Leader Election Pattern (#923)

* Fix issue #761: ThreadSafeDoubleCheckLocking.java: Instantiating by Reflection call will be successful if you do that firstly

* Create leader election module

* Create Interface of Instance and MessageManager

* Create implementations with token ring algorithm

* Change package structure.
Create basic message system.

* Implement heartbeat and heartbeat invoking message system

* Implement election message handler

* Add leader message handler

* Add main entry point

* Add comments

* Update README.md

* Fix checkstyle issue

* Add Unit Tests

* Add Unit Tests

* Add bully leader selection

* Change System.out to log print.
Add MIT license in each file.

* Add More java doc comments

* Add unit test

* Add unit tests

leader-election/src/main/java/com/iluwatar/leaderelection/bully/BullyInstance.java

@@ -0,0 +1,121 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ * <p>
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * <p>
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ * <p>
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderelection.bully;
+
+import com.iluwatar.leaderelection.AbstractInstance;
+import com.iluwatar.leaderelection.Message;
+import com.iluwatar.leaderelection.MessageManager;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Impelemetation with bully algorithm. Each instance should have a sequential id and is able to
+ * communicate with other instances in the system. Initially the instance with smallest (or largest)
+ * ID is selected to be the leader. All the other instances send heartbeat message to leader periodically
+ * to check its health. If one certain instance finds the server done, it will send an election message
+ * to all the instances of which the ID is larger. If the target instance is alive, it will return an
+ * alive message (in this sample return true) and then send election message with its ID. If not,
+ * the original instance will send leader message to all the other instances.
+ */
+public class BullyInstance extends AbstractInstance {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(BullyInstance.class);
+
+  /**
+   * Constructor of BullyInstance.
+   */
+  public BullyInstance(MessageManager messageManager, int localId, int leaderId) {
+    super(messageManager, localId, leaderId);
+  }
+
+  /**
+   * Process the heartbeat invoke message. After receiving the message, the instance will send a heartbeat
+   * to leader to check its health. If alive, it will inform the next instance to do the heartbeat. If not,
+   * it will start the election process.
+   */
+  @Override
+  protected void handleHeartbeatInvokeMessage() {
+    try {
+      boolean isLeaderAlive = messageManager.sendHeartbeatMessage(leaderId);
+      if (isLeaderAlive) {
+        LOGGER.info("Instance " + localId + "- Leader is alive.");
+        Thread.sleep(HEARTBEAT_INTERVAL);
+        messageManager.sendHeartbeatInvokeMessage(localId);
+      } else {
+        LOGGER.info("Instance " + localId + "- Leader is not alive. Start election.");
+        boolean electionResult = messageManager.sendElectionMessage(localId, String.valueOf(localId));
+        if (electionResult) {
+          LOGGER.info("Instance " + localId + "- Succeed in election. Start leader notification.");
+          messageManager.sendLeaderMessage(localId, localId);
+        }
+      }
+    } catch (InterruptedException e) {
+      LOGGER.info("Instance " + localId + "- Interrupted.");
+    }
+  }
+
+  /**
+   * Process election invoke message. Send election message to all the instances with smaller ID. If any
+   * one of them is alive, do nothing. If no instance alive, send leader message to all the alive instance
+   * and restart heartbeat.
+   */
+  @Override
+  protected void handleElectionInvokeMessage() {
+    if (!isLeader()) {
+      LOGGER.info("Instance " + localId + "- Start election.");
+      boolean electionResult = messageManager.sendElectionMessage(localId, String.valueOf(localId));
+      if (electionResult) {
+        LOGGER.info("Instance " + localId + "- Succeed in election. Start leader notification.");
+        leaderId = localId;
+        messageManager.sendLeaderMessage(localId, localId);
+        messageManager.sendHeartbeatInvokeMessage(localId);
+      }
+    }
+  }
+
+  /**
+   * Process leader message. Update local leader information.
+   */
+  @Override
+  protected void handleLeaderMessage(Message message) {
+    leaderId = Integer.valueOf(message.getContent());
+    LOGGER.info("Instance " + localId + " - Leader update done.");
+  }
+
+  private boolean isLeader() {
+    return localId == leaderId;
+  }
+
+  /**
+   * Not used in Bully instance.
+   */
+  @Override
+  protected void handleLeaderInvokeMessage() {}
+
+  @Override
+  protected void handleHeartbeatMessage(Message message) {}
+
+  @Override
+  protected void handleElectionMessage(Message message) {}
+}