教培消课怎么保证课时不重复扣、不扣成负数：一套幂等账本设计

-- 课时账户：一个学员购买的一个课时包一行
CREATE TABLE lesson_account (
  id            BIGINT PRIMARY KEY,
  student_id    BIGINT NOT NULL,
  package_id    BIGINT NOT NULL,
  total_lesson  INT NOT NULL,
  remain_lesson INT NOT NULL,
  version       INT NOT NULL DEFAULT 0,
  UNIQUE KEY uk_student_package (student_id, package_id),
  CONSTRAINT chk_remain CHECK (remain_lesson >= 0)
);

-- 消课流水：只追加，请假用红冲流水，不删除原记录
CREATE TABLE lesson_consume_flow (
  id          BIGINT PRIMARY KEY,
  biz_id      VARCHAR(64) NOT NULL,
  lesson_id   BIGINT NOT NULL,
  student_id  BIGINT NOT NULL,
  account_id  BIGINT NOT NULL,
  cost        INT NOT NULL,
  flow_type   TINYINT NOT NULL,   -- 1消课 2红冲 3补课
  status      TINYINT NOT NULL,   -- 0处理中 1成功 2已撤销
  create_time DATETIME NOT NULL,
  UNIQUE KEY uk_biz_type (biz_id, flow_type)
);

@Transactional(rollbackFor = Exception.class)
public ConsumeResult consumeLesson(ConsumeCmd cmd) {
    String bizId = cmd.getLessonId() + "_" + cmd.getStudentId();

    LessonConsume exist = flowMapper.selectByBizIdAndType(bizId, FLOW_CONSUME);
    if (exist != null) {
        return ConsumeResult.idempotent(exist.getStatus()); // 已处理过，原样返回
    }

    Lesson lesson = lessonMapper.selectByIdForUpdate(cmd.getLessonId());
    if (lesson == null || !lesson.canConsume()) {
        throw new BizException("课节状态不允许消课");
    }

    try {
        flowMapper.insert(buildConsumeFlow(cmd, bizId));
    } catch (DuplicateKeyException dup) {
        LessonConsume again = flowMapper.selectByBizIdAndType(bizId, FLOW_CONSUME);
        return ConsumeResult.idempotent(again.getStatus()); // 并发重复，被唯一索引拦下
    }

    int rows = accountMapper.deduct(cmd.getAccountId(), cmd.getCost(), cmd.getVersion());
    if (rows == 0) {
        throw new BizException("课时不足或数据已变更，请刷新重试");
    }
    return ConsumeResult.ok();
}

UPDATE lesson_account
SET remain_lesson = remain_lesson - #{cost},
    version = version + 1
WHERE id = #{accountId}
  AND remain_lesson >= #{cost}
  AND version = #{version};

@Transactional(rollbackFor = Exception.class)
public void consumeClass(Long lessonId, List<Long> studentIds) {
    List<Long> failed = new ArrayList<>();
    for (Long sid : studentIds) {
        try {
            consumeLesson(new ConsumeCmd(lessonId, sid));
        } catch (BizException e) {
            failed.add(sid); // 课时不足等业务异常记录下来
        }
    }
    if (!failed.isEmpty()) {
        throw new BizException("以下学员消课失败：" + failed);
    }
}

@Transactional(rollbackFor = Exception.class)
public void revokeConsume(String consumeBizId) {
    LessonConsume origin = flowMapper.selectByBizIdAndType(consumeBizId, FLOW_CONSUME);
    if (origin == null || origin.getStatus() != SUCCESS) {
        throw new BizException("原消课不存在或不可撤销");
    }
    flowMapper.insert(buildReverseFlow(origin));   // uk 兜底，重复红冲只生效一次
    accountMapper.refund(origin.getAccountId(), origin.getCost());
    flowMapper.markStatus(origin.getId(), REVOKED);
}