+per #3641 Storage plugin API

- Journal plugin API for storage backends with asynchronous client API (default impl: in-memory journal)
- Journal plugin API for storage backends with synchronous client API (default impl: LevelDB journal)
- Snapshot store plugin API (default impl: local filesystem snapshot store)

akka-persistence/src/main/scala/akka/persistence/JournalProtocol.scala

@@ -0,0 +1,91 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.persistence
+
+import akka.actor._
+
+/**
+ * Defines messages exchanged between processors, channels and a journal.
+ */
+private[persistence] object JournalProtocol {
+  /**
+   * Instructs a journal to mark the `persistent` message as deleted.
+   * A persistent message marked as deleted is not replayed during recovery.
+   *
+   * @param persistent persistent message.
+   */
+  case class Delete(persistent: Persistent)
+
+  /**
+   * Instructs a journal to persist a message.
+   *
+   * @param persistent message to be persisted.
+   * @param processor requesting processor.
+   */
+  case class Write(persistent: PersistentImpl, processor: ActorRef)
+
+  /**
+   * Reply message to a processor that `persistent` message has been successfully journaled.
+   *
+   * @param persistent persistent message.
+   */
+  case class WriteSuccess(persistent: PersistentImpl)
+
+  /**
+   * Reply message to a processor that `persistent` message could not be journaled.
+   *
+   * @param persistent persistent message.
+   * @param cause failure cause.
+   */
+  case class WriteFailure(persistent: PersistentImpl, cause: Throwable)
+
+  /**
+   * Instructs a journal to loop a `message` back to `processor`, without persisting the
+   * message. Looping of messages through a journal is required to preserve message order
+   * with persistent messages.
+   *
+   * @param message message to be looped through the journal.
+   * @param processor requesting processor.
+   */
+  case class Loop(message: Any, processor: ActorRef)
+
+  /**
+   * Reply message to a processor that a `message` has been looped through the journal.
+   *
+   * @param message looped message.
+   */
+  case class LoopSuccess(message: Any)
+
+  /**
+   * Instructs a journal to replay messages to `processor`.
+   *
+   * @param fromSequenceNr sequence number where replay should start.
+   * @param toSequenceNr sequence number where replay should end (inclusive).
+   * @param processorId requesting processor id.
+   * @param processor requesting processor.
+   */
+  case class Replay(fromSequenceNr: Long, toSequenceNr: Long, processorId: String, processor: ActorRef)
+
+  /**
+   * Reply message to a processor that `persistent` message has been replayed.
+   *
+   * @param persistent persistent message.
+   */
+  case class Replayed(persistent: PersistentImpl)
+
+  /**
+   * Reply message to a processor that all `persistent` messages have been replayed.
+   *
+   * @param maxSequenceNr the highest stored sequence number (for a processor).
+   */
+  case class ReplaySuccess(maxSequenceNr: Long)
+
+  /**
+   * Reply message to a processor that not all `persistent` messages could have been
+   * replayed.
+   */
+  case class ReplayFailure(cause: Throwable)
+}
+