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p

monix.execution

schedulers

package schedulers

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  1. final class AsyncScheduler extends ReferenceScheduler with BatchingScheduler

    An AsyncScheduler schedules tasks to happen in the future with the given ScheduledExecutorService and the tasks themselves are executed on the given ExecutionContext.

  2. trait BatchingScheduler extends Scheduler

    Adds trampoline execution capabilities to schedulers, when inherited.

    Adds trampoline execution capabilities to schedulers, when inherited.

    When it receives TrampolinedRunnable instances, it switches to a trampolined mode where all incoming TrampolinedRunnable are executed on the current thread.

    This is useful for light-weight callbacks. The idea is borrowed from the implementation of scala.concurrent.Future. Currently used as an optimization by Task in processing its internal callbacks.

  3. final class CanBlock extends AnyRef

    Marker for blocking operations that need to be disallowed on top of JavaScript engines, or other platforms that don't support the blocking of threads.

    Marker for blocking operations that need to be disallowed on top of JavaScript engines, or other platforms that don't support the blocking of threads.

    As sample, lets implement a low-level blocking operation; but kids, don't do this at home, since this is error prone and you already have Scala's Await.result, this sample being shown for pedagogical purposes:

    import monix.execution.schedulers.CanBlock
    import java.util.concurrent.CountDownLatch
    import scala.concurrent.{ExecutionContext, Future}
    import scala.util.Try
    
    def block[A](fa: Future[A])
      (implicit ec: ExecutionContext, permit: CanBlock): Try[A] = {
    
      var result = Option.empty[Try[A]]
      val latch = new CountDownLatch(1)
    
      fa.onComplete { r =>
        result = r
        latch.countDown()
      }
    
      latch.await()
      result.get
    }

    And then for JavaScript engines (Scala.js) you could describe the same function, with the same signature, but without any implementation, since this operation isn't supported:

    def block[A](fa: Future[A])
      (implicit ec: ExecutionContext, permit: CanBlock): Try[A] =
      throw new UnsupportedOperationException("Cannot block threads on top of JavaScript")

    Now in usage, when the caller is invoking block as described, it will work without issues on top of the JVM, but when compiled with Scala.js it will trigger a message like this:

    [error] Playground.scala:30:8: Blocking operations aren't supported
    [error] on top of JavaScript, because it cannot block threads!
    [error] Please use asynchronous API calls.
    [error]   block(Future(1))
    [error]        ^
    Annotations
    @implicitNotFound( ... )
  4. abstract class ExecutorScheduler extends SchedulerService with ReferenceScheduler with BatchingScheduler

    An ExecutorScheduler is a class for building a SchedulerService out of a Java ExecutorService.

  5. trait ReferenceScheduler extends Scheduler

    Helper for building a Scheduler.

    Helper for building a Scheduler.

    You can inherit from this class and provided a correct scheduleOnce you'll get Scheduler.scheduleWithFixedDelay and Scheduler.scheduleAtFixedRate for free.

  6. trait SchedulerService extends Scheduler

    A Scheduler type that provides methods for managing termination.

    A Scheduler type that provides methods for managing termination.

    A SchedulerService can be shut down, which will cause it to reject new tasks. The shutdown method allows previously submitted tasks to execute before terminating. The awaitTermination method allows waiting on all active tasks to finish.

    Upon termination, an executor has no tasks actively executing, no tasks awaiting execution, and no new tasks can be submitted. An unused SchedulerService should be shut down to allow reclamation of its resources.

  7. final class ShiftedRunnable extends Runnable

    Runnable that defers the execution of the given reference with an executeAsync.

    Runnable that defers the execution of the given reference with an executeAsync.

    This is useful for example when implementing scheduleOnce, to introduce a boundary between the scheduling and the execution, otherwise risk executing the runnable on the wrong thread-pool.

  8. final case class StartAsyncBatchRunnable(start: TrampolinedRunnable, s: Scheduler) extends Runnable with Serializable with Product

    Forces a real asynchronous boundary before executing the given TrampolinedRunnable.

    Forces a real asynchronous boundary before executing the given TrampolinedRunnable.

    Sometimes you want to execute multiple TrampolinedRunnable instances as a batch, with the functionality provided by schedulers implementing BatchingScheduler, however you might need the very first execution to force an asynchronous boundary.

    start

    is the TrampolinedRunnable instance that will get executed and that is supposed to trigger the execution of other trampolined runnables

    s

    is the scheduler that gets used for execution.

  9. final class TestScheduler extends ReferenceScheduler with BatchingScheduler

    Scheduler and a provider of cats.effect.Timer instances, that can simulate async boundaries and time passage, useful for testing purposes.

    Scheduler and a provider of cats.effect.Timer instances, that can simulate async boundaries and time passage, useful for testing purposes.

    Usage for simulating an ExecutionContext:

    implicit val ec = TestScheduler()
    
    ec.execute(new Runnable { def run() = println("task1") })
    
    ex.execute(new Runnable {
      def run() = {
        println("outer")
    
        ec.execute(new Runnable {
          def run() = println("inner")
        })
      }
    })
    
    // Nothing executes until `tick` gets called
    ec.tick()
    
    // Testing the resulting state
    assert(ec.state.tasks.isEmpty)
    assert(ec.state.lastReportedError == null)

    TestScheduler can also simulate the passage of time:

    val ctx = TestScheduler()
    val f = Task(1 + 1).delayExecution(10.seconds).runAsync
    
    // This only triggers immediate execution, so nothing happens yet
    ctx.tick()
    assert(f.value == None)
    
    // Simulating the passage of 5 seconds, nothing happens yet
    ctx.tick(5.seconds)
    assert(f.value == None)
    
    // Simulating another 5 seconds, now we're done!
    assert(f.value == Some(Success(2)))

    We are also able to build a cats.effect.Timer from any Scheduler and for any data type:

    val ctx = TestScheduler()
    
    val timer: Timer[IO] = ctx.timer[IO]

    We can now simulate time passage for cats.effect.IO as well:

    val io = timer.sleep(10.seconds) *> IO(1 + 1)
    val f = io.unsafeToFuture()
    
    // This invariant holds true, because our IO is async
    assert(f.value == None)
    
    // Not yet completed, because this does not simulate time passing:
    ctx.tick()
    assert(f.value == None)
    
    // Simulating time passing:
    ctx.tick(10.seconds)
    assert(f.value == Some(Success(2))

    Simulating time makes this pretty useful for testing race conditions:

    val timeoutError = new TimeoutException
    val timeout = Task.raiseError[Int](timeoutError)
      .delayExecution(10.seconds)
    
    val pair = (Task.never, timeout).parMapN(_ + _)
    
    // Not yet
    ctx.tick()
    assert(f.value == None)
    
    // Not yet
    ctx.tick(5.seconds)
    assert(f.value == None)
    
    // Good to go:
    ctx.tick(5.seconds)
    assert(f.value == Some(Failure(timeoutError)))
  10. final class TracingRunnable extends Runnable

    Wraps a Runnable into one that restores the given Local.Context upon execution of run().

    Wraps a Runnable into one that restores the given Local.Context upon execution of run().

    Used by TracingScheduler.

  11. final class TracingScheduler extends Base

    The TracingScheduler is a Scheduler implementation that wraps another Scheduler reference, but that propagates the Local.Context on async execution.

  12. final class TracingSchedulerService extends Base with SchedulerService

    The TracingScheduler is a Scheduler implementation that wraps another SchedulerService reference, with the purpose of propagating the Local.Context on async execution.

  13. final class TrampolineExecutionContext extends ExecutionContext

    A scala.concurrentExecutionContext implementation that executes runnables immediately, on the current thread, by means of a trampoline implementation.

    A scala.concurrentExecutionContext implementation that executes runnables immediately, on the current thread, by means of a trampoline implementation.

    Can be used in some cases to keep the asynchronous execution on the current thread, as an optimization, but be warned, you have to know what you're doing.

    The TrampolineExecutionContext keeps a reference to another underlying context, to which it defers for:

    • reporting errors
    • deferring the rest of the queue in problematic situations

    Deferring the rest of the queue happens:

    • in case we have a runnable throwing an exception, the rest of the tasks get re-scheduled for execution by using the underlying context
    • in case we have a runnable triggering a Scala blocking context, the rest of the tasks get re-scheduled for execution on the underlying context to prevent any deadlocks

    Thus this implementation is compatible with the scala.concurrent.BlockContext, detecting blocking blocks and reacting by forking the rest of the queue to prevent deadlocks.

  14. final class TrampolineScheduler extends Scheduler

    A Scheduler implementation that executes runnables immediately, on the current thread, by means of a trampoline implementation.

    A Scheduler implementation that executes runnables immediately, on the current thread, by means of a trampoline implementation.

    Can be used in some cases to keep the asynchronous execution on the current thread, as an optimization, but be warned, you have to know what you're doing.

    The TrampolineScheduler keeps a reference to another underlying scheduler, to which it defers for:

    • reporting errors
    • time-delayed execution
    • deferring the rest of the queue in problematic situations

    Deferring the rest of the queue happens:

    • in case we have a runnable throwing an exception, the rest of the tasks get re-scheduled for execution by using the underlying scheduler
    • in case we have a runnable triggering a Scala blocking context, the rest of the tasks get re-scheduled for execution on the underlying scheduler to prevent any deadlocks

    Thus this implementation is compatible with the scala.concurrent.BlockContext, detecting blocking blocks and reacting by forking the rest of the queue to prevent deadlocks.

  15. trait TrampolinedRunnable extends Runnable with OnCompleteRunnable

    A marker for callbacks that can be batched and executed locally (on the current thread) by means of a trampoline (if the execution context / scheduler allows it).

    A marker for callbacks that can be batched and executed locally (on the current thread) by means of a trampoline (if the execution context / scheduler allows it).

    Idea was taken from the scala.concurrent.Future implementation. Credit should be given where due.

    DO NOT use unless you know what you're doing.

    Annotations
    @FunctionalInterface()

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