Trait tokio::executor::Executor

source · [−]
pub trait Executor {
    fn spawn(
        &mut self, 
        future: Box<dyn Future<Item = (), Error = ()> + Send + 'static, Global>
    ) -> Result<(), SpawnError>;

    fn status(&self) -> Result<(), SpawnError> { ... }
}
Expand description

A value that executes futures.

The spawn function is used to submit a future to an executor. Once submitted, the executor takes ownership of the future and becomes responsible for driving the future to completion.

The strategy employed by the executor to handle the future is less defined and is left up to the Executor implementation. The Executor instance is expected to call poll on the future once it has been notified, however the “when” and “how” can vary greatly.

For example, the executor might be a thread pool, in which case a set of threads have already been spawned up and the future is inserted into a queue. A thread will acquire the future and poll it.

The Executor trait is only for futures that are Send. These are most common. There currently is no trait that describes executors that operate entirely on the current thread (i.e., are able to spawn futures that are not Send). Note that single threaded executors can still implement Executor, but only futures that are Send can be spawned via the trait.

This trait is primarily intended to implemented by executors and used to back tokio::spawn. Libraries and applications may use this trait to bound generics, but doing so will limit usage to futures that implement Send. Instead, libraries and applications are recommended to use TypedExecutor as a bound.

Errors

The spawn function returns Result with an error type of SpawnError. This error type represents the reason that the executor was unable to spawn the future. The two current represented scenarios are:

  • An executor being at capacity or full. As such, the executor is not able to accept a new future. This error state is expected to be transient.
  • An executor has been shutdown and can no longer accept new futures. This error state is expected to be permanent.

If a caller encounters an at capacity error, the caller should try to shed load. This can be as simple as dropping the future that was spawned.

If the caller encounters a shutdown error, the caller should attempt to gracefully shutdown.

Examples

use futures::future::lazy;
my_executor.spawn(Box::new(lazy(|| {
    println!("running on the executor");
    Ok(())
}))).unwrap();

Required methods

Spawns a future object to run on this executor.

future is passed to the executor, which will begin running it. The future may run on the current thread or another thread at the discretion of the Executor implementation.

Panics

Implementations are encouraged to avoid panics. However, panics are permitted and the caller should check the implementation specific documentation for more details on possible panics.

Examples
use futures::future::lazy;
my_executor.spawn(Box::new(lazy(|| {
    println!("running on the executor");
    Ok(())
}))).unwrap();

Provided methods

Provides a best effort hint to whether or not spawn will succeed.

This function may return both false positives and false negatives. If status returns Ok, then a call to spawn will probably succeed, but may fail. If status returns Err, a call to spawn will probably fail, but may succeed.

This allows a caller to avoid creating the task if the call to spawn has a high likelihood of failing.

Panics

This function must not panic. Implementers must ensure that panics do not happen.

Examples
use futures::future::lazy;

if my_executor.status().is_ok() {
    my_executor.spawn(Box::new(lazy(|| {
        println!("running on the executor");
        Ok(())
    }))).unwrap();
} else {
    println!("the executor is not in a good state");
}

Implementations on Foreign Types

Implementors