pekko/akka-docs/src/main/paradox/java/scheduler.md

Scheduler

Sometimes the need for making things happen in the future arises, and where do you go look then? Look no further than ActorSystem! There you find the scheduler method that returns an instance of akka.actor.Scheduler, this instance is unique per ActorSystem and is used internally for scheduling things to happen at specific points in time.

You can schedule sending of messages to actors and execution of tasks (functions or Runnable). You will get a Cancellable back that you can call cancel on to cancel the execution of the scheduled operation.

The scheduler in Akka is designed for high-throughput of thousands up to millions of triggers. The prime use-case being triggering Actor receive timeouts, Future timeouts, circuit breakers and other time dependent events which happen all-the-time and in many instances at the same time. The implementation is based on a Hashed Wheel Timer, which is a known datastructure and algorithm for handling such use cases, refer to the Hashed and Hierarchical Timing Wheels whitepaper by Varghese and Lauck if you'd like to understand its inner workings.

The Akka scheduler is not designed for long-term scheduling (see akka-quartz-scheduler instead for this use case) nor is it to be used for higly precise firing of the events. The maximum amount of time into the future you can schedule an event to trigger is around 8 months, which in practice is too much to be useful since this would assume the system never went down during that period. If you need long-term scheduling we highly recommend looking into alternative schedulers, as this is not the use-case the Akka scheduler is implemented for.

@@@ warning

The default implementation of Scheduler used by Akka is based on job buckets which are emptied according to a fixed schedule. It does not execute tasks at the exact time, but on every tick, it will run everything that is (over)due. The accuracy of the default Scheduler can be modified by the akka.scheduler.tick-duration configuration property.

@@@

Some examples

Schedule to send the "foo"-message to the testActor after 50ms:

@@snip SchedulerDocTest.java { #imports1 }

@@snip SchedulerDocTest.java { #schedule-one-off-message }

Schedule a Runnable, that sends the current time to the testActor, to be executed after 50ms:

@@snip SchedulerDocTest.java { #schedule-one-off-thunk }

@@@ warning

If you schedule Runnable instances you should be extra careful to not pass or close over unstable references. In practice this means that you should not call methods on the enclosing Actor from within the Runnable. If you need to schedule an invocation it is better to use the schedule() variant accepting a message and an ActorRef to schedule a message to self (containing the necessary parameters) and then call the method when the message is received.

@@@

Schedule to send the "Tick"-message to the tickActor after 0ms repeating every 50ms:

@@snip SchedulerDocTest.java { #imports1 #imports2 }

@@snip SchedulerDocTest.java { #schedule-recurring }

From akka.actor.ActorSystem

@@snip ActorSystem.scala { #scheduler }

@@@ warning

All scheduled task will be executed when the ActorSystem is terminated, i.e. the task may execute before its timeout.

@@@

The Scheduler Interface for Implementors

The actual scheduler implementation is loaded reflectively upon ActorSystem start-up, which means that it is possible to provide a different one using the akka.scheduler.implementation configuration property. The referenced class must implement the following interface:

@@snip AbstractScheduler.java { #scheduler }

The Cancellable interface

Scheduling a task will result in a Cancellable (or throw an IllegalStateException if attempted after the scheduler’s shutdown). This allows you to cancel something that has been scheduled for execution.

@@@ warning

This does not abort the execution of the task, if it had already been started. Check the return value of cancel to detect whether the scheduled task was canceled or will (eventually) have run.

@@@

@@snip Scheduler.scala { #cancellable }