Update Persistence Snapshot documentation (#22419)

* Update Persistence Snapshot documentation (#22233)

* Use copy of ExampleState as it is mutable

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

@@ -159,13 +159,21 @@ object PersistenceDocSpec {
     class MyPersistentActor extends PersistentActor {
       override def persistenceId = "my-stable-persistence-id"
 
+      def updateState(event: String): Unit = {}
+
       //#save-snapshot
       var state: Any = _
 
+      val snapShotInterval = 1000
       override def receiveCommand: Receive = {
-        case "snap"                                => saveSnapshot(state)
         case SaveSnapshotSuccess(metadata)         => // ...
         case SaveSnapshotFailure(metadata, reason) => // ...
+        case cmd: String =>
+          persist(s"evt-$cmd") { e =>
+            updateState(e)
+            if (lastSequenceNr % snapShotInterval == 0 && lastSequenceNr != 0)
+              saveSnapshot(state)
+          }
       }
       //#save-snapshot
 

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

@@ -29,14 +29,15 @@ class ExamplePersistentActor extends PersistentActor {
     case SnapshotOffer(_, snapshot: ExampleState) => state = snapshot
   }
 
+  val snapShotInterval = 1000
   val receiveCommand: Receive = {
     case Cmd(data) =>
-      persist(Evt(s"${data}-${numEvents}"))(updateState)
-      persist(Evt(s"${data}-${numEvents + 1}")) { event =>
+      persist(Evt(s"${data}-${numEvents}")) { event =>
         updateState(event)
         context.system.eventStream.publish(event)
+        if (lastSequenceNr % snapShotInterval == 0 && lastSequenceNr != 0)
+          saveSnapshot(state)
       }
-    case "snap"  => saveSnapshot(state)
     case "print" => println(state)
   }
 
@@ -51,7 +52,6 @@ object PersistentActorExample extends App {
   persistentActor ! Cmd("foo")
   persistentActor ! Cmd("baz")
   persistentActor ! Cmd("bar")
-  persistentActor ! "snap"
   persistentActor ! Cmd("buzz")
   persistentActor ! "print"