opentracing是什么？

opentracing() 是分布式跟踪系统，当我们把系统拆成服务化，分布式系统的时候，查询一个问题，很可能需要多个登录多台机器。

opentracing 定义了一套api

通过提供平台无关、厂商无关的API，使得开发人员能够方便的添加（或更换）追踪系统的实现。OpenTracing正在为全球的分布式追踪，提供统一的概念和数据标准。

opentracing 在哪里有java版？

从github，下载java版本的分支( )

从下面的图，可以看出api为定义模块，mock挡板 noop空实现 impl具体实现

opentracing 定义了哪些api?

span

一条消息，相当于一个打点信息，里面有两个重要的方法

finish 和其他

finish 用来完成span的信息，并且用于提交而其他方法用来组装span的数据

SpanContext

及传输的上下文，保存一些数据，一般loaclThread的，主要方法

Iterable<Map.Entry<String, String>> baggageItems();

Tracer

定义了两个方法,一个是往spancontext 插入参数，一个获取spancontext，相当于span集合


    
      void inject(SpanContext spanContext, Format
     
       format, C carrier);
      
        SpanContext extract(Format
       
         format, C carrier);

Format其实就是定义的几个map

public interface Format
    
      {    final class Builtin
     
       implements Format
      
        {         public final static Format
       
         TEXT_MAP = new Builtin
        
         ();	public final static Format
         
           HTTP_HEADERS = new Builtin
          
           (); public final static Format
           
             BINARY = new Builtin
            
             (); }

SpanBuilder

顾名思义，builder模式，用来构建 span, 关键是start方法，构建成功

interface SpanBuilder extends SpanContext {      /**       * A shorthand for addReference(References.CHILD_OF, parent).       */      SpanBuilder asChildOf(SpanContext parent);      /**       * A shorthand for addReference(References.CHILD_OF, parent.context()).       */      SpanBuilder asChildOf(Span parent);      /**       * Add a reference from the Span being built to a distinct (usually parent) Span. May be called multiple times to       * represent multiple such References.       *       * @param referenceType the reference type, typically one of the constants defined in References       * @param referencedContext the SpanContext being referenced; e.g., for a References.CHILD_OF referenceType, the       *                          referencedContext is the parent       *       * @see io.opentracing.References       */      SpanBuilder addReference(String referenceType, SpanContext referencedContext);      /** Same as {@link Span#setTag(String, String)}, but for the span being built. */      SpanBuilder withTag(String key, String value);      /** Same as {@link Span#setTag(String, String)}, but for the span being built. */      SpanBuilder withTag(String key, boolean value);      /** Same as {@link Span#setTag(String, String)}, but for the span being built. */      SpanBuilder withTag(String key, Number value);      /** Specify a timestamp of when the Span was started, represented in microseconds since epoch. */      SpanBuilder withStartTimestamp(long microseconds);      /** Returns the started Span. */      Span start();  }

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