分布式追踪工具 Spring cloud sleuth

1. 介绍

spring cloud sleuth是用来解决分布式中服务的跟踪。span 和trace的图解如下：

2. 实现：

• 为了实现请求跟踪，当请求发送到分布式系统的入口断点时，只需要服务跟踪框架为该请求创建一个唯一的跟踪标识，同时在分布式系统内部流转的时候，框架始终保持传递该唯一标识。直到返回给请求为止，这个ID就是TraceId。
• 为了统计各处理单元的时间延时，当请求到达各个服务组件时，或是处理逻辑到达某个状态时，也通过一个唯一标识来标记它的开始、具体过程以及结束，该标识就是Span ID。对于每个Span来说，通过记录开始Span和结束Span的时间戳，就能统计出该Span的时间延时，除了时间戳记录之外，它可以包含一些其他元数据，比如事件名称、请求信息等。
• Spring Cloud Sleuth 有一个 Sampler 策略，可以通过这个实现类来控制采样算法。采样器不会阻碍 span 相关 id 的产生，但是会对导出以及附加事件标签的相关操作造成影响。 Sleuth 默认采样算法的实现是 Reservoir sampling，具体的实现类是 PercentageBasedSampler，默认的采样比例为: 0.1(即 10%)。不过我们可以通过spring.sleuth.sampler.percentage来设置，所设置的值介于 0.0 到 1.0 之间，1.0 则表示全部采集。

2.1log实现

在日志中打印traceId
通过ThreadLocal 、MDC进行实现。

2.2 RestTemplate TraceId实现

通过实现RestTemplate拦截器接口：ClientHttpRequestInterceptor

private void setInterceptors(HttpTracing httpTracing) {
		this.template.setInterceptors(Arrays.<ClientHttpRequestInterceptor>asList(
				TracingClientHttpRequestInterceptor.create(httpTracing)));
	}

2.3. Http请求参数传递

参数传递一般通过拦截器的形式进行注入：

• X-B3-TraceId：一条请求链路Trace的唯一标识
• X-B3-SpanId：一个工作单元(Span)的唯一标识
• X-B3-ParentSpandId：标识当前工作单元所属的上一个工作单元，Root Span(请求链路的第一个工作单元)的该值为空。
• X-Span-Name：工作单元的名称

2.4 在异步调用中Tracer传递实现

• 使用切面@Aspect进行拦截
• 封装Runable 和Callble。在封装类中引入Tracer类
  举例 Scheduler：

@Aspect
public class TraceSchedulingAspect {
	private static final Log log = LogFactory.getLog(TraceSchedulingAspect.class);
	private final Tracer tracer;
	private final Pattern skipPattern;
	private final TraceKeys traceKeys;

	public TraceSchedulingAspect(Tracer tracer, Pattern skipPattern,
			TraceKeys traceKeys) {
		this.tracer = tracer;
		log.info("TraceSchedulingAspect traceId " + tracer.toString());
		this.skipPattern = skipPattern;
		this.traceKeys = traceKeys;
	}

	@Around("execution (@org.springframework.scheduling.annotation.Scheduled * *.*(..))")
	public Object traceBackgroundThread(final ProceedingJoinPoint pjp) throws Throwable {
		if (this.skipPattern.matcher(pjp.getTarget().getClass().getName()).matches()) {
			return pjp.proceed();
		}
		String spanName = SpanNameUtil.toLowerHyphen(pjp.getSignature().getName());
		Span span = startOrContinueRenamedSpan(spanName);
		log.info("traceBackgroundThread spanId:" + span);
		try(Tracer.SpanInScope ws = this.tracer.withSpanInScope(span)) {
			span.tag(this.traceKeys.getAsync().getPrefix() +
					this.traceKeys.getAsync().getClassNameKey(), pjp.getTarget().getClass().getSimpleName());
			span.tag(this.traceKeys.getAsync().getPrefix() +
					this.traceKeys.getAsync().getMethodNameKey(), pjp.getSignature().getName());
			return pjp.proceed();
		} finally {
			span.finish();
		}
	}

	private Span startOrContinueRenamedSpan(String spanName) {
		Span currentSpan = this.tracer.currentSpan();
		if (currentSpan != null) {
			return currentSpan.name(spanName);
		}
		return this.tracer.nextSpan().name(spanName);
	}

}

tracer.currentSpan()

/** * Returns the current span in scope or null if there isn't one. * 返回当前线程作用域的span，如果不存在返回null * 
When entering user code, prefer {@link #currentSpanCustomizer()} as it is a stable type and * will never return null. toSpan(currentContext);使用上下文创建一个span */
  @Nullable public Span currentSpan() {
    TraceContext currentContext = currentTraceContext.get();
    return currentContext != null ? toSpan(currentContext) : null;
  }

nextSpan()

  /** Returns a new child span if there's a {@link #currentSpan()} or a new trace if there isn't. */
  public Span nextSpan() {
    TraceContext parent = currentTraceContext.get();
    return parent == null ? newTrace() : newChild(parent);
  }

tracer.withSpanInScope()
设置span的作用域到线程上下文中：默认实现：

  /** * Makes the given span the "current span" and returns an object that exits that scope on close. * 返回一个特定范围的对象。在对象关闭之前，一直保存这。 * Calls to {@link #currentSpan()} and {@link #currentSpanCustomizer()} will affect this span * until the return value is closed. * * 
The most convenient way to use this method is via the try-with-resources idiom. * * Ex. * 
{@code * // Assume a framework interceptor uses this method to set the inbound span as current * try (SpanInScope ws = tracer.withSpanInScope(span)) { * return inboundRequest.invoke(); * } finally { * span.finish(); * } * * // An unrelated framework interceptor can now lookup the correct parent for outbound requests * Span parent = tracer.currentSpan() * Span span = tracer.nextSpan().name("outbound").start(); // parent is implicitly looked up * try (SpanInScope ws = tracer.withSpanInScope(span)) { * return outboundRequest.invoke(); * } finally { * span.finish(); * } * }
 * * 
Note: While downstream code might affect the span, calling this method, and calling close on * the result have no effect on the input. For example, calling close on the result does not * finish the span. Not only is it safe to call close, you must call close to end the scope, or * risk leaking resources associated with the scope. * * @param span span to place into scope or null to clear the scope */
  public SpanInScope withSpanInScope(@Nullable Span span) {
    return new SpanInScope(currentTraceContext.newScope(span != null ? span.context() : null));
  }

   @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();
    }
  }

4.多线程执行上下文传递

Runnables、@Async Support、线程池的支持：
重点TraceRunnable 类：

**
 * Runnable that passes Span between threads. The Span name is
 * taken either from the passed value or from the {@link SpanNamer}
 * interface.
 *
 * @author Spencer Gibb
 * @author Marcin Grzejszczak
 * @since 1.0.0
 */
public class TraceRunnable implements Runnable {

	/** * Since we don't know the exact operation name we provide a default * name for the Span */
	private static final String DEFAULT_SPAN_NAME = "async";

	private final Tracer tracer;
	private final Runnable delegate;
	private final Span span;
	private final ErrorParser errorParser;

	public TraceRunnable(Tracer tracer, SpanNamer spanNamer, ErrorParser errorParser, Runnable delegate) {
		this(tracer, spanNamer, errorParser, delegate, null);
	}

	public TraceRunnable(Tracer tracer, SpanNamer spanNamer, ErrorParser errorParser, Runnable delegate, String name) {
		this.tracer = tracer;
		this.delegate = delegate;
		String spanName = name != null ? name : spanNamer.name(delegate, DEFAULT_SPAN_NAME);
		//在主线程中执行
		this.span = this.tracer.nextSpan().name(spanName);
		this.errorParser = errorParser;
	}

	@Override
	public void run() {
	//在子线程中执行，进行传递
		Throwable error = null;
			try (SpanInScope ws = this.tracer.withSpanInScope(this.span.start())) {
				this.delegate.run();
			} catch (RuntimeException | Error e) {
				error = e;
				throw e;
			} finally {
				this.errorParser.parseErrorTags(this.span.customizer(), error);
				this.span.finish();
			}
	}
}

this.tracer.nextSpan()

  /** Returns a new child span if there's a {@link #currentSpan()} or a new trace if there isn't. */
  public Span nextSpan() {
    TraceContext parent = currentTraceContext.get();
    return parent == null ? newTrace() : newChild(parent);
  }

传递Span Context

在上下文传递时，Trace和Span ID是必传项，Baggage是个可选性。Buggage是一个key，value的map结构。在HTTP的请求头中，如果请求头开始:baggage-，则Spring sleuth可以理解。

Span initialSpan = this.tracer.nextSpan().name("span").start();
ExtraFieldPropagation.set(initialSpan.context(), "foo", "bar");
ExtraFieldPropagation.set(initialSpan.context(), "UPPER_CASE", "someValue");
}

tags

Span的中的tag是对特定span的tag。tag不是共享的，即不去传递。

参考：
https://www.baeldung.com/spring-cloud-sleuth-single-application