Spring Cloud 请求重试机制核心代码分析

场景

发布微服务的操作一般都是打完新代码的包，kill掉在跑的应用，替换新的包，启动。

spring cloud 中使用eureka为注册中心，它是允许服务列表数据的延迟性的，就是说即使应用已经不在服务列表了，客户端在一段时间内依然会请求这个地址。那么就会出现请求正在发布的地址，而导致失败。

我们会优化服务列表的刷新时间，以提高服务列表信息的时效性。但是无论怎样，都无法避免有那么一段时间是数据不一致的。

所以我们想到一个办法就是重试机制，当a机子在重启时，同个集群的b是可以正常提供服务的，如果有重试机制就可以在上面这个场景里进行重试到b而不影响正确响应。

操作

需要进行如下的操作：

ribbon:
 ReadTimeout: 10000
 ConnectTimeout: 10000
 MaxAutoRetries: 0
 MaxAutoRetriesNextServer: 1
 OkToRetryOnAllOperations: false

引入spring-retry包

<dependency>
  <groupId>org.springframework.retry</groupId>
  <artifactId>spring-retry</artifactId>
 </dependency>

以zuul为例子还需要配置开启重试：

zuul.retryable=true

遇到了问题

然而万事总没那么一帆风顺，通过测试重试机制生效了，但是并没有我想象的去请求另一台健康的机子，于是被迫去吧开源码看一看，最终发现是源码的bug，不过已经修复，升级版本即可。

代码分析

使用的版本是

spring-cloud-netflix-core:1.3.6.RELEASE

spring-retry:1.2.1.RELEASE

spring cloud 依赖版本：

<dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-dependencies</artifactId>
        <version>${spring-cloud.version}</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>

因为启用了重试，所以请求应用时会执行RetryableRibbonLoadBalancingHttpClient.execute方法：

public RibbonApacheHttpResponse execute(final RibbonApacheHttpRequest request, final IClientConfig configOverride) throws Exception {
    final RequestConfig.Builder builder = RequestConfig.custom();
    IClientConfig config = configOverride != null ? configOverride : this.config;
    builder.setConnectTimeout(config.get(
        CommonClientConfigKey.ConnectTimeout, this.connectTimeout));
    builder.setSocketTimeout(config.get(
        CommonClientConfigKey.ReadTimeout, this.readTimeout));
    builder.setRedirectsEnabled(config.get(
        CommonClientConfigKey.FollowRedirects, this.followRedirects));

    final RequestConfig requestConfig = builder.build();
    final LoadBalancedRetryPolicy retryPolicy = loadBalancedRetryPolicyFactory.create(this.getClientName(), this);
    RetryCallback retryCallback = new RetryCallback() {
      @Override
      public RibbonApacheHttpResponse doWithRetry(RetryContext context) throws Exception {
        //on retries the policy will choose the server and set it in the context
        //extract the server and update the request being made
        RibbonApacheHttpRequest newRequest = request;
        if(context instanceof LoadBalancedRetryContext) {
          ServiceInstance service = ((LoadBalancedRetryContext)context).getServiceInstance();
          if(service != null) {
            //Reconstruct the request URI using the host and port set in the retry context
            newRequest = newRequest.withNewUri(new URI(service.getUri().getScheme(),
                newRequest.getURI().getUserInfo(), service.getHost(), service.getPort(),
                newRequest.getURI().getPath(), newRequest.getURI().getQuery(),
                newRequest.getURI().getFragment()));
          }
        }
        newRequest = getSecureRequest(request, configOverride);
        HttpUriRequest httpUriRequest = newRequest.toRequest(requestConfig);
        final HttpResponse httpResponse = RetryableRibbonLoadBalancingHttpClient.this.delegate.execute(httpUriRequest);
        if(retryPolicy.retryableStatusCode(httpResponse.getStatusLine().getStatusCode())) {
          if(CloseableHttpResponse.class.isInstance(httpResponse)) {
            ((CloseableHttpResponse)httpResponse).close();
          }
          throw new RetryableStatusCodeException(RetryableRibbonLoadBalancingHttpClient.this.clientName,
              httpResponse.getStatusLine().getStatusCode());
        }
        return new RibbonApacheHttpResponse(httpResponse, httpUriRequest.getURI());
      }
    };
    return this.executeWithRetry(request, retryPolicy, retryCallback);
  }

我们发现先new 一个RetryCallback，然后执行this.executeWithRetry(request, retryPolicy, retryCallback);

而这个RetryCallback.doWithRetry的代码我们清楚看到是实际请求的代码，也就是说this.executeWithRetry方法最终还是会调用RetryCallback.doWithRetry

protected <T, E extends Throwable> T doExecute(RetryCallback<T, E> retryCallback,
      RecoveryCallback<T> recoveryCallback, RetryState state)
      throws E, ExhaustedRetryException {

    RetryPolicy retryPolicy = this.retryPolicy;
    BackOffPolicy backOffPolicy = this.backOffPolicy;

    // Allow the retry policy to initialise itself...
    RetryContext context = open(retryPolicy, state);
    if (this.logger.isTraceEnabled()) {
      this.logger.trace("RetryContext retrieved: " + context);
    }

    // Make sure the context is available globally for clients who need
    // it...
    RetrySynchronizationManager.register(context);

    Throwable lastException = null;

    boolean exhausted = false;
    try {

      // Give clients a chance to enhance the context...
      boolean running = doOpenInterceptors(retryCallback, context);

      if (!running) {
        throw new TerminatedRetryException(
            "Retry terminated abnormally by interceptor before first attempt");
      }

      // Get or Start the backoff context...
      BackOffContext backOffContext = null;
      Object resource = context.getAttribute("backOffContext");

      if (resource instanceof BackOffContext) {
        backOffContext = (BackOffContext) resource;
      }

      if (backOffContext == null) {
        backOffContext = backOffPolicy.start(context);
        if (backOffContext != null) {
          context.setAttribute("backOffContext", backOffContext);
        }
      }

      /*
       * We allow the whole loop to be skipped if the policy or context already
       * forbid the first try. This is used in the case of external retry to allow a
       * recovery in handleRetryExhausted without the callback processing (which
       * would throw an exception).
       */
      while (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {

        try {
          if (this.logger.isDebugEnabled()) {
            this.logger.debug("Retry: count=" + context.getRetryCount());
          }
          // Reset the last exception, so if we are successful
          // the close interceptors will not think we failed...
          lastException = null;
          return retryCallback.doWithRetry(context);
        }
        catch (Throwable e) {

          lastException = e;

          try {
            registerThrowable(retryPolicy, state, context, e);
          }
          catch (Exception ex) {
            throw new TerminatedRetryException("Could not register throwable",
                ex);
          }
          finally {
            doOnErrorInterceptors(retryCallback, context, e);
          }

          if (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {
            try {
              backOffPolicy.backOff(backOffContext);
            }
            catch (BackOffInterruptedException ex) {
              lastException = e;
              // back off was prevented by another thread - fail the retry
              if (this.logger.isDebugEnabled()) {
                this.logger
                    .debug("Abort retry because interrupted: count="
                        + context.getRetryCount());
              }
              throw ex;
            }
          }

          if (this.logger.isDebugEnabled()) {
            this.logger.debug(
                "Checking for rethrow: count=" + context.getRetryCount());
          }

          if (shouldRethrow(retryPolicy, context, state)) {
            if (this.logger.isDebugEnabled()) {
              this.logger.debug("Rethrow in retry for policy: count="
                  + context.getRetryCount());
            }
            throw RetryTemplate.<E>wrapIfNecessary(e);
          }

        }

        /*
         * A stateful attempt that can retry may rethrow the exception before now,
         * but if we get this far in a stateful retry there's a reason for it,
         * like a circuit breaker or a rollback classifier.
         */
        if (state != null && context.hasAttribute(GLOBAL_STATE)) {
          break;
        }
      }

      if (state == null && this.logger.isDebugEnabled()) {
        this.logger.debug(
            "Retry failed last attempt: count=" + context.getRetryCount());
      }

      exhausted = true;
      return handleRetryExhausted(recoveryCallback, context, state);

    }
    catch (Throwable e) {
      throw RetryTemplate.<E>wrapIfNecessary(e);
    }
    finally {
      close(retryPolicy, context, state, lastException == null || exhausted);
      doCloseInterceptors(retryCallback, context, lastException);
      RetrySynchronizationManager.clear();
    }
  }

在一个while循环里实现重试机制，当执行retryCallback.doWithRetry(context)出现异常的时候，就会catch异常，然后用 retryPolicy判断是否进行重试，特别注意registerThrowable(retryPolicy, state, context, e);方法，不但判断了是否重试，在重试情况下会新选出一个机子放入context，然后再去执行retryCallback.doWithRetry(context)时带入，如此就实现了换机子重试了。

但是我的配置怎么会没有换机子呢？调试代码发现registerThrowable(retryPolicy, state, context, e);选出来的机子没问题，就是新的健康的机子，但是在执行retryCallback.doWithRetry(context)代码的时候依然请求的是那台挂掉的机子。

所以我们再仔细看一下retryCallback.doWithRetry(context)的代码：

我们发现了这行代码：

newRequest = getSecureRequest(request, configOverride);
protected RibbonApacheHttpRequest getSecureRequest(RibbonApacheHttpRequest request, IClientConfig configOverride) {
    if (isSecure(configOverride)) {
      final URI secureUri = UriComponentsBuilder.fromUri(request.getUri())
          .scheme("https").build(true).toUri();
      return request.withNewUri(secureUri);
    }
    return request;
  }

newRequest在前面已经使用context构建完毕，request是上一次请求的数据，只要执行这个代码就会发现newRequest永远都会被request覆盖。看到这里我们才发现原来是一个源码bug。

issue地址：https://github.com/spring-cloud/spring-cloud-netflix/issues/2667

总结

这是一次很普通的查问题过程，在这个过程中当我发现配置没有达到我的预期时，我先查看了配置的含义，尝试多次无果，于是进行断点调试发现异常中断点后，因为场景需要一台机子健康一台机子下线，我模拟了数百次，最终才定位到了这行代码。开源项目即使是优秀的项目必然也会有bug存在，不迷信，不盲目。另一方面，阅读源码能力也是一个解决问题的重要能力，像我在找源码入口，定位代码时耗费了很多的时间。

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