+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/snapshot/SnapshotStore.scala

@@ -0,0 +1,88 @@
+/**
+ * Copyright (C) 2012-2013 Eligotech BV.
+ */
+
+package akka.persistence.snapshot
+
+import scala.concurrent.Future
+import scala.util._
+
+import akka.actor._
+import akka.pattern.pipe
+import akka.persistence._
+
+/**
+ * Abstract snapshot store.
+ */
+trait SnapshotStore extends Actor {
+  import SnapshotProtocol._
+  import context.dispatcher
+
+  final def receive = {
+    case LoadSnapshot(processorId, criteria, toSequenceNr) ⇒ {
+      val p = sender
+      loadAsync(processorId, criteria.limit(toSequenceNr)) map {
+        sso ⇒ LoadSnapshotResult(sso, toSequenceNr)
+      } recover {
+        case e ⇒ LoadSnapshotResult(None, toSequenceNr)
+      } pipeTo (p)
+    }
+    case SaveSnapshot(metadata, snapshot) ⇒ {
+      val p = sender
+      val md = metadata.copy(timestamp = System.currentTimeMillis)
+      saveAsync(md, snapshot) map {
+        _ ⇒ SaveSnapshotSuccess(md)
+      } recover {
+        case e ⇒ SaveSnapshotFailure(metadata, e)
+      } to (self, p)
+    }
+    case evt @ SaveSnapshotSuccess(metadata) ⇒ {
+      saved(metadata)
+      sender ! evt // sender is processor
+    }
+    case evt @ SaveSnapshotFailure(metadata, _) ⇒ {
+      delete(metadata)
+      sender ! evt // sender is processor
+    }
+  }
+
+  //#snapshot-store-plugin-api
+  /**
+   * Plugin API.
+   *
+   * Asynchronously loads a snapshot.
+   *
+   * @param processorId processor id.
+   * @param criteria selection criteria for loading.
+   */
+  def loadAsync(processorId: String, criteria: SnapshotSelectionCriteria): Future[Option[SelectedSnapshot]]
+
+  /**
+   * Plugin API.
+   *
+   * Asynchronously saves a snapshot.
+   *
+   * @param metadata snapshot metadata.
+   * @param snapshot snapshot.
+   */
+  def saveAsync(metadata: SnapshotMetadata, snapshot: Any): Future[Unit]
+
+  /**
+   * Plugin API.
+   *
+   * Called after successful saving of a snapshot.
+   *
+   * @param metadata snapshot metadata.
+   */
+  def saved(metadata: SnapshotMetadata)
+
+  /**
+   * Plugin API.
+   *
+   * Deletes the snapshot identified by `metadata`.
+   *
+   * @param metadata snapshot metadata.
+   */
+  def delete(metadata: SnapshotMetadata)
+  //#snapshot-store-plugin-api
+}