Supervisor

Motivation

Spawn provides multiple ways to spawn a fiber to run an action:

Spawn[F]#start: start and forget, no lifecycle management for the spawned fiber

Spawn[F]#background: ties the lifecycle of the spawned fiber to that of the fiber that invoked background

But what if we want to spawn a fiber that should outlive the scope that created it, but we still want to control its lifecycle?

A supervisor spawns fibers whose lifecycles are bound to that of the supervisor.

trait Supervisor[F[_]] {
  def supervise[A](fa: F[A]): F[Fiber[F, Throwable, A]]
}

object Supervisor {
  def apply[F[_]](await: Boolean)(implicit F: Concurrent[F]): Resource[F, Supervisor[F]]
}

Any fibers created via the supervisor will be finalized when the supervisor itself is finalized via Resource#use.

There are two finalization strategies according to the await parameter of the constructor:

true - wait for the completion of the active fibers
false - cancel the active fibers

Note: if an effect that never completes is supervised by a Supervisor with the awaiting termination policy, the termination of the Supervisor is indefinitely suspended:

import cats.effect.IO
import cats.effect.std.Supervisor

Supervisor[IO](await = true).use { supervisor =>
  supervisor.supervise(IO.never).void
}