第210集数据库快照读与当前读架构实战：MVCC机制、事务隔离、一致性读的企业级解决方案

/**
 * MVCC版本管理器
 * 管理数据版本链、ReadView、Undo日志
 */
public class MVCCVersionManager {
    private final VersionChainManager versionChainManager;
    private final ReadViewManager readViewManager;
    private final UndoLogManager undoLogManager;
    private final TransactionManager transactionManager;
    
    public MVCCVersionManager() {
        this.versionChainManager = new VersionChainManager();
        this.readViewManager = new ReadViewManager();
        this.undoLogManager = new UndoLogManager();
        this.transactionManager = new TransactionManager();
    }
    
    /**
     * 创建ReadView
     */
    public ReadView createReadView(long transactionId) {
        long maxTrxId = transactionManager.getMaxTransactionId();
        long minTrxId = transactionManager.getMinActiveTransactionId();
        Set<Long> activeTrxIds = transactionManager.getActiveTransactionIds();
        
        return new ReadView(transactionId, maxTrxId, minTrxId, activeTrxIds);
    }
    
    /**
     * 快照读操作
     */
    public Object snapshotRead(String tableName, String key, ReadView readView) {
        // 获取版本链
        VersionChain versionChain = versionChainManager.getVersionChain(tableName, key);
        
        // 根据ReadView选择可见版本
        Version visibleVersion = selectVisibleVersion(versionChain, readView);
        
        if (visibleVersion != null) {
            return visibleVersion.getData();
        }
        
        return null;
    }
    
    /**
     * 当前读操作
     */
    public Object currentRead(String tableName, String key, LockType lockType) {
        // 加锁
        Lock lock = acquireLock(tableName, key, lockType);
        
        try {
            // 获取最新版本
            VersionChain versionChain = versionChainManager.getVersionChain(tableName, key);
            Version latestVersion = versionChain.getLatestVersion();
            
            if (latestVersion != null) {
                return latestVersion.getData();
            }
            
            return null;
        } finally {
            // 根据锁类型决定是否释放锁
            if (lockType == LockType.SHARED) {
                releaseLock(lock);
            }
        }
    }
    
    /**
     * 选择可见版本
     */
    private Version selectVisibleVersion(VersionChain versionChain, ReadView readView) {
        Version current = versionChain.getLatestVersion();
        
        while (current != null) {
            if (isVersionVisible(current, readView)) {
                return current;
            }
            current = current.getPreviousVersion();
        }
        
        return null;
    }
    
    /**
     * 判断版本是否可见
     */
    private boolean isVersionVisible(Version version, ReadView readView) {
        long versionTrxId = version.getTransactionId();
        
        // 1. 如果版本的事务ID小于最小活跃事务ID，版本可见
        if (versionTrxId < readView.getMinTrxId()) {
            return true;
        }
        
        // 2. 如果版本的事务ID等于当前事务ID，版本可见
        if (versionTrxId == readView.getTransactionId()) {
            return true;
        }
        
        // 3. 如果版本的事务ID大于最大事务ID，版本不可见
        if (versionTrxId > readView.getMaxTrxId()) {
            return false;
        }
        
        // 4. 如果版本的事务ID在活跃事务列表中，版本不可见
        if (readView.getActiveTrxIds().contains(versionTrxId)) {
            return false;
        }
        
        // 5. 其他情况版本可见
        return true;
    }
    
    /**
     * 获取锁
     */
    private Lock acquireLock(String tableName, String key, LockType lockType) {
        // 实现锁获取逻辑
        return new Lock(tableName, key, lockType);
    }
    
    /**
     * 释放锁
     */
    private void releaseLock(Lock lock) {
        // 实现锁释放逻辑
    }
}

/**
 * 版本链管理器
 */
class VersionChainManager {
    private final Map<String, VersionChain> versionChains;
    
    public VersionChainManager() {
        this.versionChains = new ConcurrentHashMap<>();
    }
    
    /**
     * 获取版本链
     */
    public VersionChain getVersionChain(String tableName, String key) {
        String chainKey = tableName + ":" + key;
        return versionChains.computeIfAbsent(chainKey, k -> new VersionChain());
    }
    
    /**
     * 添加新版本
     */
    public void addVersion(String tableName, String key, Version version) {
        VersionChain chain = getVersionChain(tableName, key);
        chain.addVersion(version);
    }
    
    /**
     * 删除版本
     */
    public void removeVersion(String tableName, String key, Version version) {
        VersionChain chain = getVersionChain(tableName, key);
        chain.removeVersion(version);
    }
}

/**
 * 版本链
 */
class VersionChain {
    private Version latestVersion;
    private final List<Version> versions;
    
    public VersionChain() {
        this.versions = new CopyOnWriteArrayList<>();
    }
    
    /**
     * 添加版本
     */
    public void addVersion(Version version) {
        version.setPreviousVersion(latestVersion);
        latestVersion = version;
        versions.add(version);
    }
    
    /**
     * 移除版本
     */
    public void removeVersion(Version version) {
        versions.remove(version);
        if (latestVersion == version) {
            latestVersion = version.getPreviousVersion();
        }
    }
    
    /**
     * 获取最新版本
     */
    public Version getLatestVersion() {
        return latestVersion;
    }
    
    /**
     * 获取所有版本
     */
    public List<Version> getAllVersions() {
        return new ArrayList<>(versions);
    }
}

/**
 * 数据版本
 */
class Version {
    private final long transactionId;
    private final long timestamp;
    private final Object data;
    private final String undoLogPointer;
    private Version previousVersion;
    
    public Version(long transactionId, long timestamp, Object data, String undoLogPointer) {
        this.transactionId = transactionId;
        this.timestamp = timestamp;
        this.data = data;
        this.undoLogPointer = undoLogPointer;
    }
    
    public long getTransactionId() { return transactionId; }
    public long getTimestamp() { return timestamp; }
    public Object getData() { return data; }
    public String getUndoLogPointer() { return undoLogPointer; }
    public Version getPreviousVersion() { return previousVersion; }
    public void setPreviousVersion(Version previousVersion) { this.previousVersion = previousVersion; }
}

/**
 * ReadView管理器
 */
class ReadViewManager {
    private final Map<Long, ReadView> readViews;
    
    public ReadViewManager() {
        this.readViews = new ConcurrentHashMap<>();
    }
    
    /**
     * 创建ReadView
     */
    public ReadView createReadView(long transactionId, long maxTrxId, 
                                 long minTrxId, Set<Long> activeTrxIds) {
        ReadView readView = new ReadView(transactionId, maxTrxId, minTrxId, activeTrxIds);
        readViews.put(transactionId, readView);
        return readView;
    }
    
    /**
     * 获取ReadView
     */
    public ReadView getReadView(long transactionId) {
        return readViews.get(transactionId);
    }
    
    /**
     * 删除ReadView
     */
    public void removeReadView(long transactionId) {
        readViews.remove(transactionId);
    }
}

/**
 * ReadView
 */
class ReadView {
    private final long transactionId;
    private final long maxTrxId;
    private final long minTrxId;
    private final Set<Long> activeTrxIds;
    private final long createTime;
    
    public ReadView(long transactionId, long maxTrxId, long minTrxId, Set<Long> activeTrxIds) {
        this.transactionId = transactionId;
        this.maxTrxId = maxTrxId;
        this.minTrxId = minTrxId;
        this.activeTrxIds = new HashSet<>(activeTrxIds);
        this.createTime = System.currentTimeMillis();
    }
    
    public long getTransactionId() { return transactionId; }
    public long getMaxTrxId() { return maxTrxId; }
    public long getMinTrxId() { return minTrxId; }
    public Set<Long> getActiveTrxIds() { return activeTrxIds; }
    public long getCreateTime() { return createTime; }
}

/**
 * Undo日志管理器
 */
class UndoLogManager {
    private final Map<String, UndoLog> undoLogs;
    
    public UndoLogManager() {
        this.undoLogs = new ConcurrentHashMap<>();
    }
    
    /**
     * 创建Undo日志
     */
    public UndoLog createUndoLog(String tableName, String key, Object oldData, Object newData) {
        String logId = generateLogId();
        UndoLog undoLog = new UndoLog(logId, tableName, key, oldData, newData);
        undoLogs.put(logId, undoLog);
        return undoLog;
    }
    
    /**
     * 获取Undo日志
     */
    public UndoLog getUndoLog(String logId) {
        return undoLogs.get(logId);
    }
    
    /**
     * 删除Undo日志
     */
    public void removeUndoLog(String logId) {
        undoLogs.remove(logId);
    }
    
    private String generateLogId() {
        return "undo_" + System.currentTimeMillis() + "_" + Thread.currentThread().getId();
    }
}

/**
 * Undo日志
 */
class UndoLog {
    private final String logId;
    private final String tableName;
    private final String key;
    private final Object oldData;
    private final Object newData;
    private final long timestamp;
    
    public UndoLog(String logId, String tableName, String key, Object oldData, Object newData) {
        this.logId = logId;
        this.tableName = tableName;
        this.key = key;
        this.oldData = oldData;
        this.newData = newData;
        this.timestamp = System.currentTimeMillis();
    }
    
    public String getLogId() { return logId; }
    public String getTableName() { return tableName; }
    public String getKey() { return key; }
    public Object getOldData() { return oldData; }
    public Object getNewData() { return newData; }
    public long getTimestamp() { return timestamp; }
}

/**
 * 事务管理器
 */
class TransactionManager {
    private final AtomicLong nextTransactionId;
    private final Set<Long> activeTransactions;
    private final Map<Long, Transaction> transactions;
    
    public TransactionManager() {
        this.nextTransactionId = new AtomicLong(1);
        this.activeTransactions = ConcurrentHashMap.newKeySet();
        this.transactions = new ConcurrentHashMap<>();
    }
    
    /**
     * 开始事务
     */
    public long beginTransaction() {
        long transactionId = nextTransactionId.getAndIncrement();
        Transaction transaction = new Transaction(transactionId);
        transactions.put(transactionId, transaction);
        activeTransactions.add(transactionId);
        return transactionId;
    }
    
    /**
     * 提交事务
     */
    public void commitTransaction(long transactionId) {
        Transaction transaction = transactions.get(transactionId);
        if (transaction != null) {
            transaction.commit();
            activeTransactions.remove(transactionId);
            transactions.remove(transactionId);
        }
    }
    
    /**
     * 回滚事务
     */
    public void rollbackTransaction(long transactionId) {
        Transaction transaction = transactions.get(transactionId);
        if (transaction != null) {
            transaction.rollback();
            activeTransactions.remove(transactionId);
            transactions.remove(transactionId);
        }
    }
    
    /**
     * 获取最大事务ID
     */
    public long getMaxTransactionId() {
        return nextTransactionId.get() - 1;
    }
    
    /**
     * 获取最小活跃事务ID
     */
    public long getMinActiveTransactionId() {
        return activeTransactions.stream().mapToLong(Long::longValue).min().orElse(0);
    }
    
    /**
     * 获取活跃事务ID列表
     */
    public Set<Long> getActiveTransactionIds() {
        return new HashSet<>(activeTransactions);
    }
}

/**
 * 事务
 */
class Transaction {
    private final long transactionId;
    private final long startTime;
    private TransactionStatus status;
    private final List<Operation> operations;
    
    public Transaction(long transactionId) {
        this.transactionId = transactionId;
        this.startTime = System.currentTimeMillis();
        this.status = TransactionStatus.ACTIVE;
        this.operations = new CopyOnWriteArrayList<>();
    }
    
    public void commit() {
        this.status = TransactionStatus.COMMITTED;
    }
    
    public void rollback() {
        this.status = TransactionStatus.ROLLED_BACK;
    }
    
    public long getTransactionId() { return transactionId; }
    public long getStartTime() { return startTime; }
    public TransactionStatus getStatus() { return status; }
    public List<Operation> getOperations() { return operations; }
}

enum TransactionStatus {
    ACTIVE, COMMITTED, ROLLED_BACK
}

/**
 * 操作
 */
class Operation {
    private final OperationType type;
    private final String tableName;
    private final String key;
    private final Object data;
    
    public Operation(OperationType type, String tableName, String key, Object data) {
        this.type = type;
        this.tableName = tableName;
        this.key = key;
        this.data = data;
    }
    
    public OperationType getType() { return type; }
    public String getTableName() { return tableName; }
    public String getKey() { return key; }
    public Object getData() { return data; }
}

enum OperationType {
    INSERT, UPDATE, DELETE, SELECT
}

/**
 * 锁
 */
class Lock {
    private final String tableName;
    private final String key;
    private final LockType lockType;
    private final long transactionId;
    private final long acquireTime;
    
    public Lock(String tableName, String key, LockType lockType) {
        this.tableName = tableName;
        this.key = key;
        this.lockType = lockType;
        this.transactionId = Thread.currentThread().getId(); // 简化实现
        this.acquireTime = System.currentTimeMillis();
    }
    
    public String getTableName() { return tableName; }
    public String getKey() { return key; }
    public LockType getLockType() { return lockType; }
    public long getTransactionId() { return transactionId; }
    public long getAcquireTime() { return acquireTime; }
}

enum LockType {
    SHARED, EXCLUSIVE
}

/**
 * 事务隔离级别管理器
 * 实现不同隔离级别下的快照读和当前读
 */
public class TransactionIsolationManager {
    private final MVCCVersionManager mvccManager;
    private final LockManager lockManager;
    private final TransactionManager transactionManager;
    
    public TransactionIsolationManager() {
        this.mvccManager = new MVCCVersionManager();
        this.lockManager = new LockManager();
        this.transactionManager = new TransactionManager();
    }
    
    /**
     * 执行查询操作（快照读）
     */
    public Object executeQuery(long transactionId, IsolationLevel isolationLevel, 
                             String tableName, String key) {
        switch (isolationLevel) {
            case READ_UNCOMMITTED:
                return executeReadUncommitted(transactionId, tableName, key);
            case READ_COMMITTED:
                return executeReadCommitted(transactionId, tableName, key);
            case REPEATABLE_READ:
                return executeRepeatableRead(transactionId, tableName, key);
            case SERIALIZABLE:
                return executeSerializable(transactionId, tableName, key);
            default:
                throw new IllegalArgumentException("不支持的隔离级别: " + isolationLevel);
        }
    }
    
    /**
     * 执行更新操作（当前读）
     */
    public Object executeUpdate(long transactionId, IsolationLevel isolationLevel, 
                               String tableName, String key, Object newData) {
        switch (isolationLevel) {
            case READ_UNCOMMITTED:
                return executeUpdateReadUncommitted(transactionId, tableName, key, newData);
            case READ_COMMITTED:
                return executeUpdateReadCommitted(transactionId, tableName, key, newData);
            case REPEATABLE_READ:
                return executeUpdateRepeatableRead(transactionId, tableName, key, newData);
            case SERIALIZABLE:
                return executeUpdateSerializable(transactionId, tableName, key, newData);
            default:
                throw new IllegalArgumentException("不支持的隔离级别: " + isolationLevel);
        }
    }
    
    /**
     * READ UNCOMMITTED - 读未提交
     */
    private Object executeReadUncommitted(long transactionId, String tableName, String key) {
        // 读未提交：读取最新数据，不加锁
        return mvccManager.currentRead(tableName, key, LockType.SHARED);
    }
    
    /**
     * READ COMMITTED - 读已提交
     */
    private Object executeReadCommitted(long transactionId, String tableName, String key) {
        // 读已提交：每次读取都创建新的ReadView
        ReadView readView = mvccManager.createReadView(transactionId);
        return mvccManager.snapshotRead(tableName, key, readView);
    }
    
    /**
     * REPEATABLE READ - 可重复读
     */
    private Object executeRepeatableRead(long transactionId, String tableName, String key) {
        // 可重复读：事务开始时创建ReadView，整个事务期间使用同一个ReadView
        ReadView readView = mvccManager.getReadView(transactionId);
        if (readView == null) {
            readView = mvccManager.createReadView(transactionId);
        }
        return mvccManager.snapshotRead(tableName, key, readView);
    }
    
    /**
     * SERIALIZABLE - 串行化
     */
    private Object executeSerializable(long transactionId, String tableName, String key) {
        // 串行化：使用当前读，加排他锁
        return mvccManager.currentRead(tableName, key, LockType.EXCLUSIVE);
    }
    
    /**
     * READ UNCOMMITTED - 更新操作
     */
    private Object executeUpdateReadUncommitted(long transactionId, String tableName, 
                                               String key, Object newData) {
        // 读未提交：直接更新，不加锁
        return updateData(transactionId, tableName, key, newData);
    }
    
    /**
     * READ COMMITTED - 更新操作
     */
    private Object executeUpdateReadCommitted(long transactionId, String tableName, 
                                            String key, Object newData) {
        // 读已提交：加排他锁后更新
        lockManager.acquireLock(tableName, key, LockType.EXCLUSIVE, transactionId);
        try {
            return updateData(transactionId, tableName, key, newData);
        } finally {
            lockManager.releaseLock(tableName, key, transactionId);
        }
    }
    
    /**
     * REPEATABLE READ - 更新操作
     */
    private Object executeUpdateRepeatableRead(long transactionId, String tableName, 
                                              String key, Object newData) {
        // 可重复读：加排他锁后更新
        lockManager.acquireLock(tableName, key, LockType.EXCLUSIVE, transactionId);
        try {
            return updateData(transactionId, tableName, key, newData);
        } finally {
            lockManager.releaseLock(tableName, key, transactionId);
        }
    }
    
    /**
     * SERIALIZABLE - 更新操作
     */
    private Object executeUpdateSerializable(long transactionId, String tableName, 
                                            String key, Object newData) {
        // 串行化：加排他锁后更新
        lockManager.acquireLock(tableName, key, LockType.EXCLUSIVE, transactionId);
        try {
            return updateData(transactionId, tableName, key, newData);
        } finally {
            lockManager.releaseLock(tableName, key, transactionId);
        }
    }
    
    /**
     * 更新数据
     */
    private Object updateData(long transactionId, String tableName, String key, Object newData) {
        // 获取旧数据
        Object oldData = mvccManager.currentRead(tableName, key, LockType.EXCLUSIVE);
        
        // 创建Undo日志
        UndoLog undoLog = mvccManager.getUndoLogManager().createUndoLog(tableName, key, oldData, newData);
        
        // 创建新版本
        Version newVersion = new Version(transactionId, System.currentTimeMillis(), newData, undoLog.getLogId());
        
        // 添加到版本链
        mvccManager.getVersionChainManager().addVersion(tableName, key, newVersion);
        
        return newData;
    }
}

/**
 * 锁管理器
 */
class LockManager {
    private final Map<String, Lock> locks;
    private final Map<Long, Set<String>> transactionLocks;
    
    public LockManager() {
        this.locks = new ConcurrentHashMap<>();
        this.transactionLocks = new ConcurrentHashMap<>();
    }
    
    /**
     * 获取锁
     */
    public void acquireLock(String tableName, String key, LockType lockType, long transactionId) {
        String lockKey = tableName + ":" + key;
        
        Lock existingLock = locks.get(lockKey);
        if (existingLock != null) {
            // 检查锁兼容性
            if (!isLockCompatible(existingLock.getLockType(), lockType)) {
                // 等待锁释放
                waitForLock(lockKey);
            }
        }
        
        // 创建新锁
        Lock newLock = new Lock(tableName, key, lockType);
        locks.put(lockKey, newLock);
        
        // 记录事务锁
        transactionLocks.computeIfAbsent(transactionId, k -> ConcurrentHashMap.newKeySet())
                .add(lockKey);
    }
    
    /**
     * 释放锁
     */
    public void releaseLock(String tableName, String key, long transactionId) {
        String lockKey = tableName + ":" + key;
        
        Lock lock = locks.get(lockKey);
        if (lock != null && lock.getTransactionId() == transactionId) {
            locks.remove(lockKey);
            
            // 从事务锁记录中移除
            Set<String> transactionLockSet = transactionLocks.get(transactionId);
            if (transactionLockSet != null) {
                transactionLockSet.remove(lockKey);
            }
        }
    }
    
    /**
     * 检查锁兼容性
     */
    private boolean isLockCompatible(LockType existingLockType, LockType requestedLockType) {
        if (existingLockType == LockType.SHARED && requestedLockType == LockType.SHARED) {
            return true; // 共享锁与共享锁兼容
        }
        return false; // 其他情况不兼容
    }
    
    /**
     * 等待锁释放
     */
    private void waitForLock(String lockKey) {
        // 实现锁等待逻辑
        try {
            Thread.sleep(10); // 简化的等待实现
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}

enum IsolationLevel {
    READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE
}

/**
 * 一致性读管理器
 * 实现不同场景下的一致性读策略
 */
public class ConsistencyReadManager {
    private final MVCCVersionManager mvccManager;
    private final TransactionIsolationManager isolationManager;
    private final ConsistencyStrategy strategy;
    
    public ConsistencyReadManager() {
        this.mvccManager = new MVCCVersionManager();
        this.isolationManager = new TransactionIsolationManager();
        this.strategy = new ConsistencyStrategy();
    }
    
    /**
     * 执行一致性读
     */
    public Object executeConsistencyRead(ConsistencyReadRequest request) {
        switch (request.getConsistencyLevel()) {
            case STRONG_CONSISTENCY:
                return executeStrongConsistencyRead(request);
            case EVENTUAL_CONSISTENCY:
                return executeEventualConsistencyRead(request);
            case SESSION_CONSISTENCY:
                return executeSessionConsistencyRead(request);
            case CAUSAL_CONSISTENCY:
                return executeCausalConsistencyRead(request);
            default:
                throw new IllegalArgumentException("不支持的一致性级别: " + request.getConsistencyLevel());
        }
    }
    
    /**
     * 强一致性读
     */
    private Object executeStrongConsistencyRead(ConsistencyReadRequest request) {
        // 强一致性：读取最新数据，加锁
        return mvccManager.currentRead(request.getTableName(), request.getKey(), LockType.SHARED);
    }
    
    /**
     * 最终一致性读
     */
    private Object executeEventualConsistencyRead(ConsistencyReadRequest request) {
        // 最终一致性：读取快照数据，不加锁
        ReadView readView = mvccManager.createReadView(request.getTransactionId());
        return mvccManager.snapshotRead(request.getTableName(), request.getKey(), readView);
    }
    
    /**
     * 会话一致性读
     */
    private Object executeSessionConsistencyRead(ConsistencyReadRequest request) {
        // 会话一致性：同一会话内保持一致性
        String sessionId = request.getSessionId();
        ReadView sessionReadView = getSessionReadView(sessionId);
        
        if (sessionReadView == null) {
            sessionReadView = mvccManager.createReadView(request.getTransactionId());
            setSessionReadView(sessionId, sessionReadView);
        }
        
        return mvccManager.snapshotRead(request.getTableName(), request.getKey(), sessionReadView);
    }
    
    /**
     * 因果一致性读
     */
    private Object executeCausalConsistencyRead(ConsistencyReadRequest request) {
        // 因果一致性：考虑操作之间的因果关系
        CausalContext causalContext = request.getCausalContext();
        ReadView causalReadView = createCausalReadView(causalContext);
        
        return mvccManager.snapshotRead(request.getTableName(), request.getKey(), causalReadView);
    }
    
    /**
     * 获取会话ReadView
     */
    private ReadView getSessionReadView(String sessionId) {
        // 实现会话ReadView获取
        return null;
    }
    
    /**
     * 设置会话ReadView
     */
    private void setSessionReadView(String sessionId, ReadView readView) {
        // 实现会话ReadView设置
    }
    
    /**
     * 创建因果ReadView
     */
    private ReadView createCausalReadView(CausalContext causalContext) {
        // 实现因果ReadView创建
        return null;
    }
}

/**
 * 一致性读请求
 */
class ConsistencyReadRequest {
    private final long transactionId;
    private final String tableName;
    private final String key;
    private final ConsistencyLevel consistencyLevel;
    private final String sessionId;
    private final CausalContext causalContext;
    
    public ConsistencyReadRequest(long transactionId, String tableName, String key, 
                                ConsistencyLevel consistencyLevel) {
        this.transactionId = transactionId;
        this.tableName = tableName;
        this.key = key;
        this.consistencyLevel = consistencyLevel;
        this.sessionId = null;
        this.causalContext = null;
    }
    
    public ConsistencyReadRequest(long transactionId, String tableName, String key, 
                                ConsistencyLevel consistencyLevel, String sessionId) {
        this.transactionId = transactionId;
        this.tableName = tableName;
        this.key = key;
        this.consistencyLevel = consistencyLevel;
        this.sessionId = sessionId;
        this.causalContext = null;
    }
    
    public ConsistencyReadRequest(long transactionId, String tableName, String key, 
                                ConsistencyLevel consistencyLevel, CausalContext causalContext) {
        this.transactionId = transactionId;
        this.tableName = tableName;
        this.key = key;
        this.consistencyLevel = consistencyLevel;
        this.causalContext = causalContext;
    }
    
    public long getTransactionId() { return transactionId; }
    public String getTableName() { return tableName; }
    public String getKey() { return key; }
    public ConsistencyLevel getConsistencyLevel() { return consistencyLevel; }
    public String getSessionId() { return sessionId; }
    public CausalContext getCausalContext() { return causalContext; }
}

enum ConsistencyLevel {
    STRONG_CONSISTENCY, EVENTUAL_CONSISTENCY, SESSION_CONSISTENCY, CAUSAL_CONSISTENCY
}

/**
 * 因果上下文
 */
class CausalContext {
    private final Map<String, Long> vectorClock;
    private final long timestamp;
    
    public CausalContext(Map<String, Long> vectorClock) {
        this.vectorClock = new HashMap<>(vectorClock);
        this.timestamp = System.currentTimeMillis();
    }
    
    public Map<String, Long> getVectorClock() { return vectorClock; }
    public long getTimestamp() { return timestamp; }
}

/**
 * 一致性策略
 */
class ConsistencyStrategy {
    private final Map<ConsistencyLevel, ConsistencyPolicy> policies;
    
    public ConsistencyStrategy() {
        this.policies = new HashMap<>();
        initializePolicies();
    }
    
    private void initializePolicies() {
        policies.put(ConsistencyLevel.STRONG_CONSISTENCY, new StrongConsistencyPolicy());
        policies.put(ConsistencyLevel.EVENTUAL_CONSISTENCY, new EventualConsistencyPolicy());
        policies.put(ConsistencyLevel.SESSION_CONSISTENCY, new SessionConsistencyPolicy());
        policies.put(ConsistencyLevel.CAUSAL_CONSISTENCY, new CausalConsistencyPolicy());
    }
    
    public ConsistencyPolicy getPolicy(ConsistencyLevel level) {
        return policies.get(level);
    }
}

/**
 * 一致性策略接口
 */
interface ConsistencyPolicy {
    Object executeRead(String tableName, String key, ReadView readView);
    boolean isConsistent(Object data, ReadView readView);
}

/**
 * 强一致性策略
 */
class StrongConsistencyPolicy implements ConsistencyPolicy {
    @Override
    public Object executeRead(String tableName, String key, ReadView readView) {
        // 强一致性：读取最新数据
        return null; // 实现逻辑
    }
    
    @Override
    public boolean isConsistent(Object data, ReadView readView) {
        // 强一致性：数据必须是最新的
        return true;
    }
}

/**
 * 最终一致性策略
 */
class EventualConsistencyPolicy implements ConsistencyPolicy {
    @Override
    public Object executeRead(String tableName, String key, ReadView readView) {
        // 最终一致性：读取快照数据
        return null; // 实现逻辑
    }
    
    @Override
    public boolean isConsistent(Object data, ReadView readView) {
        // 最终一致性：允许读取历史数据
        return true;
    }
}

/**
 * 会话一致性策略
 */
class SessionConsistencyPolicy implements ConsistencyPolicy {
    @Override
    public Object executeRead(String tableName, String key, ReadView readView) {
        // 会话一致性：同一会话内保持一致性
        return null; // 实现逻辑
    }
    
    @Override
    public boolean isConsistent(Object data, ReadView readView) {
        // 会话一致性：会话内数据一致
        return true;
    }
}

/**
 * 因果一致性策略
 */
class CausalConsistencyPolicy implements ConsistencyPolicy {
    @Override
    public Object executeRead(String tableName, String key, ReadView readView) {
        // 因果一致性：考虑操作因果关系
        return null; // 实现逻辑
    }
    
    @Override
    public boolean isConsistent(Object data, ReadView readView) {
        // 因果一致性：保持因果关系
        return true;
    }
}

/**
 * 分布式一致性读管理器
 * 实现分布式环境下的快照读和当前读
 */
public class DistributedConsistencyReadManager {
    private final ClusterManager clusterManager;
    private final ReplicationManager replicationManager;
    private final ConsistencyReadManager consistencyReadManager;
    private final VectorClockManager vectorClockManager;
    
    public DistributedConsistencyReadManager() {
        this.clusterManager = new ClusterManager();
        this.replicationManager = new ReplicationManager();
        this.consistencyReadManager = new ConsistencyReadManager();
        this.vectorClockManager = new VectorClockManager();
    }
    
    /**
     * 执行分布式快照读
     */
    public Object executeDistributedSnapshotRead(DistributedReadRequest request) {
        // 选择读取节点
        List<Node> readNodes = selectReadNodes(request);
        
        // 执行快照读
        Map<Node, Object> results = new HashMap<>();
        for (Node node : readNodes) {
            Object result = executeSnapshotReadOnNode(node, request);
            results.put(node, result);
        }
        
        // 合并结果
        return mergeResults(results, request.getConsistencyLevel());
    }
    
    /**
     * 执行分布式当前读
     */
    public Object executeDistributedCurrentRead(DistributedReadRequest request) {
        // 选择主节点
        Node masterNode = clusterManager.getMasterNode();
        
        // 执行当前读
        return executeCurrentReadOnNode(masterNode, request);
    }
    
    /**
     * 选择读取节点
     */
    private List<Node> selectReadNodes(DistributedReadRequest request) {
        List<Node> availableNodes = clusterManager.getAvailableNodes();
        
        switch (request.getReadStrategy()) {
            case MASTER_ONLY:
                return Arrays.asList(clusterManager.getMasterNode());
            case SLAVE_ONLY:
                return clusterManager.getSlaveNodes();
            case ANY_NODE:
                return availableNodes;
            case CONSISTENT_NODE:
                return selectConsistentNodes(availableNodes, request);
            default:
                return availableNodes;
        }
    }
    
    /**
     * 选择一致性节点
     */
    private List<Node> selectConsistentNodes(List<Node> nodes, DistributedReadRequest request) {
        // 根据一致性要求选择节点
        List<Node> consistentNodes = new ArrayList<>();
        
        for (Node node : nodes) {
            if (isNodeConsistent(node, request)) {
                consistentNodes.add(node);
            }
        }
        
        return consistentNodes;
    }
    
    /**
     * 检查节点一致性
     */
    private boolean isNodeConsistent(Node node, DistributedReadRequest request) {
        // 检查节点的向量时钟
        VectorClock nodeClock = vectorClockManager.getNodeClock(node.getId());
        VectorClock requestClock = request.getVectorClock();
        
        return vectorClockManager.isConsistent(nodeClock, requestClock);
    }
    
    /**
     * 在节点上执行快照读
     */
    private Object executeSnapshotReadOnNode(Node node, DistributedReadRequest request) {
        try {
            // 创建ReadView
            ReadView readView = createDistributedReadView(node, request);
            
            // 执行快照读
            return consistencyReadManager.executeConsistencyRead(
                    new ConsistencyReadRequest(
                            request.getTransactionId(),
                            request.getTableName(),
                            request.getKey(),
                            request.getConsistencyLevel()
                    )
            );
        } catch (Exception e) {
            System.err.println("在节点 " + node.getId() + " 上执行快照读失败: " + e.getMessage());
            return null;
        }
    }
    
    /**
     * 在节点上执行当前读
     */
    private Object executeCurrentReadOnNode(Node node, DistributedReadRequest request) {
        try {
            // 执行当前读
            return consistencyReadManager.executeConsistencyRead(
                    new ConsistencyReadRequest(
                            request.getTransactionId(),
                            request.getTableName(),
                            request.getKey(),
                            ConsistencyLevel.STRONG_CONSISTENCY
                    )
            );
        } catch (Exception e) {
            System.err.println("在节点 " + node.getId() + " 上执行当前读失败: " + e.getMessage());
            return null;
        }
    }
    
    /**
     * 创建分布式ReadView
     */
    private ReadView createDistributedReadView(Node node, DistributedReadRequest request) {
        // 获取节点的最大事务ID
        long maxTrxId = clusterManager.getNodeMaxTransactionId(node.getId());
        
        // 获取活跃事务列表
        Set<Long> activeTrxIds = clusterManager.getNodeActiveTransactionIds(node.getId());
        
        // 创建ReadView
        return new ReadView(request.getTransactionId(), maxTrxId, 0, activeTrxIds);
    }
    
    /**
     * 合并结果
     */
    private Object mergeResults(Map<Node, Object> results, ConsistencyLevel consistencyLevel) {
        switch (consistencyLevel) {
            case STRONG_CONSISTENCY:
                return mergeStrongConsistencyResults(results);
            case EVENTUAL_CONSISTENCY:
                return mergeEventualConsistencyResults(results);
            case SESSION_CONSISTENCY:
                return mergeSessionConsistencyResults(results);
            case CAUSAL_CONSISTENCY:
                return mergeCausalConsistencyResults(results);
            default:
                return mergeDefaultResults(results);
        }
    }
    
    /**
     * 合并强一致性结果
     */
    private Object mergeStrongConsistencyResults(Map<Node, Object> results) {
        // 强一致性：返回最新结果
        return results.values().stream()
                .filter(Objects::nonNull)
                .findFirst()
                .orElse(null);
    }
    
    /**
     * 合并最终一致性结果
     */
    private Object mergeEventualConsistencyResults(Map<Node, Object> results) {
        // 最终一致性：返回任意结果
        return results.values().stream()
                .filter(Objects::nonNull)
                .findFirst()
                .orElse(null);
    }
    
    /**
     * 合并会话一致性结果
     */
    private Object mergeSessionConsistencyResults(Map<Node, Object> results) {
        // 会话一致性：返回会话相关结果
        return results.values().stream()
                .filter(Objects::nonNull)
                .findFirst()
                .orElse(null);
    }
    
    /**
     * 合并因果一致性结果
     */
    private Object mergeCausalConsistencyResults(Map<Node, Object> results) {
        // 因果一致性：返回因果相关结果
        return results.values().stream()
                .filter(Objects::nonNull)
                .findFirst()
                .orElse(null);
    }
    
    /**
     * 合并默认结果
     */
    private Object mergeDefaultResults(Map<Node, Object> results) {
        // 默认：返回第一个非空结果
        return results.values().stream()
                .filter(Objects::nonNull)
                .findFirst()
                .orElse(null);
    }
}

/**
 * 分布式读请求
 */
class DistributedReadRequest {
    private final long transactionId;
    private final String tableName;
    private final String key;
    private final ConsistencyLevel consistencyLevel;
    private final ReadStrategy readStrategy;
    private final VectorClock vectorClock;
    
    public DistributedReadRequest(long transactionId, String tableName, String key, 
                                ConsistencyLevel consistencyLevel, ReadStrategy readStrategy) {
        this.transactionId = transactionId;
        this.tableName = tableName;
        this.key = key;
        this.consistencyLevel = consistencyLevel;
        this.readStrategy = readStrategy;
        this.vectorClock = null;
    }
    
    public DistributedReadRequest(long transactionId, String tableName, String key, 
                                ConsistencyLevel consistencyLevel, ReadStrategy readStrategy, 
                                VectorClock vectorClock) {
        this.transactionId = transactionId;
        this.tableName = tableName;
        this.key = key;
        this.consistencyLevel = consistencyLevel;
        this.readStrategy = readStrategy;
        this.vectorClock = vectorClock;
    }
    
    public long getTransactionId() { return transactionId; }
    public String getTableName() { return tableName; }
    public String getKey() { return key; }
    public ConsistencyLevel getConsistencyLevel() { return consistencyLevel; }
    public ReadStrategy getReadStrategy() { return readStrategy; }
    public VectorClock getVectorClock() { return vectorClock; }
}

enum ReadStrategy {
    MASTER_ONLY, SLAVE_ONLY, ANY_NODE, CONSISTENT_NODE
}

/**
 * 向量时钟管理器
 */
class VectorClockManager {
    private final Map<String, VectorClock> nodeClocks;
    
    public VectorClockManager() {
        this.nodeClocks = new ConcurrentHashMap<>();
    }
    
    /**
     * 获取节点向量时钟
     */
    public VectorClock getNodeClock(String nodeId) {
        return nodeClocks.get(nodeId);
    }
    
    /**
     * 更新节点向量时钟
     */
    public void updateNodeClock(String nodeId, VectorClock clock) {
        nodeClocks.put(nodeId, clock);
    }
    
    /**
     * 检查一致性
     */
    public boolean isConsistent(VectorClock clock1, VectorClock clock2) {
        if (clock1 == null || clock2 == null) {
            return true;
        }
        
        return clock1.isConsistentWith(clock2);
    }
}

/**
 * 向量时钟
 */
class VectorClock {
    private final Map<String, Long> clock;
    
    public VectorClock() {
        this.clock = new ConcurrentHashMap<>();
    }
    
    public VectorClock(Map<String, Long> clock) {
        this.clock = new ConcurrentHashMap<>(clock);
    }
    
    /**
     * 递增时钟
     */
    public void increment(String nodeId) {
        clock.put(nodeId, clock.getOrDefault(nodeId, 0L) + 1);
    }
    
    /**
     * 更新时钟
     */
    public void update(String nodeId, long value) {
        clock.put(nodeId, Math.max(clock.getOrDefault(nodeId, 0L), value));
    }
    
    /**
     * 检查一致性
     */
    public boolean isConsistentWith(VectorClock other) {
        if (other == null) {
            return true;
        }
        
        // 检查是否存在因果关系
        boolean thisHappensBeforeOther = this.happensBefore(other);
        boolean otherHappensBeforeThis = other.happensBefore(this);
        
        return !thisHappensBeforeOther && !otherHappensBeforeThis;
    }
    
    /**
     * 检查happens-before关系
     */
    public boolean happensBefore(VectorClock other) {
        if (other == null) {
            return false;
        }
        
        boolean allLessOrEqual = true;
        boolean atLeastOneLess = false;
        
        Set<String> allNodes = new HashSet<>(clock.keySet());
        allNodes.addAll(other.clock.keySet());
        
        for (String nodeId : allNodes) {
            long thisValue = clock.getOrDefault(nodeId, 0L);
            long otherValue = other.clock.getOrDefault(nodeId, 0L);
            
            if (thisValue > otherValue) {
                allLessOrEqual = false;
                break;
            } else if (thisValue < otherValue) {
                atLeastOneLess = true;
            }
        }
        
        return allLessOrEqual && atLeastOneLess;
    }
    
    public Map<String, Long> getClock() {
        return new HashMap<>(clock);
    }
}

/**
 * 集群管理器
 */
class ClusterManager {
    private final List<Node> nodes;
    private Node masterNode;
    
    public ClusterManager() {
        this.nodes = new CopyOnWriteArrayList<>();
    }
    
    public void addNode(Node node) {
        nodes.add(node);
        if (node.isMaster()) {
            masterNode = node;
        }
    }
    
    public void removeNode(String nodeId) {
        nodes.removeIf(node -> node.getId().equals(nodeId));
        if (masterNode != null && masterNode.getId().equals(nodeId)) {
            masterNode = null;
        }
    }
    
    public List<Node> getAvailableNodes() {
        return nodes.stream()
                .filter(Node::isAvailable)
                .collect(Collectors.toList());
    }
    
    public List<Node> getSlaveNodes() {
        return nodes.stream()
                .filter(node -> !node.isMaster() && node.isAvailable())
                .collect(Collectors.toList());
    }
    
    public Node getMasterNode() {
        return masterNode;
    }
    
    public long getNodeMaxTransactionId(String nodeId) {
        // 实现获取节点最大事务ID
        return 0;
    }
    
    public Set<Long> getNodeActiveTransactionIds(String nodeId) {
        // 实现获取节点活跃事务ID列表
        return new HashSet<>();
    }
}

/**
 * 节点
 */
class Node {
    private final String id;
    private final String host;
    private final int port;
    private final boolean isMaster;
    private volatile boolean available;
    
    public Node(String id, String host, int port, boolean isMaster) {
        this.id = id;
        this.host = host;
        this.port = port;
        this.isMaster = isMaster;
        this.available = true;
    }
    
    public String getId() { return id; }
    public String getHost() { return host; }
    public int getPort() { return port; }
    public boolean isMaster() { return isMaster; }
    public boolean isAvailable() { return available; }
    
    public void setAvailable(boolean available) { this.available = available; }
}

/**
 * 复制管理器
 */
class ReplicationManager {
    private final ClusterManager clusterManager;
    
    public ReplicationManager() {
        this.clusterManager = new ClusterManager();
    }
    
    public void replicateData(String tableName, String key, Object data) {
        // 实现数据复制逻辑
    }
    
    public void syncData(String tableName, String key) {
        // 实现数据同步逻辑
    }
}