#Hinted Handoff

// Hint 数据结构（简化版）
public class HintedWrite {
    private String key;                    // 数据的键
    private byte[] value;                  // 数据的值
    private int targetNodeId;              // 本应写入的目标节点 ID
    private long hintedAt;                 // 记录 Hint 的时间戳
    private long expiresAt;                // Hint 过期时间

    // 计算 Hint 的有效期
    public boolean isExpired() {
        return System.currentTimeMillis() > expiresAt;
    }

    // 归还数据时的元信息
    private Map<String, String> metadata;  // 版本号、TTL 等
}

// Cassandra Hint Replay 简化实现
public class HintReplayer {
    private final File hintsDirectory;
    private final Duration maxHintAge = Duration.ofHours(4);

    public void replayHints(Node recoveredNode) {
        File[] hintFiles = listHintFilesFor(recoveredNode.getId());

        for (File hintFile : hintFiles) {
            try {
                HintedWrite hint = deserializeHint(hintFile);

                // 检查 Hint 是否过期
                if (hint.isExpired()) {
                    LOG.info("Hint {} expired, skipping", hint.getKey());
                    deleteHintFile(hintFile);
                    continue;
                }

                // 归还数据到目标节点
                boolean success = writeToTargetNode(hint);

                if (success) {
                    deleteHintFile(hintFile);
                    LOG.info("Hint {} replayed successfully", hint.getKey());
                } else {
                    // 写入失败，稍后重试
                    LOG.warn("Failed to replay hint {}, will retry", hint.getKey());
                }

            } catch (Exception e) {
                LOG.error("Error replaying hint {}", hintFile, e);
            }
        }
    }
}

public class WriteHandler {
    public WriteResult handleWrite(Request request) {
        List<Node> targets = getTargetNodes(request.getKey());

        // 第一阶段：尝试写入所有目标节点
        List<Future<WriteResponse>> futures = writeToAllNodes(targets, request);

        // 第二阶段：检查成功数量
        int successCount = waitForResponses(futures);

        if (successCount >= W) {
            return WriteResult.success();
        }

        // 第三阶段：降级处理
        if (successCount >= 1) {
            // 至少有一个成功，触发 Hinted Handoff
            triggerHintedHandoff(request, targets, futures);
            return WriteResult.success();  // 用户视角成功
        }

        return WriteResult.failure("Write quorum not met");
    }
}

// 问题场景：账户余额操作
// Hint 1: 余额 +100
// Hint 2: 余额 -50
// 如果重放顺序错误：先 -50 再 +100，结果不同

// 解决：使用单调递增的序列号
public class AccountHint {
    private long sequenceNumber;  // 全局单调递增序列号
    private Operation operation;   // 操作类型
    private BigDecimal delta;     // 变更金额

    public boolean canApply(Account current) {
        return this.sequenceNumber > current.getLastSequence();
    }
}