Added a messaging service implementation on top of IOLoop. Added ability to easily switch between netty and io loop (default is netty)

Change-Id: Id9af0756bf0a542f832f3611b486b2ac680b91e4
diff --git a/core/api/src/main/java/org/onosproject/store/cluster/messaging/ClusterMessage.java b/core/api/src/main/java/org/onosproject/store/cluster/messaging/ClusterMessage.java
index 9f4d867..46560e4 100644
--- a/core/api/src/main/java/org/onosproject/store/cluster/messaging/ClusterMessage.java
+++ b/core/api/src/main/java/org/onosproject/store/cluster/messaging/ClusterMessage.java
@@ -15,7 +15,6 @@
  */
 package org.onosproject.store.cluster.messaging;
 
-import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 import java.util.Objects;
@@ -35,6 +34,7 @@
     private final NodeId sender;
     private final MessageSubject subject;
     private final byte[] payload;
+    private transient byte[] response;
 
     /**
      * Creates a cluster message.
@@ -77,13 +77,21 @@
     }
 
     /**
-     * Sends a response to the sender.
+     * Records the response to be sent to the sender.
      *
-     * @param data payload response.
-     * @throws IOException when I/O exception of some sort has occurred
+     * @param data response payload
      */
-    public void respond(byte[] data) throws IOException {
-        throw new IllegalStateException("One can only respond to message received from others.");
+    public void respond(byte[] data) {
+        response = data;
+    }
+
+    /**
+     * Returns the response to be sent to the sender.
+     *
+     * @return response bytes
+     */
+    public byte[] response() {
+        return response;
     }
 
     @Override
diff --git a/core/api/src/main/java/org/onosproject/store/cluster/messaging/Endpoint.java b/core/api/src/main/java/org/onosproject/store/cluster/messaging/Endpoint.java
new file mode 100644
index 0000000..2ac50df
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/store/cluster/messaging/Endpoint.java
@@ -0,0 +1,75 @@
+/*
+ * Copyright 2014-2015 Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.store.cluster.messaging;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import java.util.Objects;
+
+import org.onlab.packet.IpAddress;
+
+import com.google.common.base.MoreObjects;
+
+/**
+ * Representation of a TCP/UDP communication end point.
+ */
+public final class Endpoint {
+
+    private final int port;
+    private final IpAddress ip;
+
+    public Endpoint(IpAddress host, int port) {
+        this.ip = checkNotNull(host);
+        this.port = port;
+    }
+
+    public IpAddress host() {
+        return ip;
+    }
+
+    public int port() {
+        return port;
+    }
+
+    @Override
+    public String toString() {
+        return MoreObjects.toStringHelper(getClass())
+                .add("ip", ip)
+                .add("port", port)
+                .toString();
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(ip, port);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+        if (obj == null) {
+            return false;
+        }
+        if (getClass() != obj.getClass()) {
+            return false;
+        }
+        Endpoint that = (Endpoint) obj;
+        return Objects.equals(this.port, that.port) &&
+               Objects.equals(this.ip, that.ip);
+    }
+}
diff --git a/core/api/src/main/java/org/onosproject/store/cluster/messaging/MessagingService.java b/core/api/src/main/java/org/onosproject/store/cluster/messaging/MessagingService.java
new file mode 100644
index 0000000..3fe335b
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/store/cluster/messaging/MessagingService.java
@@ -0,0 +1,69 @@
+/*
+ * Copyright 2014-2015 Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.store.cluster.messaging;
+
+import java.io.IOException;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executor;
+import java.util.function.Consumer;
+import java.util.function.Function;
+
+/**
+ * Interface for low level messaging primitives.
+ */
+public interface MessagingService {
+
+    /**
+     * Sends a message asynchronously to the specified communication end point.
+     * The message is specified using the type and payload.
+     * @param ep end point to send the message to.
+     * @param type type of message.
+     * @param payload message payload bytes.
+     * @throws IOException when I/O exception of some sort has occurred
+     */
+    void sendAsync(Endpoint ep, String type, byte[] payload) throws IOException;
+
+    /**
+     * Sends a message synchronously and waits for a response.
+     * @param ep end point to send the message to.
+     * @param type type of message.
+     * @param payload message payload.
+     * @return a response future
+     */
+    CompletableFuture<byte[]> sendAndReceive(Endpoint ep, String type, byte[] payload);
+
+    /**
+     * Registers a new message handler for message type.
+     * @param type message type.
+     * @param handler message handler
+     * @param executor executor to use for running message handler logic.
+     */
+    void registerHandler(String type, Consumer<byte[]> handler, Executor executor);
+
+    /**
+     * Registers a new message handler for message type.
+     * @param type message type.
+     * @param handler message handler
+     * @param executor executor to use for running message handler logic.
+     */
+    void registerHandler(String type, Function<byte[], byte[]> handler, Executor executor);
+
+    /**
+     * Unregister current handler, if one exists for message type.
+     * @param type message type
+     */
+    void unregisterHandler(String type);
+}