微服务进阶的3个硬核问题：分布式事务、可观测性与契约治理

// 1. 本地事务：订单入库 + 消息落库（同库，保证原子性）
@Transactional
public void createOrder(OrderDTO order) {
    orderMapper.insert(order);
    messageMapper.insert(new PendingMessage(
        "inventory", 
        "deduct", 
        order.getSkuId(), 
        order.getQuantity()
    ));
}

// 2. 定时任务：轮询未发送消息，调用库存服务（带指数退避重试）
@Scheduled(fixedDelay = 5000)
public void dispatchPendingMessages() {
    List<PendingMessage> messages = messageMapper.selectUnsent(100);
    for (PendingMessage msg : messages) {
        try {
            inventoryClient.deduct(msg.getSkuId(), msg.getQty());
            messageMapper.markSent(msg.getId());
        } catch (Exception e) {
            msg.retryCount++;
            msg.nextRetryTime = now + 2 ^ msg.retryCount * 1000;
            messageMapper.update(msg);
        }
    }
}

// 3. 每日对账：拉取订单和库存流水，自动发现不一致并告警
@Scheduled(cron = "0 1 0 * * ?")
public void dailyReconciliation() {
    List<Order> orders = orderMapper.selectYesterday();
    List<InventoryLog> logs = inventoryLogMapper.selectYesterday();
    // 比对两边数据，发现差异自动生成工单
    List<Mismatch> mismatches = compare(orders, logs);
    if (!mismatches.isEmpty()) {
        alertService.send("对账异常", mismatches);
    }
}

# 改造前：纯文本日志，无法检索
logger.info("Order created, orderId: 12345")

# 改造后：结构化日志，自带trace_id
logger.info(
    "Order created",
    extra={
        "trace_id": trace_id,
        "order_id": "12345",
        "user_id": "67890",
        "cost_ms": 120,
        "service": "order-service"
    }
)

@Component
public class TraceInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, 
                            HttpServletResponse response, 
                            Object handler) {
        // 从上游提取或生成新的traceId
        String traceId = request.getHeader("X-Trace-Id");
        if (StringUtils.isEmpty(traceId)) {
            traceId = UUID.randomUUID().toString();
        }
        // 放入MDC，日志框架自动打印
        MDC.put("trace_id", traceId);
        // 传递给下游服务
        response.setHeader("X-Trace-Id", traceId);
        return true;
    }
}

// 服务提供方定义契约（order-service）
Contract.make {
    description "获取订单详情接口，必须返回订单ID为Long类型"
    request {
        method GET()
        url "/order/10001"
        headers {
            header "Content-Type": "application/json"
        }
    }
    response {
        status 200
        body([
            orderId: 10001L,        // Long类型，明确声明
            amount: 99.50,
            status: "PAID"
        ])
        headers {
            header "Content-Type": "application/json"
        }
    }
}

<!-- Maven插件配置：自动生成契约测试 -->
<plugin>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-contract-maven-plugin</artifactId>
    <version>4.0.0</version>
</plugin>