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
2026-05-19 03:26:07 +00:00 · 2019-10-08 23:29:59 +08:00
parent 41b8d80479
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+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * 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:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * 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.ring;
+
+import com.iluwatar.leaderelection.AbstractInstance;
+import com.iluwatar.leaderelection.Message;
+import com.iluwatar.leaderelection.MessageManager;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * Implementation with token ring algorithm. The instances in the system are organized as a ring.
+ * Each instance should have a sequential id and the instance with smallest (or largest) id should
+ * be the initial 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 the next alive instance in the ring, which contains its own ID. Then the next instance
+ * add its ID into the message and pass it to the next. After all the alive instances' ID are add
+ * to the message, the message is send back to the first instance and it will choose the instance
+ * with smallest ID to be the new leader, and then send a leader message to other instances to
+ * inform the result.
+ */
+public class RingInstance extends AbstractInstance {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(RingInstance.class);
+
+  /**
+   * Constructor of RingInstance.
+   */
+  public RingInstance(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(this.leaderId);
+      if (isLeaderAlive) {
+        LOGGER.info("Instance " + localId + "- Leader is alive. Start next heartbeat in 5 second.");
+        Thread.sleep(HEARTBEAT_INTERVAL);
+        messageManager.sendHeartbeatInvokeMessage(this.localId);
+      } else {
+        LOGGER.info("Instance " + localId + "- Leader is not alive. Start election.");
+        messageManager.sendElectionMessage(this.localId, String.valueOf(this.localId));
+      }
+    } catch (InterruptedException e) {
+      LOGGER.info("Instance " + localId + "- Interrupted.");
+    }
+  }
+
+  /**
+   * Process election message. If the local ID is contained in the ID list, the instance will select the
+   * alive instance with smallest ID to be the new leader, and send the leader inform message. If not,
+   * it will add its local ID to the list and send the message to the next instance in the ring.
+   */
+  @Override
+  protected void handleElectionMessage(Message message) {
+    String content = message.getContent();
+    LOGGER.info("Instance " + localId + " - Election Message: " + content);
+    List<Integer> candidateList =
+        Arrays.stream(content.trim().split(","))
+                 .map(Integer::valueOf)
+                 .sorted()
+                 .collect(Collectors.toList());
+    if (candidateList.contains(localId)) {
+      int newLeaderId = candidateList.get(0);
+      LOGGER.info("Instance " + localId + " - New leader should be " + newLeaderId + ".");
+      messageManager.sendLeaderMessage(localId, newLeaderId);
+    } else {
+      content += "," + localId;
+      messageManager.sendElectionMessage(localId, content);
+    }
+  }
+
+  /**
+   * Process leader Message. The instance will set the leader ID to be the new one and send the message to
+   * the next instance until all the alive instance in the ring is informed.
+   */
+  @Override
+  protected void handleLeaderMessage(Message message) {
+    int newLeaderId = Integer.valueOf(message.getContent());
+    if (this.leaderId != newLeaderId) {
+      LOGGER.info("Instance " + localId + " - Update leaderID");
+      this.leaderId = newLeaderId;
+      messageManager.sendLeaderMessage(localId, newLeaderId);
+    } else {
+      LOGGER.info("Instance " + localId + " - Leader update done. Start heartbeat.");
+      messageManager.sendHeartbeatInvokeMessage(localId);
+    }
+  }
+
+  /**
+   * Not used in Ring instance.
+   */
+  @Override
+  protected void handleLeaderInvokeMessage() {}
+
+  @Override
+  protected void handleHeartbeatMessage(Message message) {}
+
+  @Override
+  protected void handleElectionInvokeMessage() {}
+
+}