Spring Cloud Gateway 雪崩了，我 TM 人傻了

Posted 2021-09-29 zhxdick

大家好，我又人傻了。这次的经验告诉我们，出来写代码偷的懒，迟早要还的。

问题现象与背景

昨晚我们的网关雪崩了一段时间，现象是：

1.不断有各种微服务报异常：在写 HTTP 响应的时候，连接已经关闭：

reactor.netty.http.client.PrematureCloseException: Connection prematurely closed BEFORE response 

2.同时还有请求还没读取完，连接已经关闭的异常：

org.springframework.http.converter.HttpMessageNotReadableException: I/O error while reading input message; nested exception is java.io.IOException: UT000128: Remote peer closed connection before all data could be read 

3.前端不断有请求超时的报警，504 Gateway Time-out

4.网关进程不断健康检查失败而被重启

5.重启后的网关进程，立刻请求数量激增，每个实例峰值 2000 qps，闲时每个实例 500 qps，忙时由于有扩容也能保持每个实例在 1000 qps 以内，然后健康检查接口就很长时间没有响应，导致实例不断重启

其中，1 和 2 的问题应该是应为网关不断重启，并且由于某些原因优雅关闭失败导致强制关闭，强制关闭导致连接被强制断开从而有 1 和 2 相关的异常。

我们的网关是基于 Spring Cloud Gateway 实现的，并且有自动根据 CPU 负载扩容的机制。奇怪的是在请求数量彪增的时候，CPU 利用率并没有提高很多，保持在 60% 左右，由于 CPU 负载没有达到扩容的界限，所以一直没有自动扩容。为了快速解决问题，我们手动扩容了几个网关实例，将网关单实例负载控制在了 1000 以内，暂时解决了问题。

问题分析

为了彻底解决这个问题，我们使用 JFR 分析。首先先根据已知的线索去分析：

1. Spring Cloud Gateway 是基于 Spring-WebFlux 实现的异步响应式网关，http 业务线程是有限的（默认是 2 * 可以使用的 CPU 个数，我们这里是 4）。
2. 网关进程不断健康检查失败，健康检查调用的是 /actuator/health 接口，这个接口一直超时。

健康检查接口超时一般有两个原因：

1. 健康检查接口检查某个组件的时候，阻塞住了。例如数据库如果卡住，那么可能数据库健康检查会一直没有返回。
2. http 线程池没来得及处理健康检查请求，请求就超时了。

我们可以先去看 JFR 中的定时堆栈，看是否有 http 线程卡在健康检查上面。查看出问题后的线程堆栈，重点关注那 4 个 http 线程，结果发现这 4 个线程的堆栈基本一样，都是在执行 Redis 命令：

"reactor-http-nio-1" #68 daemon prio=5 os_prio=0 cpu=70832.99ms elapsed=199.98s tid=0x0000ffffb2f8a740 nid=0x69 waiting on condition  [0x0000fffe8adfc000]
   java.lang.Thread.State: TIMED_WAITING (parking)
    at jdk.internal.misc.Unsafe.park(java.base@11.0.8/Native Method)
    - parking to wait for  <0x00000007d50eddf8> (a java.util.concurrent.CompletableFuture$Signaller)
    at java.util.concurrent.locks.LockSupport.parkNanos(java.base@11.0.8/LockSupport.java:234)
    at java.util.concurrent.CompletableFuture$Signaller.block(java.base@11.0.8/CompletableFuture.java:1798)
    at java.util.concurrent.ForkJoinPool.managedBlock(java.base@11.0.8/ForkJoinPool.java:3128)
    at java.util.concurrent.CompletableFuture.timedGet(java.base@11.0.8/CompletableFuture.java:1868)
    at java.util.concurrent.CompletableFuture.get(java.base@11.0.8/CompletableFuture.java:2021)
    at io.lettuce.core.protocol.AsyncCommand.await(AsyncCommand.java:83)
    at io.lettuce.core.internal.Futures.awaitOrCancel(Futures.java:244)
    at io.lettuce.core.FutureSyncInvocationHandler.handleInvocation(FutureSyncInvocationHandler.java:75)
    at io.lettuce.core.internal.AbstractInvocationHandler.invoke(AbstractInvocationHandler.java:80)
    at com.sun.proxy.$Proxy245.get(Unknown Source)
    at org.springframework.data.redis.connection.lettuce.LettuceStringCommands.get(LettuceStringCommands.java:68)
    at org.springframework.data.redis.connection.DefaultedRedisConnection.get(DefaultedRedisConnection.java:267)
    at org.springframework.data.redis.connection.DefaultStringRedisConnection.get(DefaultStringRedisConnection.java:406)
    at org.springframework.data.redis.core.DefaultValueOperations$1.inRedis(DefaultValueOperations.java:57)
    at org.springframework.data.redis.core.AbstractOperations$ValueDeserializingRedisCallback.doInRedis(AbstractOperations.java:60)
    at org.springframework.data.redis.core.RedisTemplate.execute(RedisTemplate.java:222)
    at org.springframework.data.redis.core.RedisTemplate.execute(RedisTemplate.java:189)
    at org.springframework.data.redis.core.AbstractOperations.execute(AbstractOperations.java:96)
    at org.springframework.data.redis.core.DefaultValueOperations.get(DefaultValueOperations.java:53)
    at com.jojotech.apigateway.filter.AccessCheckFilter.traced(AccessCheckFilter.java:196)
    at com.jojotech.apigateway.filter.AbstractTracedFilter.filter(AbstractTracedFilter.java:39)
    at org.springframework.cloud.gateway.handler.FilteringWebHandler$GatewayFilterAdapter.filter(FilteringWebHandler.java:137)
    at org.springframework.cloud.gateway.filter.OrderedGatewayFilter.filter(OrderedGatewayFilter.java:44)
    at org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain.lambda$filter$0(FilteringWebHandler.java:117)
    at org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain$$Lambda$1478/0x0000000800b84c40.get(Unknown Source)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:44)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at com.jojotech.apigateway.common.TracedMono.subscribe(TracedMono.java:24)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at com.jojotech.apigateway.common.TracedMono.subscribe(TracedMono.java:24)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at com.jojotech.apigateway.common.TracedMono.subscribe(TracedMono.java:24)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at com.jojotech.apigateway.common.TracedMono.subscribe(TracedMono.java:24)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at com.jojotech.apigateway.common.TracedMono.subscribe(TracedMono.java:24)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at com.jojotech.apigateway.common.TracedMono.subscribe(TracedMono.java:24)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:255)
    at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:51)
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:157)
    at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:73)
    at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:82)
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.innerNext(FluxConcatMap.java:281)
    at reactor.core.publisher.FluxConcatMap$ConcatMapInner.onNext(FluxConcatMap.java:860)
    at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:120)
    at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:73)
    at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1815)
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:151)
    at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:120)
    at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:82)
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.innerNext(FluxConcatMap.java:281)
    at reactor.core.publisher.FluxConcatMap$ConcatMapInner.onNext(FluxConcatMap.java:860)
    at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79)
    at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onNext(MonoPeekTerminal.java:180)
    at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1815)
    at reactor.core.publisher.MonoFilterWhen$MonoFilterWhenMain.onNext(MonoFilterWhen.java:149)
    at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2397)
    at reactor.core.publisher.MonoFilterWhen$MonoFilterWhenMain.onSubscribe(MonoFilterWhen.java:112)
    at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:54)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:448)
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onNext(FluxConcatMap.java:250)
    at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.onNext(FluxDematerialize.java:98)
    at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.onNext(FluxDematerialize.java:44)
    at reactor.core.publisher.FluxIterable$IterableSubscription.slowPath(FluxIterable.java:270)
    at reactor.core.publisher.FluxIterable$IterableSubscription.request(FluxIterable.java:228)
    at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.request(FluxDematerialize.java:127)
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:235)
    at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.onSubscribe(FluxDematerialize.java:77)
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:164)
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:86)
    at reactor.core.publisher.InternalFluxOperator.subscribe(InternalFluxOperator.java:62)
    at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:54)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:448)
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:218)
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:164)
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:86)
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at org.springframework.cloud.sleuth.instrument.web.TraceWebFilter$MonoWebFilterTrace.subscribe(TraceWebFilter.java:184)
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
    at reactor.core.publisher.Mono.subscribe(Mono.java:4150)
    at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:255)
    at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:51)
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
    at reactor.netty.http.server.HttpServer$HttpServerHandle.onStateChange(HttpServer.java:915)
    at reactor.netty.ReactorNetty$CompositeConnectionObserver.onStateChange(ReactorNetty.java:654)
    at reactor.netty.transport.ServerTransport$ChildObserver.onStateChange(ServerTransport.java:478)
    at reactor.netty.http.server.HttpServerOperations.onInboundNext(HttpServerOperations.java:526)
    at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:94)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
    at reactor.netty.http.server.HttpTrafficHandler.channelRead(HttpTrafficHandler.java:209)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
    at reactor.netty.http.server.logging.AccessLogHandlerH1.channelRead(AccessLogHandlerH1.java:59)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
    at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436)
    at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:324)
    at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:296)
    at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
    at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
    at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
    at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166)
    at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:719)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:655)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:581)
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493)
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
    at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    at java.lang.Thread.run(java.base@11.0.8/Thread.java:834)

发现 http 线程没有卡在健康检查，同时其他线程也没有任何和健康检查相关的堆栈（异步环境下，健康检查也是异步的，其中某些过程可能交给其他线程）。所以，健康检查请求应该是还没被执行就超时取消了。

那么为啥会这样呢？于此同时，我还发现这里用的是 RedisTemplate，是 spring-data-redis 的同步 Redis API。我猛然想起来之前写这里的代码的时候，因为只是验证一个 key 是否存在和修改 key 的过期时间，偷懒没有用异步 API。这里是不是因为使用同步 API 阻塞了 http 线程导致的雪崩呢？

我们来验证下这个猜想：我们的项目中 redis 操作是通过 spring-data-redis + Lettuce 连接池，启用并且增加了关于 Lettuce 命令的 JFR 监控，可以参考我的这篇文章：这个 Redis 连接池的新监控方式针不戳~我再加一点佐料，截至目前我的 pull request 已经合并，这个特性会在 6.2.x 版本发布。我们看下出问题时间附近的 Redis 命令采集，如下图所示：

我们来简单计算下执行 Redis 命令导致的阻塞时间（我们的采集是 10s 一次，count 是命令次数，时间单位是微秒）：使用这里的命令个数乘以 50% 的中位数，除以 10（因为是 10s），得出每秒因为执行 Redis 命令导致的阻塞时间：

32*152=4864
1*860=860
5*163=815
32*176=5632
1*178=178
16959*168=2849112
774*176=136224
3144*166=521904
17343*179=3104397
702*166=116532
总和 6740518
6740518 / 10 = 674051.8 us = 0.67s

这个仅仅是使用中位数计算的阻塞时间，从图上的分布其实可以看出真正的值应该比这个大，这样很有可能每秒需要在 Redis 同步接口上阻塞的时间就超过了 1s，不断地请求，请求没有减少，从而导致了请求越积越多，最后雪崩。

并且由于是阻塞接口，线程很多时间消耗在等待 io 了，所以 CPU 上不去，导致没有自动扩容。业务高峰时，由于有设定好的预先扩容，导致网关单实例没有达到出问题的压力，所以没问题。

解决问题

我们来改写原有代码，使用同步 spring-data-redis Api 原有代码是(其实就是 spring-cloud-gateway 的 Filter 接口的核心方法 public Mono<Void> traced(ServerWebExchange exchange, GatewayFilterChain chain) 的方法体)：

if (StringUtils.isBlank(token)) {
    //如果 token 不存在，则根据路径决定继续请求还是返回需要登录的状态码
    return continueOrUnauthorized(path, exchange, chain, headers);
} else {
    try {
        String accessTokenValue = redisTemplate.opsForValue().get(token);
        if (StringUtils.isNotBlank(accessTokenValue)) {
            //如果 accessTokenValue 不为空，则续期 4 小时，保证登录用户只要有操作就不会让 token 过期
            Long expire = redisTemplate.getExpire(token);
            log.info("accessTokenValue = {}, expire = {}", accessTokenValue, expire);
            if (expire != null && expire < 4 * 60 * 60) {
                redisTemplate.expire(token, 4, TimeUnit.HOURS);
            }

            //解析，获取 userId
            JSONObject accessToken = JSON.parseObject(accessTokenValue);
            String userId = accessToken.getString("userId");
            //如果 userId 不为空才合法
            if (StringUtils.isNotBlank(userId)) {
                //解析 Token 
                HttpHeaders newHeaders = parse(accessToken);
                //继续请求
                return FilterUtil.changeRequestHeader(exchange, chain, newHeaders);
            }
        }
    } catch (Exception e) {
        log.error("read accessToken error: {}", e.getMessage(), e);
    }
    //如果 token 不合法，则根据路径决定继续请求还是返回需要登录的状态码
    return continueOrUnauthorized(path, exchange, chain, headers);
}

改成使用异步：

if (StringUtils.isBlank(token)) {
    return continueOrUnauthorized(path, exchange, chain, headers);
} else {
    HttpHeaders finalHeaders = headers;
    //必须使用 tracedPublisherFactory 包裹，否则链路信息会丢失，这里参考我的另一篇文章：Spring Cloud Gateway 没有链路信息，我 TM 人傻了
    return tracedPublisherFactory.getTracedMono(
            redisTemplate.opsForValue().get(token)
                    //必须切换线程，否则后续线程使用的还是 Redisson 的线程，如果耗时长则会影响其他使用 Redis 的业务，并且这个耗时也算在 Redis 连接命令超时中
                    .publishOn(Schedulers.parallel()),
            exchange
    ).doOnSuccess(accessTokenValue -> {
        if (accessTokenValue != null) {
            //accessToken续期,4小时
            tracedPublisherFactory.getTracedMono(redisTemplate.getExpire(token).publishOn(Schedulers.parallel()), exchange).doOnSuccess(expire -> {
                log.info("accessTokenValue = {}, expire = {}", accessTokenValue, expire);
                if (expire != null && expire.toHours() < 4) {
                    redisTemplate.expire(token, Duration.ofHours(4)).subscribe();
                }
            }).subscribe();
        }
    })
    //必须转换成非 null，否则 flatmap 不会执行；也不能在末尾用 switchIfEmpty，因为整体返回的是 Mono<Void> 本来里面承载的就是空的，会导致每个请求发送两遍。
    .defaultIfEmpty("")
    .flatMap(accessTokenValue -> {
        try {
            if (StringUtils.isNotBlank(accessTokenValue)) {
                JSONObject accessToken = JSON.parseObject(accessTokenValue);
                String userId = accessToken.getString("userId");
                if (StringUtils.isNotBlank(userId)) {
                    //解析 Token 
                    HttpHeaders newHeaders = parse(accessToken);
                    //继续请求
                    return FilterUtil.changeRequestHeader(exchange, chain, newHeaders);
                }
            }
            return continueOrUnauthorized(path, exchange, chain, finalHeaders);
        } catch (Exception e) {
            log.error("read accessToken error: {}", e.getMessage(), e);
            return continueOrUnauthorized(path, exchange, chain, finalHeaders);
        }
    });
}

这里有几个注意点：

1. Spring-Cloud-Sleuth 对于 Spring-WebFlux 中做的链路追踪优先，如果我们在 Filter 中创建新的 Flux 或者 Mono，这里面是没有链路信息的，需要我们手动加入。这个可以参考我的另一篇文章：Spring Cloud Gateway 没有链路信息，我 TM 人傻了
2. spring-data-redis + Lettuce 连接池的组合，对于异步接口，我们最好在获取响应之后切换成别的线程池执行，否则后续线程使用的还是 Redisson 的线程，如果耗时长则会影响其他使用 Redis 的业务，并且这个耗时也算在 Redis 连接命令超时中
3. Project Reactor 如果中间结果有 null 值，则后面的 flatmap、map 等流操作就不执行了。如果在这里终止，前端收到的响应是有问题的。所以中间结果我们要在每一步考虑 null 问题。
4. spring-cloud-gateway 的核心 GatewayFilter 接口，核心方法返回的是 Mono<Void>。Mono<Void> 本来里面承载的就是空的，导致我们不能使用末尾的 switchIfEmpty 来简化中间步骤的 null，如果用了会导致每个请求发送两遍。

这样修改后，压测了下网关，单实例 2w qps 请求也没有出现这个问题了。