#Replicated Load-Balanced Services 多副本负载均衡

public class MasterSlaveRouting {
    private final DataSource master;
    private final List<DataSource> slaves;
    private final LoadBalancer loadBalancer;

    public Connection getConnection(boolean readOnly) {
        if (readOnly) {
            // 读操作：路由到从副本
            DataSource slave = loadBalancer.select(slaves);
            return slave.getConnection();
        } else {
            // 写操作：路由到主副本
            return master.getConnection();
        }
    }
}

// Spring 配置
@Bean
public DataSource routingDataSource(
        @Qualifier("masterDataSource") DataSource master,
        @Qualifier("slaveDataSource1") DataSource slave1,
        @Qualifier("slaveDataSource2") DataSource slave2) {

    Map<Object, Object> targetDataSources = new HashMap<>();
    targetDataSources.put("master", master);
    targetDataSources.put("slave1", slave1);
    targetDataSources.put("slave2", slave2);

    RoutingDataSource routingDataSource = new RoutingDataSource();
    routingDataSource.setTargetDataSources(targetDataSources);
    routingDataSource.setDefaultTargetDataSource(master);

    return routingDataSource;
}

public class ReplicationAwareRouting {
    // 检测主从延迟
    public boolean isReplicationLagging(DataSource slave) {
        try (Connection conn = slave.getConnection();
             Statement stmt = conn.createStatement()) {

            ResultSet rs = stmt.executeQuery(
                "SHOW SLAVE STATUS"
            );
            if (rs.next()) {
                long secondsBehindMaster = rs.getLong("Seconds_Behind_Master");
                return secondsBehindMaster > 5;  // 延迟超过 5 秒
            }
        }
        return false;
    }

    // 读操作：根据延迟选择合适的从副本
    public Connection getReadConnection() {
        List<DataSource> healthySlaves = slaves.stream()
            .filter(s -> !isReplicationLagging(s))
            .collect(Collectors.toList());

        // 如果所有从副本都有延迟，强制读主副本
        if (healthySlaves.isEmpty()) {
            return master.getConnection();
        }

        return loadBalancer.select(healthySlaves).getConnection();
    }
}

public class ConflictResolution {
    // Last-Writer-Wins：时间戳决定，简单的策略
    public Record resolveLastWriterWins(Record local, Record remote) {
        return local.getVersion() > remote.getVersion() ? local : remote;
    }

    // 版本向量：记录每个副本的更新历史
    public Record resolveVersionVector(Record local, Record remote) {
        Map<String, Long> localVersions = local.getVersionVector();
        Map<String, Long> remoteVersions = remote.getVersionVector();

        // 如果一个版本包含另一个版本，接受新版本
        if (containsAll(localVersions, remoteVersions)) {
            return local;
        } else if (containsAll(remoteVersions, localVersions)) {
            return remote;
        }

        // 真正的冲突：并发修改
        // 使用业务规则或随机策略解决
        return resolveBusinessRule(local, remote);
    }

    // 业务规则解决冲突
    private Record resolveBusinessRule(Record local, Record remote) {
        // 示例：取金额最大的
        if (local instanceof Payment && remote instanceof Payment) {
            Payment localPayment = (Payment) local;
            Payment remotePayment = (Payment) remote;
            return localPayment.getAmount() > remotePayment.getAmount()
                ? local : remote;
        }

        // 默认策略
        return local;
    }
}

public class QuorumReplication {
    private final List<Replica> replicas;
    private final int n;  // 总副本数
    private final int w;  // 写入确认数
    private final int r;  // 读取确认数

    public QuorumReplication(int replicas, int w, int r) {
        this.n = replicas;
        this.w = w;
        this.r = r;
    }

    public void write(String key, String value) {
        // 写入所有副本
        List<CompletableFuture<Boolean>> futures = replicas.stream()
            .map(r -> CompletableFuture.supplyAsync(() -> r.write(key, value)))
            .collect(Collectors.toList());

        // 等待 W 个副本确认
        long successCount = futures.stream()
            .mapToLong(f -> f.join() ? 1 : 0)
            .sum();

        if (successCount < w) {
            throw new WriteFailureException("写入失败：只有 " + successCount + " 个副本确认");
        }
    }

    public String read(String key) {
        // 并行读取 R 个副本
        List<CompletableFuture<ReplicaValue>> futures = replicas.stream()
            .limit(r)  // 读取 R 个副本
            .map(r -> CompletableFuture.supplyAsync(() -> r.read(key)))
            .collect(Collectors.toList());

        // 对比 R 个副本的值
        List<ReplicaValue> values = futures.stream()
            .map(CompletableFuture::join)
            .collect(Collectors.toList());

        // 返回最新值（基于版本号）
        return values.stream()
            .max(Comparator.comparing(ReplicaValue::getVersion))
            .map(ReplicaValue::getValue)
            .orElseThrow(() -> new ReadFailureException("读取失败"));
    }
}

public class ConsistencyLevelRouting {
    private final ConsistencyLevel defaultLevel = ConsistencyLevel.QUORUM;

    public ReadResult read(String key, ConsistencyLevel level) {
        switch (level) {
            case STRONG:
                // 读取所有副本，取最新
                return readAllAndMerge(key);
            case QUORUM:
                // 读取 Quorum 副本
                return readQuorum(key);
            case ONE:
                // 只读一个副本，最快但不保证最新
                return readOne(key);
            default:
                return readQuorum(key);
        }
    }
}