【Zookeeper】源码分析之Leader选举（二）之FastLeaderElection

一、前言

　　前面学习了Leader选举的总体框架，接着来学习Zookeeper中默认的选举策略，FastLeaderElection。

二、FastLeaderElection源码分析

　　2.1 类的继承关系　

public class FastLeaderElection implements Election {}

　　说明：FastLeaderElection实现了Election接口，其需要实现接口中定义的lookForLeader方法和shutdown方法，其是标准的Fast Paxos算法的实现，各服务器之间基于TCP协议进行选举。

　　2.2 类的内部类

　　FastLeaderElection有三个较为重要的内部类，分别为Notification、ToSend、Messenger。

　　1. Notification类　

    static public class Notification {
        /*
         * Format version, introduced in 3.4.6
         */
        
        public final static int CURRENTVERSION = 0x1; 
        int version;
                
        /*
         * Proposed leader
         */
        // 被推选的leader的id
        long leader;

        /*
         * zxid of the proposed leader
         */
        // 被推选的leader的事务id
        long zxid;

        /*
         * Epoch
         */
        // 推选者的选举周期
        long electionEpoch;

        /*
         * current state of sender
         */
        // 推选者的状态
        QuorumPeer.ServerState state;

        /*
         * Address of sender
         */
        // 推选者的id
        long sid;

        /*
         * epoch of the proposed leader
         */
        // 被推选者的选举周期
        long peerEpoch;
        
        @Override
        public String toString() {
            return new String(Long.toHexString(version) + " (message format version), " 
                    + leader + " (n.leader), 0x"
                    + Long.toHexString(zxid) + " (n.zxid), 0x"
                    + Long.toHexString(electionEpoch) + " (n.round), " + state
                    + " (n.state), " + sid + " (n.sid), 0x"
                    + Long.toHexString(peerEpoch) + " (n.peerEpoch) ");
        }
    }
    
    static ByteBuffer buildMsg(int state,
            long leader,
            long zxid,
            long electionEpoch,
            long epoch) {
        byte requestBytes[] = new byte[40];
        ByteBuffer requestBuffer = ByteBuffer.wrap(requestBytes);

        /*
         * Building notification packet to send 
         */

        requestBuffer.clear();
        requestBuffer.putInt(state);
        requestBuffer.putLong(leader);
        requestBuffer.putLong(zxid);
        requestBuffer.putLong(electionEpoch);
        requestBuffer.putLong(epoch);
        requestBuffer.putInt(Notification.CURRENTVERSION);
        
        return requestBuffer;
    }

Notification