Tasks
Now that we know what Futures are, we want to run them!
In async-std
, the task
module is responsible for this. The simplest way is using the block_on
function:
extern crate async_std; use async_std::{fs::File, io, prelude::*, task}; async fn read_file(path: &str) -> io::Result<String> { let mut file = File::open(path).await?; let mut contents = String::new(); file.read_to_string(&mut contents).await?; Ok(contents) } fn main() { let reader_task = task::spawn(async { let result = read_file("data.csv").await; match result { Ok(s) => println!("{}", s), Err(e) => println!("Error reading file: {:?}", e) } }); println!("Started task!"); task::block_on(reader_task); println!("Stopped task!"); }
This asks the runtime baked into async_std
to execute the code that reads a file. Let's go one by one, though, inside to outside.
#![allow(unused)] fn main() { extern crate async_std; use async_std::{fs::File, io, prelude::*, task}; async fn read_file(path: &str) -> io::Result<String> { let mut file = File::open(path).await?; let mut contents = String::new(); file.read_to_string(&mut contents).await?; Ok(contents) } async { let result = read_file("data.csv").await; match result { Ok(s) => println!("{}", s), Err(e) => println!("Error reading file: {:?}", e) } }; }
This is an async
block. Async blocks are necessary to call async
functions, and will instruct the compiler to include all the relevant instructions to do so. In Rust, all blocks return a value and async
blocks happen to return a value of the kind Future
.
But let's get to the interesting part:
#![allow(unused)] fn main() { extern crate async_std; use async_std::task; task::spawn(async { }); }
spawn
takes a Future
and starts running it on a Task
. It returns a JoinHandle
. Futures in Rust are sometimes called cold Futures. You need something that starts running them. To run a Future, there may be some additional bookkeeping required, e.g. whether it's running or finished, where it is being placed in memory and what the current state is. This bookkeeping part is abstracted away in a Task
.
A Task
is similar to a Thread
, with some minor differences: it will be scheduled by the program instead of the operating system kernel, and if it encounters a point where it needs to wait, the program itself is responsible for waking it up again. We'll talk a little bit about that later. An async_std
task can also have a name and an ID, just like a thread.
For now, it is enough to know that once you have spawn
ed a task, it will continue running in the background. The JoinHandle
is itself a future that will finish once the Task
has run to conclusion. Much like with threads
and the join
function, we can now call block_on
on the handle to block the program (or the calling thread, to be specific) and wait for it to finish.
Tasks in async_std
Tasks in async_std
are one of the core abstractions. Much like Rust's thread
s, they provide some practical functionality over the raw concept. Tasks
have a relationship to the runtime, but they are in themselves separate. async_std
tasks have a number of desirable properties:
- They are allocated in one single allocation
- All tasks have a backchannel, which allows them to propagate results and errors to the spawning task through the
JoinHandle
- They carry useful metadata for debugging
- They support task local storage
async_std
s task API handles setup and teardown of a backing runtime for you and doesn't rely on a runtime being explicitly started.
Blocking
Task
s are assumed to run concurrently, potentially by sharing a thread of execution. This means that operations blocking an operating system thread, such as std::thread::sleep
or io function from Rust's std
library will stop execution of all tasks sharing this thread. Other libraries (such as database drivers) have similar behaviour. Note that blocking the current thread is not in and of itself bad behaviour, just something that does not mix well with the concurrent execution model of async-std
. Essentially, never do this:
extern crate async_std; use async_std::task; fn main() { task::block_on(async { // this is std::fs, which blocks std::fs::read_to_string("test_file"); }) }
If you want to mix operation kinds, consider putting such blocking operations on a separate thread
.
Errors and panics
Tasks report errors through normal patterns: If they are fallible, their Output
should be of kind Result<T,E>
.
In case of panic
, behaviour differs depending on whether there's a reasonable part that addresses the panic
. If not, the program aborts.
In practice, that means that block_on
propagates panics to the blocking component:
extern crate async_std; use async_std::task; fn main() { task::block_on(async { panic!("test"); }); }
thread 'async-task-driver' panicked at 'test', examples/panic.rs:8:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
While panicking a spawned task will abort:
#![allow(unused)] fn main() { extern crate async_std; use async_std::task; use std::time::Duration; task::spawn(async { panic!("test"); }); task::block_on(async { task::sleep(Duration::from_millis(10000)).await; }) }
thread 'async-task-driver' panicked at 'test', examples/panic.rs:8:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
Aborted (core dumped)
That might seem odd at first, but the other option would be to silently ignore panics in spawned tasks. The current behaviour can be changed by catching panics in the spawned task and reacting with custom behaviour. This gives users the choice of panic handling strategy.
Conclusion
async_std
comes with a useful Task
type that works with an API similar to std::thread
. It covers error and panic behaviour in a structured and defined way.
Tasks are separate concurrent units and sometimes they need to communicate. That's where Stream
s come in.