Clarify supervision of singleton actors (#25681)

* Clarify supervision of singleton actors
* Document supervision for typed singletons

akka-cluster-typed/src/test/java/jdocs/akka/cluster/typed/SingletonCompileOnlyTest.java

@@ -4,16 +4,15 @@
 
 package jdocs.akka.cluster.typed;
 
-import akka.actor.typed.ActorRef;
-import akka.actor.typed.ActorSystem;
-import akka.actor.typed.Behavior;
-import akka.actor.typed.Props;
+import akka.actor.typed.*;
 import akka.actor.typed.javadsl.Behaviors;
 
 //#import
 import akka.cluster.typed.ClusterSingleton;
 import akka.cluster.typed.ClusterSingletonSettings;
 
+import java.time.Duration;
+
 //#import
 
 public class SingletonCompileOnlyTest {
@@ -49,21 +48,41 @@ public class SingletonCompileOnlyTest {
   public static void example() {
 
     ActorSystem system = ActorSystem.create(
-      Behaviors.empty(), "SingletonExample"
+            Behaviors.empty(), "SingletonExample"
     );
 
     //#singleton
     ClusterSingleton singleton = ClusterSingleton.get(system);
     // Start if needed and provide a proxy to a named singleton
     ActorRef<CounterCommand> proxy = singleton.spawn(
-      counter("TheCounter", 0),
+            counter("TheCounter", 0),
+            "GlobalCounter",
+            Props.empty(),
+            ClusterSingletonSettings.create(system),
+            new GoodByeCounter()
+    );
+
+    proxy.tell(new Increment());
+    //#singleton
+
+  }
+
+  public static void backoff() {
+
+    ActorSystem system = ActorSystem.create(
+      Behaviors.empty(), "SingletonExample"
+    );
+
+    //#backoff
+    ClusterSingleton singleton = ClusterSingleton.get(system);
+    ActorRef<CounterCommand> proxy = singleton.spawn(
+      Behaviors.supervise(counter("TheCounter", 0))
+              .onFailure(SupervisorStrategy.restartWithBackoff(Duration.ofSeconds(1), Duration.ofSeconds(10), 0.2)),
       "GlobalCounter",
       Props.empty(),
       ClusterSingletonSettings.create(system),
       new GoodByeCounter()
     );
-
-    proxy.tell(new Increment());
-    //#singleton
+    //#backoff
   }
 }

akka-cluster-typed/src/test/scala/docs/akka/cluster/typed/SingletonCompileOnlySpec.scala

@@ -4,11 +4,9 @@
 
 package docs.akka.cluster.typed
 
-import akka.actor.typed.ActorRef
-import akka.actor.typed.ActorSystem
-import akka.actor.typed.Behavior
-import akka.actor.typed.Props
+import akka.actor.typed.{ ActorRef, ActorSystem, Behavior, Props, SupervisorStrategy }
 import akka.actor.typed.scaladsl.Behaviors
+import scala.concurrent.duration._
 
 object SingletonCompileOnlySpec {
 
@@ -39,7 +37,8 @@ object SingletonCompileOnlySpec {
   val singletonManager = ClusterSingleton(system)
   // Start if needed and provide a proxy to a named singleton
   val proxy: ActorRef[CounterCommand] = singletonManager.spawn(
-    behavior = counter("TheCounter", 0),
+    behavior = Behaviors.supervise(counter("TheCounter", 0))
+      .onFailure[Exception](SupervisorStrategy.restart),
     "GlobalCounter",
     Props.empty,
     ClusterSingletonSettings(system),
@@ -49,4 +48,14 @@ object SingletonCompileOnlySpec {
   proxy ! Increment
   //#singleton
 
+  //#backoff
+  val proxyBackOff: ActorRef[CounterCommand] = singletonManager.spawn(
+    behavior = Behaviors.supervise(counter("TheCounter", 0))
+      .onFailure[Exception](SupervisorStrategy.restartWithBackoff(1.second, 10.seconds, 0.2)),
+    "GlobalCounter",
+    Props.empty,
+    ClusterSingletonSettings(system),
+    terminationMessage = GoodByeCounter
+  )
+  //#backoff
 }

akka-docs/src/main/paradox/cluster-singleton.md

@@ -160,7 +160,15 @@ with different settings if needed.
 
 ## Supervision
 
-Sometimes it is useful to add supervision for the Cluster Singleton itself. To accomplish this you need to add a parent supervisor actor which will be used to create the 'real' singleton instance. Below is an example implementation (credit to [this StackOverflow answer](https://stackoverflow.com/a/36716708/779513))
+There are two actors that could potentially be supervised. For the `consumer` singleton created above these would be: 
+
+* Cluster singleton manager e.g. `/user/consumer` which runs on every node in the cluster
+* The user actor e.g. `/user/consumer/singleton` which the manager starts on the oldest node
+
+The Cluster singleton manager actor should not have its supervision strategy changed as it should always be running.
+However it is sometimes useful to add supervision for the user actor. 
+To accomplish this add a parent supervisor actor which will be used to create the 'real' singleton instance. 
+Below is an example implementation (credit to [this StackOverflow answer](https://stackoverflow.com/a/36716708/779513))
 
 Scala
 :  @@snip [ClusterSingletonSupervision.scala](/akka-docs/src/test/scala/docs/cluster/singleton/ClusterSingletonSupervision.scala) { #singleton-supervisor-actor }

akka-docs/src/main/paradox/typed/cluster-singleton.md

@@ -57,6 +57,23 @@ Scala
 Java
 :  @@snip [SingletonCompileOnlyTest.java](/akka-cluster-typed/src/test/java/jdocs/akka/cluster/typed/SingletonCompileOnlyTest.java) { #import #singleton }
 
+## Supervision
+
+The default @ref[supervision strategy](./fault-tolerance.md) when an exception is thrown is for an actor to be stopped. 
+The above example overrides this to `restart` to ensure it is always running. Another option would be to restart with 
+a backoff: 
+
+
+Scala
+:  @@snip [SingletonCompileOnlySpec.scala](/akka-cluster-typed/src/test/scala/docs/akka/cluster/typed/SingletonCompileOnlySpec.scala) { #backoff}
+
+Java
+:  @@snip [SingletonCompileOnlyTest.java](/akka-cluster-typed/src/test/java/jdocs/akka/cluster/typed/SingletonCompileOnlyTest.java) { #backoff}
+
+Be aware that this means there will be times when the singleton won't be running as restart is delayed.
+See @ref[Fault Tolerance](./fault-tolerance.md) for a full list of supervision options.
+
+
 ## Accessing singleton of another data centre
 
 TODO