!per #3631 Snapshotting

- capture and save snapshots of processor state
- start processor recovery from saved snapshots
- snapshot storage on local filesystem
- snapshot store completely isolated from journal
- LevelDB journal modularized (and completely re-rwritten)
- In-memory journal removed

akka-samples/akka-sample-persistence/src/main/java/sample/persistence/japi/SnapshotExample.java

@@ -0,0 +1,73 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package sample.persistence.japi;
+
+import java.io.Serializable;
+import java.util.ArrayList;
+
+import akka.actor.*;
+import akka.persistence.*;
+
+public class SnapshotExample {
+    public static class ExampleState implements Serializable {
+        private final ArrayList<String> received;
+
+        public ExampleState() {
+            this(new ArrayList<String>());
+        }
+
+        public ExampleState(ArrayList<String> received) {
+            this.received = received;
+        }
+
+        public ExampleState copy() {
+            return new ExampleState(new ArrayList<String>(received));
+        }
+
+        public void update(String s) {
+            received.add(s);
+        }
+
+        @Override
+        public String toString() {
+            return received.toString();
+        }
+    }
+
+    public static class ExampleProcessor extends UntypedProcessor {
+        private ExampleState state = new ExampleState();
+
+        @Override
+        public void onReceive(Object message) throws Exception {
+            if (message instanceof Persistent) {
+                Persistent persistent = (Persistent)message;
+                state.update(String.format("%s-%d", persistent.payload(), persistent.sequenceNr()));
+            } else if (message instanceof SnapshotOffer) {
+                ExampleState s = (ExampleState)((SnapshotOffer)message).snapshot();
+                System.out.println("offered state = " + s);
+                state = s;
+            } else if (message.equals("print")) {
+                System.out.println("current state = " + state);
+            } else if (message.equals("snap")) {
+                saveSnapshot(state.copy());
+            }
+        }
+    }
+
+    public static void main(String... args) throws Exception {
+        final ActorSystem system = ActorSystem.create("example");
+        final ActorRef processor = system.actorOf(Props.create(ExampleProcessor.class), "processor-3-java");
+
+        processor.tell(Persistent.create("a"), null);
+        processor.tell(Persistent.create("b"), null);
+        processor.tell("snap", null);
+        processor.tell(Persistent.create("c"), null);
+        processor.tell(Persistent.create("d"), null);
+        processor.tell("print", null);
+
+        Thread.sleep(1000);
+        system.shutdown();
+    }
+}