Akka/Scala example source code file (TimerBasedThrottler.scala)

This example Akka source code file (TimerBasedThrottler.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Akka and Scala source code examples by using tags.

All credit for the original source code belongs to akka.io; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Akka tags/keywords

active, akka, collection, concurrent, data, duration, event, idle, int, q, rate, state, time, timerbasedthrottler, utilities

The TimerBasedThrottler.scala Akka example source code

/**
 * Copyright (C) 2009-2014 Typesafe Inc. <http://www.typesafe.com>
 */

package akka.contrib.throttle

import scala.concurrent.duration.{ Duration, FiniteDuration }
import scala.util.control.NonFatal
import scala.collection.immutable.{ Queue ⇒ Q }
import akka.actor.{ ActorRef, Actor, FSM }
import Throttler._
import TimerBasedThrottler._
import java.util.concurrent.TimeUnit
import akka.AkkaException

/**
 * @see [[akka.contrib.throttle.TimerBasedThrottler]]
 * @see [[akka.contrib.throttle.Throttler.Rate]]
 * @see [[akka.contrib.throttle.Throttler.SetRate]]
 * @see [[akka.contrib.throttle.Throttler.SetTarget]]
 */
object Throttler {
  /**
   * A rate used for throttling.
   *
   * Scala API: There are some shorthands available to construct rates:
   * {{{
   *  import java.util.concurrent.TimeUnit._
   *  import scala.concurrent.duration.{ Duration, FiniteDuration }
   *
   *  val rate1 = 1 msgsPer (1, SECONDS)
   *  val rate2 = 1 msgsPer Duration(1, SECONDS)
   *  val rate3 = 1 msgsPer (1 seconds)
   *  val rate4 = 1 msgsPerSecond
   *  val rate5 = 1 msgsPerMinute
   *  val rate6 = 1 msgsPerHour
   * }}}
   *
   * @param numberOfCalls the number of calls that may take place in a period
   * @param duration the length of the period
   * @see [[akka.contrib.throttle.Throttler]]
   */
  final case class Rate(val numberOfCalls: Int, val duration: FiniteDuration) {
    /**
     * The duration in milliseconds.
     */
    def durationInMillis(): Long = duration.toMillis
  }

  /**
   * Set the target of a throttler.
   *
   * You may change a throttler's target at any time.
   *
   * Notice that the messages sent by the throttler to the target will have the original sender (and
   * not the throttler) as the sender. (In Akka terms, the throttler `forward`s the message.)
   *
   * @param target if `target` is `None`, the throttler will stop delivering messages and the messages already received
   *  but not yet delivered, as well as any messages received in the future will be queued
   *  and eventually be delivered when a new target is set. If `target` is not `None`, the currently queued messages
   *  as well as any messages received in the future will be delivered to the new target at a rate not exceeding the current throttler's rate.
   */
  final case class SetTarget(target: Option[ActorRef]) {
    /**
     * Java API:
     * @param target if `target` is `null`, the throttler will stop delivering messages and the messages already received
     *  but not yet delivered, as well as any messages received in the future will be queued
     *  and eventually be delivered when a new target is set. If `target` is not `null`, the currently queued messages
     *  as well as any messages received in the future will be delivered to the new target at a rate not exceeding
     *  the current throttler's rate.
     */
    def this(target: ActorRef) = this(Option(target))
  }

  /**
   * Set the rate of a throttler.
   *
   * You may change a throttler's rate at any time.
   *
   * @param rate the rate at which messages will be delivered to the target of the throttler
   */
  final case class SetRate(rate: Rate)

  import language.implicitConversions

  /**
   * Helper for some syntactic sugar.
   *
   * @see [[akka.contrib.throttle.Throttler.Rate]]
   */
  implicit class RateInt(val numberOfCalls: Int) extends AnyVal {
    def msgsPer(duration: Int, timeUnit: TimeUnit) = Rate(numberOfCalls, Duration(duration, timeUnit))
    def msgsPer(duration: FiniteDuration) = Rate(numberOfCalls, duration)
    def msgsPerSecond = Rate(numberOfCalls, Duration(1, TimeUnit.SECONDS))
    def msgsPerMinute = Rate(numberOfCalls, Duration(1, TimeUnit.MINUTES))
    def msgsPerHour = Rate(numberOfCalls, Duration(1, TimeUnit.HOURS))
  }

}

/**
 * INTERNAL API
 */
private[throttle] object TimerBasedThrottler {
  case object Tick

  // States of the FSM: A `TimerBasedThrottler` is in state `Active` iff the timer is running.
  sealed trait State
  case object Idle extends State
  case object Active extends State

  // Messages, as we queue them to be sent later
  final case class Message(message: Any, sender: ActorRef)

  // The data of the FSM
  final case class Data(target: Option[ActorRef],
                        callsLeftInThisPeriod: Int,
                        queue: Q[Message])
}

/**
 * A throttler that uses a timer to control the message delivery rate.
 *
 * == Throttling ==
 * A <em>throttler</em> is an actor that is defined through a <em>target actor</em> and a <em>rate</em>
 * (of type [[akka.contrib.throttle.Throttler.Rate]]). You set or change the target and rate at any time through the
 * [[akka.contrib.throttle.Throttler.SetTarget]] and [[akka.contrib.throttle.Throttler.SetRate]]
 * messages, respectively. When you send the throttler any other message `msg`, it will
 * put the message `msg` into an internal queue and eventually send all queued messages to the target, at
 * a speed that respects the given rate. If no target is currently defined then the messages will be queued
 * and will be delivered as soon as a target gets set.
 *
 * A throttler understands actor messages of type
 * [[akka.contrib.throttle.Throttler.SetTarget]], [[akka.contrib.throttle.Throttler.SetRate]], in
 * addition to any other messages, which the throttler will consider as messages to be sent to
 * the target.
 *
 * == Transparency ==
 * Notice that the throttler `forward`s messages, i.e., the target will see the original message sender
 * (and not the throttler) as the sender of the message.
 *
 * == Persistence ==
 * Throttlers usually use an internal queue to keep the messages that need to be sent to the target.
 * You therefore cannot rely on the throttler's inbox size in order to learn how much messages are
 * outstanding.
 *
 * It is left to the implementation whether the internal queue is persisted over application restarts or
 * actor failure.
 *
 * == Processing messages ==
 * The target should process messages as fast as possible. If the target requires substantial time to
 * process messages, it should distribute its work to other actors (using for example something like
 * a `BalancingDispatcher`), otherwise the resulting system will always work <em>below</em>
 * the threshold rate.
 *
 * <em>Example:</em> Suppose the throttler has a rate of 3msg/s and the target requires 1s to process a message.
 * This system will only process messages at a rate of 1msg/s: the target will receive messages at at most 3msg/s
 * but as it handles them synchronously and each of them takes 1s, its inbox will grow and grow. In such
 * a situation, the target should <em>distribute</em> its messages to a set of worker actors so that individual messages
 * can be handled in parallel.
 *
 * ==Example==
 * For example, if you set a rate like "3 messages in 1 second", the throttler
 * will send the first three messages immediately to the target actor but will need to impose a delay before
 * sending out further messages:
 * {{{
 *   // A simple actor that prints whatever it receives
 *   class Printer extends Actor {
 *     def receive = {
 *       case x => println(x)
 *     }
 *   }
 *
 *   val printer = system.actorOf(Props[Printer], "printer")
 *
 *   // The throttler for this example, setting the rate
 *   val throttler = system.actorOf(Props(classOf[TimerBasedThrottler], 3 msgsPer 1.second))
 *
 *   // Set the target
 *   throttler ! SetTarget(Some(printer))
 *   // These three messages will be sent to the printer immediately
 *   throttler ! "1"
 *   throttler ! "2"
 *   throttler ! "3"
 *   // These two will wait at least until 1 second has passed
 *   throttler ! "4"
 *   throttler ! "5"
 * }}}
 *
 * ==Implementation notes==
 * This throttler implementation internally installs a timer that repeats every `rate.durationInMillis` and enables `rate.numberOfCalls`
 * additional calls to take place. A `TimerBasedThrottler` uses very few system resources, provided the rate's duration is not too
 * fine-grained (which would cause a lot of timer invocations); for example, it does not store the calling history
 * as other throttlers may need to do.
 *
 * However, a `TimerBasedThrottler` only provides ''weak guarantees'' on the rate (see also
 * <a href='http://letitcrash.com/post/28901663062/throttling-messages-in-akka-2'>this blog post</a>):
 *
 *  - Only ''delivery'' times are taken into account: if, for example, the throttler is used to throttle
 *    requests to an external web service then only the start times of the web requests are considered.
 *    If a web request takes very long on the server then more than `rate.numberOfCalls`-many requests
 *    may be observed on the server in an interval of duration `rate.durationInMillis()`.
 *  - There may be intervals of duration `rate.durationInMillis()` that contain more than `rate.numberOfCalls`
 *    message deliveries: a `TimerBasedThrottler` only makes guarantees for the intervals
 *    of its ''own'' timer, namely that no more than `rate.numberOfCalls`-many messages are delivered within such intervals. Other intervals on the
 *    timeline may contain more calls.
 *
 * For some applications, these guarantees may not be sufficient.
 *
 * ==Known issues==
 *
 *  - If you change the rate using `SetRate(rate)`, the actual rate may in fact be higher for the
 *    overlapping period (i.e., `durationInMillis()`) of the new and old rate. Therefore,
 *    changing the rate frequently is not recommended with the current implementation.
 *  - The queue of messages to be delivered is not persisted in any way; actor or system failure will
 *    cause the queued messages to be lost.
 *
 * @see [[akka.contrib.throttle.Throttler]]
 */
class TimerBasedThrottler(var rate: Rate) extends Actor with FSM[State, Data] {
  startWith(Idle, Data(None, rate.numberOfCalls, Q()))

  // Idle: no messages, or target not set
  when(Idle) {
    // Set the rate
    case Event(SetRate(rate), d) ⇒
      this.rate = rate
      stay using d.copy(callsLeftInThisPeriod = rate.numberOfCalls)

    // Set the target
    case Event(SetTarget(t @ Some(_)), d) if !d.queue.isEmpty ⇒
      goto(Active) using deliverMessages(d.copy(target = t))
    case Event(SetTarget(t), d) ⇒
      stay using d.copy(target = t)

    // Queuing
    case Event(msg, d @ Data(None, _, queue)) ⇒
      stay using d.copy(queue = queue.enqueue(Message(msg, context.sender())))
    case Event(msg, d @ Data(Some(_), _, Seq())) ⇒
      goto(Active) using deliverMessages(d.copy(queue = Q(Message(msg, context.sender()))))
    // Note: The case Event(msg, t @ Data(Some(_), _, _, Seq(_*))) should never happen here.
  }

  when(Active) {
    // Set the rate
    case Event(SetRate(rate), d) ⇒
      this.rate = rate
      // Note: this should be improved (see "Known issues" in class comments)
      stopTimer()
      startTimer(rate)
      stay using d.copy(callsLeftInThisPeriod = rate.numberOfCalls)

    // Set the target (when the new target is None)
    case Event(SetTarget(None), d) ⇒
      // Note: We do not yet switch to state `Inactive` because we need the timer to tick once more before
      stay using d.copy(target = None)

    // Set the target (when the new target is not None)
    case Event(SetTarget(t @ Some(_)), d) ⇒
      stay using d.copy(target = t)

    // Tick after a `SetTarget(None)`: take the additional permits and go to `Idle`
    case Event(Tick, d @ Data(None, _, _)) ⇒
      goto(Idle) using d.copy(callsLeftInThisPeriod = rate.numberOfCalls)

    // Period ends and we have no more messages: take the additional permits and go to `Idle`
    case Event(Tick, d @ Data(_, _, Seq())) ⇒
      goto(Idle) using d.copy(callsLeftInThisPeriod = rate.numberOfCalls)

    // Period ends and we get more occasions to send messages
    case Event(Tick, d @ Data(_, _, _)) ⇒
      stay using deliverMessages(d.copy(callsLeftInThisPeriod = rate.numberOfCalls))

    // Queue a message (when we cannot send messages in the current period anymore)
    case Event(msg, d @ Data(_, 0, queue)) ⇒
      stay using d.copy(queue = queue.enqueue(Message(msg, context.sender())))

    // Queue a message (when we can send some more messages in the current period)
    case Event(msg, d @ Data(_, _, queue)) ⇒
      stay using deliverMessages(d.copy(queue = queue.enqueue(Message(msg, context.sender()))))
  }

  onTransition {
    case Idle -> Active ⇒ startTimer(rate)
    case Active -> Idle ⇒ stopTimer()
  }

  initialize()

  private def startTimer(rate: Rate) = setTimer("morePermits", Tick, rate.duration, true)
  private def stopTimer() = cancelTimer("morePermits")

  /**
   * Send as many messages as we can (while respecting the rate) to the target and
   * return the state data (with the queue containing the remaining ones).
   */
  private def deliverMessages(data: Data): Data = {
    val queue = data.queue
    val nrOfMsgToSend = scala.math.min(queue.length, data.callsLeftInThisPeriod)

    queue.take(nrOfMsgToSend).foreach(x ⇒ data.target.get.tell(x.message, x.sender))

    data.copy(queue = queue.drop(nrOfMsgToSend), callsLeftInThisPeriod = data.callsLeftInThisPeriod - nrOfMsgToSend)
  }
}