Leader 选举机制#

public class RaftConfig {
    // 心跳间隔：Leader 向 Follower 发送心跳的频率
    public static final int HEARTBEAT_INTERVAL_MS = 150;

    // 选举超时范围：Follower 等待心跳的最大时间
    // 150ms ~ 300ms 之间的随机值
    public static final int ELECTION_TIMEOUT_MIN_MS = 150;
    public static final int ELECTION_TIMEOUT_MAX_MS = 300;
}

public class RaftNode {
    /**
     * 投票规则：
     * 1. Term 必须 >= 当前 Term（否则拒绝，可能是旧 Leader）
     * 2. 当前 Term 还没投过票，或投给了同一个 Candidate
     * 3. Candidate 的日志必须比本地日志「更新」
     */
    public boolean canVoteFor(CandidateInfo candidate) {
        // 规则 1
        if (candidate.term < currentTerm) {
            return false;
        }

        // 规则 2
        if (currentTerm > votedFor && !votedFor.equals(candidate.id)) {
            return false;
        }

        // 规则 3：比较谁的日志更新
        LogEntry myLastLog = log.getLast();
        LogEntry candidateLastLog = candidate.getLastLog();

        // 「更新」的定义：
        // - 最后日志的 term 更大，或
        // - term 相同但 index 更大
        if (candidateLastLog.term < myLastLog.term) {
            return false;
        }
        if (candidateLastLog.term == myLastLog.term &&
            candidateLastLog.index < myLastLog.index) {
            return false;
        }

        return true;
    }
}

// 随机选择选举超时
public class RaftFollower implements Runnable {
    private final Random random = new Random();
    private long electionTimeout;

    public void resetElectionTimeout() {
        // 在 [150ms, 300ms] 范围内随机选择
        electionTimeout = 150 + random.nextInt(151);
        lastHeartbeat = System.currentTimeMillis();
    }

    public void run() {
        while (running) {
            long elapsed = System.currentTimeMillis() - lastHeartbeat;
            if (elapsed > electionTimeout) {
                // 超时，发起选举
                startElection();
            }
            Thread.sleep(10);
        }
    }
}

public class RaftNode {
    public void onElectionTimeout() {
        // 发送 Pre-Vote 请求
        int preVoteYes = 0;
        for (Follower f : followers) {
            boolean willVote = f.preVoteCheck();
            if (willVote) preVoteYes++;
        }

        // 只有多数派同意，才真正发起选举
        if (preVoteYes > totalNodes / 2) {
            startRealElection();
        }
    }
}

public class RaftNode {
    public void handleAppendEntries(Request request) {
        // 发现更高 Term，说明有更新的 Leader
        if (request.term > currentTerm) {
            currentTerm = request.term;
            state = NodeState.FOLLOWER;
            votedFor = null;
        }
    }
}

public class RaftLeader {
    private volatile long leaseExpiry;

    // 发送心跳时，同时带上 lease 时间
    public void sendHeartbeat() {
        AppendEntriesRequest req = new AppendEntriesRequest(
            currentTerm,
            nodeId,
            prevLogIndex,
            prevLogTerm,
            entries,
            commitIndex,
            leaseDurationMs   // Lease 时长
        );
        for (Follower f : followers) {
            sendRequest(f, req);
        }
        leaseExpiry = System.currentTimeMillis() + leaseDurationMs;
    }
}

public class RaftFollower {
    public void onHeartbeatReceived(Request request) {
        lastHeartbeat = System.currentTimeMillis();

        // 如果 lease 还没过期，不发起选举
        if (System.currentTimeMillis() < leaseExpiry) {
            electionTimeout = leaseExpiry - System.currentTimeMillis() + MARGIN;
        }
    }
}