Tasks

Now that we know what Futures are, we want to run them!

In async-std, the tasks 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_variables)]
#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_variables)]
#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 spawned 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 threads, 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_stds task API handles setup and teardown of a backing runtime for you and doesn't rely on a runtime being explicitly started.

Blocking

Tasks 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 panicing a spawned task will abort:


# #![allow(unused_variables)]
#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 Streams come in.