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play-scheduler

This project demonstrates three different ways to trigger background tasks using Play framework. Dependencies and plugins:

  • Scala 2.13.3
  • Play Framework 2.8.2
  • SBT 1.3.13

Three different ways

We have to choose three flavors:

  • scheduleOnce
  • scheduleAtFixedRate
  • scheduleWithFixedDelay

scheduleOnce

This is the simplest one, and you know the result just reading its name: the task will be run only once. The task will be executed only once, and you can define some delay to wait before executing the task.

scheduleAtFixedRate & scheduleWithFixedDelay

Both look similar so what's the difference? Let's see some examples.

Method scheduleAtFixedRate takes this two parameters (among others):

  • initialDelay
  • interval

Let's say your Play application starts at 10:00 am, and you set intialDelay to 2 seconds and interval to 3 seconds. Also your task takes 1 second to complete. Then you will have something like this:

10:00 Play application starts
10:02 start executing your task for the first time
10:03 your task completes
10:05 start executing your task for the second time (10:02 + 3 seconds)
10:06 your task completes again
...

Method scheduleWithFixedDelay takes similar parameters (among others):

  • initialDelay
  • delay

Again let's say your Play application starts at 10:00 am, and you set intialDelay to 2 seconds and interval to 3 seconds:

10:00 Play application starts
10:02 start executing your task for the first time
10:03 your task completes
10:06 start executing your task for the second time (10:03 + 3 seconds)
10:07 your task completes again
...

In this case the next time to execute will be computed using the date and time when task completes.

Implementation

scheduleAtFixedRate

First we need to tell to Play to execute something when it starts. To do that we add this to application.conf:

play.modules.enabled += "tasks.BackgroundTaskAtFixedRate"

It means in package tasks class BackgroundTaskAtFixedRate will be executed at start.

We need to define the class like this:

class BackgroundTaskAtFixedRate extends SimpleModule(bind[SetTaskToRunAtFixedRate].toSelf.eagerly())

The class SetTaskToRunAtFixedRate will contain the code we want to run, in this case we are going to use Akka scheduler included in Play:

class SetTaskToRunAtFixedRate @Inject()(actorSystem: ActorSystem)  (
  implicit executionContext: ExecutionContext
) {

  // This will set a task that will be first executed after 2 seconds (parameter initialDelay).
  // and then will be executed every 3 seconds (parameter interval).

  actorSystem.log.info("[SetTaskToRunAtFixedRate]")
  actorSystem.log.info("[SetTaskToRunAtFixedRate] " + new java.util.Date())

  actorSystem.scheduler
    .scheduleAtFixedRate(initialDelay = 2 seconds, interval = 3 seconds)(
      new TaskAtFixedRateRunnable(actorSystem)
    )

}

Here we are passing to method scheduleAtFixedRate 2 seconds as initial delay and 3 seconds as interval for each execution. Also the task is passed as an instance of class TaskAtFixedRateRunnable.

class TaskAtFixedRateRunnable @Inject()(actorSystem: ActorSystem)
     extends Runnable {
     override def run(): Unit = {
       actorSystem.log.info("[TaskAtFixedRateRunnable] Executing something...")
       actorSystem.log.info("[TaskAtFixedRateRunnable] Start: " + new java.util.Date())
       Thread.sleep(1000)
       actorSystem.log.info("[TaskAtFixedRateRunnable] End: " + new java.util.Date())
     }
   }

We are not doing nothing special, just loggin message to show the use of the scheduler.

If you run the application with sbt run you will:

2020-07-11 20:49:19 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [SetTaskToRunAtFixedRate]
2020-07-11 20:49:19 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [SetTaskToRunAtFixedRate] Sat Jul 11 20:49:19 UYT 2020
2020-07-11 20:49:21 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Executing something...
2020-07-11 20:49:21 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Start: Sat Jul 11 20:49:21 UYT 2020
2020-07-11 20:49:22 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] End: Sat Jul 11 20:49:22 UYT 2020
2020-07-11 20:49:24 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Executing something...
2020-07-11 20:49:24 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Start: Sat Jul 11 20:49:24 UYT 2020
2020-07-11 20:49:25 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] End: Sat Jul 11 20:49:25 UYT 2020
2020-07-11 20:49:27 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Executing something...
2020-07-11 20:49:27 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Start: Sat Jul 11 20:49:27 UYT 2020
2020-07-11 20:49:28 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] End: Sat Jul 11 20:49:28 UYT 2020
2020-07-11 20:49:30 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Executing something...
2020-07-11 20:49:30 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Start: Sat Jul 11 20:49:30 UYT 2020
2020-07-11 20:49:31 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] End: Sat Jul 11 20:49:31 UYT 2020
2020-07-11 20:49:33 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Executing something...
2020-07-11 20:49:33 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskAtFixedRateRunnable] Start: Sat Jul 11 20:49:33 UYT 2020
...

First two lines shows class SetTaskToRunAtFixedRate invoking method scheduleAtFixedRate. Then every 3 seconds the app logs the task being executed.

scheduleWithFixedDelay

This is very similar to method scheduleAtFixedRate:

  • Add an entry in application.conf pointing to tasks.BackgroundTaskWithFixedDelay.
  • Create the class BackgroundTaskWithFixedDelay.
  • Create a new class SetTaskToRunWithFixedDelay which will be invoked by BackgroundTaskWithFixedDelay.
  • This class will set the scheduler invoking method scheduleWithFixedDelay.
  • The scheduler will execute the task of a new class in TaskWithFixedDelayRunnable.

After running the application the output is:

2020-07-11 21:44:30 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [SetTaskToRunWithFixedDelay]
2020-07-11 21:44:30 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [SetTaskToRunWithFixedDelay] Sat Jul 11 21:44:30 UYT 2020

2020-07-11 21:44:32 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] Executing something...
2020-07-11 21:44:32 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] Start: Sat Jul 11 21:44:32 UYT 2020
2020-07-11 21:44:33 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] End: Sat Jul 11 21:44:33 UYT 2020
2020-07-11 21:44:36 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] Executing something...
2020-07-11 21:44:36 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] Start: Sat Jul 11 21:44:36 UYT 2020
2020-07-11 21:44:37 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] End: Sat Jul 11 21:44:37 UYT 2020
2020-07-11 21:44:40 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] Executing something...
2020-07-11 21:44:40 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] Start: Sat Jul 11 21:44:40 UYT 2020
2020-07-11 21:44:41 INFO  akka.actor.ActorSystemImpl akka.actor.ActorSystemImpl(application) [TaskWithFixedDelayRunnable] End: Sat Jul 11 21:44:41 UYT 2020

scheduleOnce

Finally, we added a new feature in the app that will trigger a task but it will only run once. The steps are the same:

  • Add an entry in application.conf pointing to tasks.BackgroundTaskOnce.
  • Create the class BackgroundTaskOnce.
  • Create a new class SetTaskToRunOnce which will be invoked by BackgroundTaskOnce.
  • This class will set the scheduler invoking method scheduleOnce.
  • The scheduler will execute the task of a new class in TaskToRunOnce.