Merge pull request akka#17925 from ktoso/wip-move-BackoffSupervisor-k…

…toso

!per +act akka#17842 move BackoffSupervisor to akka.pattern

akka-persistence/src/main/scala/akka/persistence/BackoffSupervisor.scala → akka-actor/src/main/scala/akka/pattern/BackoffSupervisor.scala

@@ -1,12 +1,11 @@
 /**
  * Copyright (C) 2015 Typesafe Inc. <http://www.typesafe.com>
  */
-package akka.persistence
+package akka.pattern
 
 import scala.concurrent.duration.FiniteDuration
 import scala.concurrent.forkjoin.ThreadLocalRandom
 import akka.actor.Actor
-import akka.actor.ActorLogging
 import akka.actor.ActorRef
 import akka.actor.DeadLetterSuppression
 import akka.actor.Props
@@ -17,15 +16,17 @@ import scala.concurrent.duration.Duration
 object BackoffSupervisor {
 
   /**
+   * Props for creating an [[BackoffSupervisor]] actor.
+   *
    * @param childProps the [[akka.actor.Props]] of the child actor that
    *   will be started and supervised
    * @param childName name of the child actor
    * @param minBackoff minimum (initial) duration until the child actor will
    *   started again, if it is terminated
    * @param maxBackoff the exponential back-off is capped to this duration
    * @param randomFactor after calculation of the exponential back-off an additional
-   *   random delay based on this factor is added, e.g. 0.2 adds up to 20%
-   *   delay
+   *   random delay based on this factor is added, e.g. `0.2` adds up to `20%` delay.
+   *   In order to skip this additional delay pass in `0`.
    */
   def props(
     childProps: Props,
@@ -64,8 +65,14 @@ object BackoffSupervisor {
 
 /**
  * This actor can be used to supervise a child actor and start it again
- * after a back-off duration if the child actor is stopped. This is useful
- * for persistent actors, which are stopped in case of persistence failures.
+ * after a back-off duration if the child actor is stopped.
+ *
+ * This is useful in situations where the re-start of the child actor should be
+ * delayed e.g. in order to give an external resource time to recover before the
+ * child actor tries contacting it again (after being restarted).
+ *
+ * Specifically this pattern is useful for for persistent actors,
+ * which are stopped in case of persistence failures.
  * Just restarting them immediately would probably fail again (since the data
  * store is probably unavailable). It is better to try again after a delay.
  *
@@ -81,13 +88,13 @@ object BackoffSupervisor {
  * actors hit the backend resource at the same time.
  *
  * You can retrieve the current child `ActorRef` by sending `BackoffSupervisor.GetCurrentChild`
- * message to this actor and it will reply with [[BackoffSupervisor.CurrentChild]] containing the
- * `ActorRef` of the current child, if any.
+ * message to this actor and it will reply with [[akka.pattern.BackoffSupervisor.CurrentChild]]
+ * containing the `ActorRef` of the current child, if any.
  *
  * The `BackoffSupervisor` forwards all other messages to the child, if it is currently running.
  *
  * The child can stop itself and send a [[akka.actor.PoisonPill]] to the parent supervisor
- * if it want to do an intentional stop.
+ * if it wants to do an intentional stop.
  */
 final class BackoffSupervisor(
   childProps: Props,
@@ -119,7 +126,7 @@ final class BackoffSupervisor(
         if (restartCount >= 30) // Duration overflow protection (> 100 years)
           maxBackoff
         else
-          (maxBackoff.min(minBackoff * math.pow(2, restartCount)) * rnd) match {
+          maxBackoff.min(minBackoff * math.pow(2, restartCount)) * rnd match {
             case f: FiniteDuration ⇒ f
             case _                 ⇒ maxBackoff
           }

akka-cluster-sharding/src/main/scala/akka/cluster/sharding/ClusterSharding.scala

@@ -20,7 +20,7 @@ import akka.actor.PoisonPill
 import akka.actor.Props
 import akka.cluster.Cluster
 import akka.cluster.singleton.ClusterSingletonManager
-import akka.persistence.BackoffSupervisor
+import akka.pattern.BackoffSupervisor
 import akka.util.ByteString
 import akka.pattern.ask
 import akka.dispatch.Dispatchers

akka-cluster-sharding/src/multi-jvm/scala/akka/cluster/sharding/ClusterShardingSpec.scala

@@ -11,6 +11,7 @@ import language.postfixOps
 import scala.concurrent.duration._
 import com.typesafe.config.ConfigFactory
 import akka.actor._
+import akka.pattern.BackoffSupervisor
 import akka.cluster.Cluster
 import akka.persistence.PersistentActor
 import akka.persistence.Persistence
@@ -26,7 +27,7 @@ import java.io.File
 import org.apache.commons.io.FileUtils
 import akka.cluster.singleton.ClusterSingletonManager
 import akka.cluster.singleton.ClusterSingletonManagerSettings
-import akka.persistence.BackoffSupervisor
+import akka.pattern.BackoffSupervisor
 
 object ClusterShardingSpec extends MultiNodeConfig {
   val controller = role("controller")

akka-docs/rst/general/supervision.rst

@@ -199,6 +199,37 @@ external resource, which may also be one of its own children. If a third party
 terminates a child by way of the ``system.stop(child)`` method or sending a
 :class:`PoisonPill`, the supervisor might well be affected.
 
+.. _backoff-supervisor:
+
+Delayed restarts with the BackoffSupervisor pattern
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Provided as a build-in pattern the ``akka.pattern.BackoffSupervisor`` actor implements the so-called
+*exponential backoff supervision strategy*, which can be used to death-watch an actor,
+and when it terminates try to start it again, each time with a growing time delay between those restarts.
+
+This pattern is useful when the started actor fails because some external resource is not available,
+and we need to give it some time to start-up again. One of the prime examples when this is useful is
+when a :ref:`PersistentActor <persistence-scala>` fails with an persistence failure - which indicates that
+the database may be down or overloaded, in such situations it makes most sense to give it a little bit of time
+to recover before the peristent actor is restarted.
+
+The following Scala snippet shows how to create a backoff supervisor which will start the given echo actor
+in increasing intervals of 3, 6, 12, 24 and finally 30 seconds:
+
+.. includecode:: ../scala/code/docs/pattern/BackoffSupervisorDocSpec.scala#backoff
+
+The above is equivalent to this Java code:
+
+.. includecode:: ../java/code/docs/pattern/BackoffSupervisorDocTest.java#backoff-imports
+.. includecode:: ../java/code/docs/pattern/BackoffSupervisorDocTest.java#backoff
+
+Using a ``randomFactor`` to add a little bit of additional variance to the backoff intervals
+is highly recommended, in order to avoid multiple actors re-start at the exact same point in time,
+for example because they were stopped due to a shared resource such as a database going down
+and re-starting after the same configured interval. By adding additional randomness to the
+re-start intervals the actors will start in slightly different points in time, thus avoiding
+large spikes of traffic hitting the recovering shared database or other resource that they all need to contact.
+
 One-For-One Strategy vs. All-For-One Strategy
 ---------------------------------------------
 

akka-docs/rst/java/code/docs/pattern/BackoffSupervisorDocTest.java

@@ -0,0 +1,32 @@
+/**
+ * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package docs.pattern;
+
+import akka.actor.*;
+import akka.pattern.BackoffSupervisor;
+import akka.testkit.TestActors.EchoActor;
+//#backoff-imports
+import scala.concurrent.duration.Duration;
+//#backoff-imports
+
+import java.util.concurrent.TimeUnit;
+
+public class BackoffSupervisorDocTest {
+
+  void example (ActorSystem system) {
+    //#backoff
+    final Props childProps = Props.create(EchoActor.class);
+
+    final Props  supervisorProps = BackoffSupervisor.props(
+      childProps,
+      "myEcho",
+      Duration.create(3, TimeUnit.SECONDS),
+      Duration.create(30, TimeUnit.SECONDS),
+      0.2); // adds 20% "noise" to vary the intervals slightly
+
+    system.actorOf(supervisorProps, "echoSupervisor");
+    //#backoff
+  }
+
+}

akka-docs/rst/java/code/docs/persistence/LambdaPersistenceDocTest.java

@@ -10,6 +10,7 @@
 import akka.actor.Props;
 import akka.japi.Procedure;
 import akka.japi.pf.ReceiveBuilder;
+import akka.pattern.BackoffSupervisor;
 import akka.persistence.*;
 import scala.Option;
 import scala.concurrent.duration.Duration;

akka-docs/rst/java/code/docs/persistence/PersistenceDocTest.java

@@ -5,6 +5,8 @@
 package docs.persistence;
 
 import java.util.concurrent.TimeUnit;
+
+import akka.pattern.BackoffSupervisor;
 import scala.concurrent.duration.Duration;
 import akka.actor.ActorPath;
 import akka.actor.ActorRef;

akka-docs/rst/java/lambda-persistence.rst

@@ -304,7 +304,7 @@ and the actor will unconditionally be stopped.
 The reason that it cannot resume when persist fails is that it is unknown if the even was actually
 persisted or not, and therefore it is in an inconsistent state. Restarting on persistent failures 
 will most likely fail anyway, since the journal is probably unavailable. It is better to stop the 
-actor and after a back-off timeout start it again. The ``akka.persistence.BackoffSupervisor`` actor
+actor and after a back-off timeout start it again. The ``akka.pattern.BackoffSupervisor`` actor
 is provided to support such restarts.
 
 .. includecode:: code/docs/persistence/LambdaPersistenceDocTest.java#backoff

akka-docs/rst/java/persistence.rst

@@ -307,7 +307,7 @@ and the actor will unconditionally be stopped.
 The reason that it cannot resume when persist fails is that it is unknown if the even was actually
 persisted or not, and therefore it is in an inconsistent state. Restarting on persistent failures 
 will most likely fail anyway, since the journal is probably unavailable. It is better to stop the 
-actor and after a back-off timeout start it again. The ``akka.persistence.BackoffSupervisor`` actor
+actor and after a back-off timeout start it again. The ``akka.pattern.BackoffSupervisor`` actor
 is provided to support such restarts.
 
 .. includecode:: code/docs/persistence/PersistenceDocTest.java#backoff

akka-docs/rst/scala/code/docs/pattern/BackoffSupervisorDocSpec.scala

@@ -0,0 +1,31 @@
+/**
+ * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.pattern
+
+import akka.actor.{ActorSystem, Props}
+import akka.pattern.BackoffSupervisor
+import akka.testkit.TestActors.EchoActor
+
+class BackoffSupervisorDocSpec {
+
+  class BackoffSupervisorDocSpecExample {
+    val system: ActorSystem = ???
+    import scala.concurrent.duration._
+
+    //#backoff
+    val childProps = Props(classOf[EchoActor])
+
+    val supervisor = BackoffSupervisor.props(
+      childProps,
+      childName = "myEcho",
+      minBackoff = 3.seconds,
+      maxBackoff = 30.seconds,
+      randomFactor = 0.2) // adds 20% "noise" to vary the intervals slightly
+
+    system.actorOf(supervisor, name = "echoSupervisor")
+    //#backoff
+  }
+
+}

akka-docs/rst/scala/code/docs/persistence/PersistenceDocSpec.scala

@@ -5,6 +5,7 @@
 package docs.persistence
 
 import akka.actor.{ Actor, ActorRef, ActorSystem, Props }
+import akka.pattern.BackoffSupervisor
 import akka.persistence._
 
 import scala.concurrent.duration._

akka-docs/rst/scala/persistence.rst

@@ -293,7 +293,7 @@ and the actor will unconditionally be stopped.
 The reason that it cannot resume when persist fails is that it is unknown if the even was actually
 persisted or not, and therefore it is in an inconsistent state. Restarting on persistent failures 
 will most likely fail anyway, since the journal is probably unavailable. It is better to stop the 
-actor and after a back-off timeout start it again. The ``akka.persistence.BackoffSupervisor`` actor
+actor and after a back-off timeout start it again. The ``akka.pattern.BackoffSupervisor`` actor
 is provided to support such restarts.
 
 .. includecode:: code/docs/persistence/PersistenceDocSpec.scala#backoff

akka-persistence/src/test/scala/akka/persistence/BackoffSupervisorSpec.scala

@@ -4,6 +4,8 @@
 
 package akka.persistence
 
+import akka.pattern.BackoffSupervisor
+
 import scala.concurrent.duration._
 import akka.actor._
 import akka.testkit._