! }
//!
//! // The `rx` half of the channel returns `None` once **all** `tx` clones
//! // drop. To ensure `None` is returned, drop the handle owned by the
//! // current task. If this `tx` handle is not dropped, there will always
//! // be a single outstanding `tx` handle.
//! drop(tx);
//!
//! while let Some(res) = rx.recv().await {
//! socket.write_all(res).await?;
//! }
//!
//! Ok(())
//! }
//! ```
//!
//! The [`mpsc`][mpsc] and [`oneshot`][oneshot] channels can be combined to
//! provide a request / response type synchronization pattern with a shared
//! resource. A task is spawned to synchronize a resource and waits on commands
//! received on a [`mpsc`][mpsc] channel. Each command includes a
//! [`oneshot`][oneshot] `Sender` on which the result of the command is sent.
//!
//! **Example:** use a task to synchronize a `u64` counter. Each task sends an
//! "fetch and increment" command. The counter value **before** the increment is
//! sent over the provided `oneshot` channel.
//!
//! ```
//! use tokio::sync::{oneshot, mpsc};
//! use Command::Increment;
//!
//! enum Command {
//! Increment,
//! // Other commands can be added here
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let (cmd_tx, mut cmd_rx) = mpsc::channel::<(Command, oneshot::Sender<u64>)>(100);
//!
//! // Spawn a task to manage the counter
//! tokio::spawn(async move {
//! let mut counter: u64 = 0;
//!
//! while let Some((cmd, response)) = cmd_rx.recv().await {
//! match cmd {
//! Increment => {
//! let prev = counter;
//! counter += 1;
//! response.send(prev).unwrap();
//! }
//! }
//! }
//! });
//!
//! let mut join_handles = vec![];
//!
//! // Spawn tasks that will send the increment command.
//! for _ in 0..10 {
//! let mut cmd_tx = cmd_tx.clone();
//!
//! join_handles.push(tokio::spawn(async move {
//! let (resp_tx, resp_rx) = oneshot::channel();
//!
//! cmd_tx.send((Increment, resp_tx)).await.ok().unwrap();
//! let res = resp_rx.await.unwrap();
//!
//! println!("previous value = {}", res);
//! }));
//! }
//!
//! // Wait for all tasks to complete
//! for join_handle in join_handles.drain(..) {
//! join_handle.await.unwrap();
//! }
//! }
//! ```
//!
//! ## `broadcast` channel
//!
//! The [`broadcast` channel[broadcast] supports sending **many** values from
//! **many** producers to **many** consumers. Each consumer will receive
//! **each** value. This channel can be used to implement "fan out" style
//! patterns common with pub / sub or "chat" systems.
//!
//! This channel tends to be used less often than `oneshot` and `mpsc` but still
//! has its use cases.
//!
//! Basic usage
//!
//! ```
//! use tokio::sync::broadcast;
//!
//! #[tokio::main]
//! async fn main() {
//! let (tx, mut rx1) = broadcast::channel(16);
//! let mut rx2 = tx.subscribe();
//!
//! tokio::spawn(async move {
//! assert_eq!(rx1.recv().await.unwrap(), 10);
//! assert_eq!(rx1.recv().await.unwrap(), 20);
//! });
//!
//! tokio::spawn(async move {
//! assert_eq!(rx2.recv().await.unwrap(), 10);
//! assert_eq!(rx2.recv().await.unwrap(), 20);
//! });
//!
//! tx.send(10).unwrap();
//! tx.send(20).unwrap();
//! }
//! ```
//!
//! ## `watch` channel
//!
//! The [`watch` channel][watch] supports sending **many** values from a
//! **single** producer to **many** consumers. However, only the **most recent**
//! value is stored in the channel. Consumers are notified when a new value is
//! sent, but there is no guarantee that consumers will see **all** values.
//!
//! The [`watch` channel] is similar to a [`broadcast` channel] with capacity 1.
//!
//! Use cases for the [`watch` channel] include broadcasting configuration
//! changes or signalling program state changes, such as transitioning to
//! shutdown.
//!
//! **Example:** use a `watch` channel to notify tasks of configuration changes.
//! In this example, a configuration file is checked periodically. When the file
//! changes, the configuration changes are signalled to consumers.
//!
//! ```
//! use tokio::sync::watch;
//! use tokio::time::{self, Duration, Instant};
//!
//! use std::io;
//!
//! #[derive(Debug, Clone, Eq, PartialEq)]
//! struct Config {
//! timeout: Duration,
//! }
//!
//! impl Config {
//! async fn load_from_file() -> io::Result<Config> {
//! // file loading and deserialization logic here
//! # Ok(Config { timeout: Duration::from_secs(1) })
//! }
//! }
//!
//! async fn my_async_operation() {
//! // Do something here
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! // Load initial configuration value
//! let mut config = Config::load_from_file().await.unwrap();
//!
//! // Create the watch channel, initialized with the loaded configuration
//! let (tx, rx) = watch::channel(config.clone());
//!
//! // Spawn a task to monitor the file.
//! tokio::spawn(async move {
//! loop {
//! // Wait 10 seconds between checks
//! time::delay_for(Duration::from_secs(10)).await;
//!
//! // Load the configuration file
//! let new_config = Config::load_from_file().await.unwrap();
//!
//! // If the configuration changed, send the new config value
//! // on the watch channel.
//! if new_config != config {
//! tx.broadcast(new_config.clone()).unwrap();
//! config = new_config;
//! }
//! }
//! });
//!
//! let mut handles = vec![];
//!
//! // Spawn tasks that runs the async operation for at most `timeout`. If
//! // the timeout elapses, restart the operation.
//! //
//! // The task simultaneously watches the `Config` for changes. When the
//! // timeout duration changes, the timeout is updated without restarting
//! // the in-flight operation.
//! for _ in 0..5 {
//! // Clone a config watch handle for use in this task
//! let mut rx = rx.clone();
//!
//! let handle = tokio::spawn(async move {
//! // Start the initial operation and pin the future to the stack.
//! // Pinning to the stack is required to resume the operation
//! // across multiple calls to `select!`
//! let op = my_async_operation();
//! tokio::pin!(op);
//!
//! // Receive the **initial** configuration value. As this is the
//! // first time the config is received from the watch, it will
//! // always complete immediatedly.
//! let mut conf = rx.recv().await.unwrap();
//!
//! let mut op_start = Instant::now();
//! let mut delay = time::delay_until(op_start + conf.timeout);
//!
//! loop {
//! tokio::select! {
//! _ = &mut delay => {
//! // The operation elapsed. Restart it
//! op.set(my_async_operation());
//!
//! // Track the new start time
//! op_start = Instant::now();
//!
//! // Restart the timeout
//! delay = time::delay_until(op_start + conf.timeout);
//! }
//! new_conf = rx.recv() => {
//! conf = new_conf.unwrap();
//!
//! // The configuration has been updated. Update the
//! // `delay` using the new `timeout` value.
//! delay.reset(op_start + conf.timeout);
//! }
//! _ = &mut op => {
//! // The operation completed!
//! return
//! }
//! }
//! }
//! });
//!
//! handles.push(handle);
//! }
//!
//! for handle in handles.drain(..) {
//! handle.await.unwrap();
//! }
//! }
//! ```
//!
//! # State synchronization
//!
//! The remainding synchronization primitives focus on synchronizing state.
//! These are asynchronous equivalents to versions provided by `std`. They
//! operate in a similar way as their `std` counterparts parts but will wait
//! asynchronously instead of blocking the thread.
//!
//! * [`Barrier`][Barrier] Ensures multiple tasks will wait for each other to
//! reach a point in the program, before continuing execution all together.
//!
//! * [`Mutex`][Mutex] Mutual Exclusion mechanism, which ensures that at most
//! one thread at a time is able to access some data.
//!
//! * [`RwLock`][RwLock] Provides a mutual exclusion mechanism which allows
//! multiple readers at the same time, while allowing only one writer at a
//! time. In some cases, this can be more efficient than a mutex.
cfg_sync! {
mod barrier;
tokio/src/sync/watch.rs
+32
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@@ -258,6 +258,38 @@ impl<T> Receiver<T> {
impl<T: Clone> Receiver<T> {
/// Attempts to clone the latest value sent via the channel.
///
/// If this is the first time the function is called on a `Receiver`
/// instance, then the function completes immediately with the **current**
/// value held by the channel. On the next call, the function waits until
/// a new value is sent in the channel.
///
/// `None` is returned if the `Sender` half is dropped.
///
/// # Examples
///
/// ```
/// use tokio::sync::watch;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx) = watch::channel("hello");
///
/// let v = rx.recv().await.unwrap();
/// assert_eq!(v, "hello");
///
/// tokio::spawn(async move {
/// tx.broadcast("goodbye").unwrap();
/// });
///
/// // Waits for the new task to spawn and send the value.
/// let v = rx.recv().await.unwrap();
/// assert_eq!(v, "goodbye");
///
/// let v = rx.recv().await;
/// assert!(v.is_none());
/// }
/// ```
pub async fn recv(&mut self) -> Option<T> {
poll_fn(|cx| {
let v_ref = ready!(self.poll_recv_ref(cx));