架构模式：Saga、Outbox 与 Strangler Fig

这一课你将能做到

1. 用 Saga 模式实现"提交-判题-更新练习记录-发通知"的分布式事务
2. 用 Outbox 模式保证事件消息"发了就存，存了必达"
3. 用 Strangler Fig 模式把老单体系统逐步拆成微服务

为什么要学

架构风格告诉你"怎么搭骨架"，架构模式告诉你"遇到具体问题怎么解决"。判题平台里，一次提交要动数据库、调判题引擎、写练习记录、发 WebSocket，这些步骤跨了多个服务，怎么保证一致性？Outbox 解决"消息发了但数据库没存"的问题，Strangler Fig 解决"老系统不敢拆"的问题。

先建立图像

Saga                    Outbox                   Strangler Fig
──────                  ──────                   ─────────────
1. 提交入库              业务操作 + Outbox 事件     老单体
2. 发 JudgeEvent  →    同事务写入               ↓ 新功能走新服务
3. 判题完成   ← 消费    Outbox 轮询投递 MQ       老功能逐步迁移
4. 更新记录   ← 消费    ↓                       ↓
5. 通知用户   ← 消费    事件最终一致             新服务完全接管

跟着做

步骤 1：Saga 编排——判题分布式事务

Saga 把大事务拆成小事务，每步失败都有补偿：

public class SubmissionSaga {
    private final SubmissionRepository submissionRepo;
    private final JudgeClient judgeClient;
    private final PracticeRecordService practiceService;
    private final NotificationService notifyService;

    public void execute(SubmitCommand cmd) {
        // 步骤 1：保存提交
        Submission submission = submissionRepo.save(Submission.create(cmd));
        try {
            // 步骤 2：发起判题
            JudgeResult result = judgeClient.judge(submission);
            submission.markAs(result);

            // 步骤 3：更新练习记录
            practiceService.record(cmd.userId(), cmd.lessonId(), result);

            // 步骤 4：通知用户
            notifyService.notifyJudged(cmd.userId(), result);
        } catch (Exception e) {
            // 补偿：标记提交为失败，不发通知
            submission.markAs(SubmissionStatus.FAILED);
            submissionRepo.save(submission);
            throw new SubmissionSagaException("判题流程失败", e);
        }
    }
}

每个步骤是本地事务，失败时执行补偿操作（回滚已完成的步骤）。

步骤 2：Outbox 模式——事件可靠投递

判题完成事件要发 MQ，但"先存数据库"和"后发 MQ"之间可能宕机。Outbox 把事件和业务数据放在同一个本地事务里：

@Entity
public class OutboxEvent {
    @Id private Long id;
    private String aggregateType;   // "Submission"
    private Long aggregateId;       // submissionId
    private String eventType;       // "JudgeFinished"
    private String payload;         // JSON 事件体
    private LocalDateTime createdAt;
    private Boolean published;      // 是否已投递 MQ
}

@Service
public class JudgeService {
    private final SubmissionRepository submissionRepo;
    private final OutboxEventRepository outboxRepo;

    @Transactional
    public void finishJudge(SubmissionId id, JudgeResult result) {
        Submission s = submissionRepo.findById(id).orElseThrow();
        s.markAs(result);
        submissionRepo.save(s);

        // 同事务写入 Outbox，保证原子性
        OutboxEvent event = new OutboxEvent("Submission", id.value(), "JudgeFinished", toJson(result));
        outboxRepo.save(event);
    }
}

@Component
public class OutboxPublisher {
    @Scheduled(fixedDelay = 1000)
    public void publishPendingEvents() {
        List<OutboxEvent> pending = outboxRepo.findByPublishedFalse();
        for (OutboxEvent e : pending) {
            mqTemplate.send(e.getEventType(), e.getPayload());
            e.markPublished();
            outboxRepo.save(e);
        }
    }
}