spring-cloud Sleuth

一直没弄明白sleuth的tracerContext是如何创建和传递的，闲来无事研究了一下。由于对sleuth的源码不熟悉，准备通过debug brave.Tracer的nextId()方法，查看方法调用栈来找来龙去脉。

首先创建两个service A和B，记作srvA、srvB，在srvA中添加testA controller，sevB中添加testB controller，testA中通过Feign调用testB。

1. 先看当用户通过浏览器调用srvA的时候，srvA是作为server的。

configuration:
TraceWebServletAutoConfiguration==>TracingFilter
TraceHttpAutoConfiguration==>HttpTracing
TraceAutoConfiguration==>Tracing
SleuthLogAutoConfiguration.Slf4jConfiguration==>CurrentTraceContext

配置中，TracingFilter在实例化时需要一个HttpTracing：

public static Filter create(HttpTracing httpTracing) {
    return new TracingFilter(httpTracing);
  }

  //Servlet运行时类
  final ServletRuntime servlet = ServletRuntime.get();
  //Slf4jCurrentTraceContext
  final CurrentTraceContext currentTraceContext;
  final Tracer tracer;
  final HttpServerHandler<HttpServletRequest, HttpServletResponse> handler;
  //TraceContext的数据提取器
  final TraceContext.Extractor<HttpServletRequest> extractor;

  TracingFilter(HttpTracing httpTracing) {
    tracer = httpTracing.tracing().tracer();
    currentTraceContext = httpTracing.tracing().currentTraceContext();
    handler = HttpServerHandler.create(httpTracing, ADAPTER);
    extractor = httpTracing.tracing().propagation().extractor(GETTER);
  }

HttpTracing Builder模式构造时接收一个Tracing：

Tracing tracing;
    //客户端span解析器
    HttpClientParser clientParser;
    String serverName;
    //服务端span解析器
    HttpServerParser serverParser;
    HttpSampler clientSampler, serverSampler;

    Builder(Tracing tracing) {
      if (tracing == null) throw new NullPointerException("tracing == null");
      final ErrorParser errorParser = tracing.errorParser();
      this.tracing = tracing;
      this.serverName = "";
      // override to re-use any custom error parser from the tracing component
      this.clientParser = new HttpClientParser() {
        @Override protected ErrorParser errorParser() {
          return errorParser;
        }
      };
      this.serverParser = new HttpServerParser() {
        @Override protected ErrorParser errorParser() {
          return errorParser;
        }
      };
      this.clientSampler = HttpSampler.TRACE_ID;
      this.serverSampler(HttpSampler.TRACE_ID);
    }

Tracing实例化：

@Bean
    @ConditionalOnMissingBean
    // NOTE: stable bean name as might be used outside sleuth
    Tracing tracing(@Value("${spring.zipkin.service.name:${spring.application.name:default}}") String serviceName,
            Propagation.Factory factory,
            CurrentTraceContext currentTraceContext,
            Reporter<zipkin2.Span> reporter,
            Sampler sampler,
            ErrorParser errorParser,
            SleuthProperties sleuthProperties
    ) {
        return Tracing.newBuilder()
                .sampler(sampler)
                .errorParser(errorParser)
                .localServiceName(serviceName)
                //ExtraFieldPropagation.Factory
                .propagationFactory(factory)
                .currentTraceContext(currentTraceContext)
                .spanReporter(adjustedReporter(reporter))
                .traceId128Bit(sleuthProperties.isTraceId128())
                .supportsJoin(sleuthProperties.isSupportsJoin())
                .build();
    }

下面看TracingFilter的doFilter：

@Override
  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
      throws IOException, ServletException {
    HttpServletRequest httpRequest = (HttpServletRequest) request;
    HttpServletResponse httpResponse = servlet.httpResponse(response);

    // Prevent duplicate spans for the same request
    TraceContext context = (TraceContext) request.getAttribute(TraceContext.class.getName());
    if (context != null) {
      // A forwarded request might end up on another thread, so make sure it is scoped
      Scope scope = currentTraceContext.maybeScope(context);
      try {
        chain.doFilter(request, response);
      } finally {
        scope.close();
      }
      return;
    }

    Span span = handler.handleReceive(extractor, httpRequest);

    // Add attributes for explicit access to customization or span context
    request.setAttribute(SpanCustomizer.class.getName(), span.customizer());
    request.setAttribute(TraceContext.class.getName(), span.context());

    Throwable error = null;
    Scope scope = currentTraceContext.newScope(span.context());
    try {
      // any downstream code can see Tracer.currentSpan() or use Tracer.currentSpanCustomizer()
      chain.doFilter(httpRequest, httpResponse);
    } catch (IOException | ServletException | RuntimeException | Error e) {
      error = e;
      throw e;
    } finally {
      scope.close();
      if (servlet.isAsync(httpRequest)) { // we don't have the actual response, handle later
        servlet.handleAsync(handler, httpRequest, httpResponse, span);
      } else { // we have a synchronous response, so we can finish the span
        handler.handleSend(ADAPTER.adaptResponse(httpRequest, httpResponse), error, span);
      }
    }
  }

在SleuthLogAutoConfiguration中如果有slfj的包，则注入CurrentTraceContext：

@Configuration
    @ConditionalOnClass(MDC.class)
    @EnableConfigurationProperties(SleuthSlf4jProperties.class)
    protected static class Slf4jConfiguration {

        @Bean
        @ConditionalOnProperty(value = "spring.sleuth.log.slf4j.enabled", matchIfMissing = true)
        @ConditionalOnMissingBean
        public CurrentTraceContext slf4jSpanLogger() {
            return Slf4jCurrentTraceContext.create();
        }
       
        ...
     }

Slf4jCurrentTraceContext中，delegate就是CurrentTraceContext.Default.inheritable()：

public static final class Default extends CurrentTraceContext {
    static final ThreadLocal<TraceContext> DEFAULT = new ThreadLocal<>();
    // Inheritable as Brave 3's ThreadLocalServerClientAndLocalSpanState was inheritable
    static final InheritableThreadLocal<TraceContext> INHERITABLE = new InheritableThreadLocal<>();

    final ThreadLocal<TraceContext> local;

    //静态方法create，local对象为ThreadLocal类型 
    /** Uses a non-inheritable static thread local */
    public static CurrentTraceContext create() {
      return new Default(DEFAULT);
    }

    //local对象为InheritableThreadLocal类型
    //官方文档指出，inheritable方法在线程池的环境中需谨慎使用，可能会取出错误的TraceContext，这样会导致Span等信息会记录并关联到错误的traceId上
    /**
     * Uses an inheritable static thread local which allows arbitrary calls to {@link
     * Thread#start()} to automatically inherit this context. This feature is available as it is was
     * the default in Brave 3, because some users couldn't control threads in their applications.
     *
     * <p>This can be a problem in scenarios such as thread pool expansion, leading to data being
     * recorded in the wrong span, or spans with the wrong parent. If you are impacted by this,
     * switch to {@link #create()}.
     */
    public static CurrentTraceContext inheritable() {
      return new Default(INHERITABLE);
    }

    Default(ThreadLocal<TraceContext> local) {
      if (local == null) throw new NullPointerException("local == null");
      this.local = local;
    }

    @Override public TraceContext get() {
      return local.get();
    }

    //替换当前TraceContext，close方法将之前的TraceContext设置回去
    //Scope接口继承了Closeable接口，在try中使用会自动调用close方法，为了避免用户忘记close方法，还提供了Runnable，Callable，Executor，ExecutorService包装方法
    @Override public Scope newScope(@Nullable TraceContext currentSpan) {
      final TraceContext previous = local.get();
      local.set(currentSpan);
      class DefaultCurrentTraceContextScope implements Scope {
        @Override public void close() {
          local.set(previous);
        }
      }
      return new DefaultCurrentTraceContextScope();
    }
  }

Slf4jCurrentTraceContext的delegate使用的就是一个InheritableThreadLocal，InheritableThreadLocal在创建子线程的时候，会将父线程的inheritableThreadLocals继承下来。这样就实现了TraceContext在父子线程中的传递。

看一下CurrentTraceContext的maybeScope：

//返回一个新的scope，如果当前scope就是传入的scope，返回一个空scope
  public Scope maybeScope(@Nullable TraceContext currentSpan) {
    //获取当前TraceContext
    TraceContext currentScope = get();
    //如果传入的TraceContext为空，且当前TraceContext为空返回空scope
    if (currentSpan == null) {
      if (currentScope == null) return Scope.NOOP;
      return newScope(null);
    }
    return currentSpan.equals(currentScope) ? Scope.NOOP : newScope(currentSpan);
  }

TracingFilter中HttpServerHandler解析Request：

请输入代码

2.srvA请求到servB时作为Client。

TraceLoadBalancerFeignClient-->LoadBalancerFeignClient-->FeignLoadBalancer-->LazyTracingFeignClient-->Client