diff options
| -rw-r--r-- | tokio/src/sync/mod.rs | 417 | ||||
| -rw-r--r-- | tokio/src/sync/watch.rs | 32 |
2 files changed, 438 insertions, 11 deletions
diff --git a/tokio/src/sync/mod.rs b/tokio/src/sync/mod.rs index beb96920..029d626a 100644 --- a/tokio/src/sync/mod.rs +++ b/tokio/src/sync/mod.rs @@ -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; diff --git a/tokio/src/sync/watch.rs b/tokio/src/sync/watch.rs index 3e945563..59e3eec0 100644 --- a/tokio/src/sync/watch.rs +++ b/tokio/src/sync/watch.rs @@ -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)); |