Zipkin Brave源碼解讀-Tracing（全鏈路跟蹤埋點）（一）

最近在研究全鏈路跟蹤，因某些原因選用了 ZipKin 的 Brave 作為埋點工具，ZipKin不使用，本系列僅做 Brave(版本=5.6.0) 部分源碼解讀萨醒，其他內(nèi)容不涉及。

對全鏈路不熟悉的苇倡，先學(xué)習(xí) opentracing 的標(biāo)準(zhǔn)規(guī)范：https://opentracing-contrib.github.io/opentracing-specification-zh/specification.html

先來看一個簡單埋點Demo：

import brave.Span;
import brave.Tracer;
import brave.Tracer.SpanInScope;
import brave.Tracing;
import brave.propagation.B3Propagation;
import brave.propagation.ExtraFieldPropagation;

public class Atest {
    public static void main(String[] args) {
        Tracing tracing = Tracing.newBuilder().localServiceName("test")
                .propagationFactory(ExtraFieldPropagation.newFactory(B3Propagation.FACTORY, "user-name"))
                .build();

        Tracer tracer = tracing.tracer();
        Span root = tracer.nextSpan().name("root").start();//創(chuàng)建跨度 root
        SpanInScope scope = null;
        try {
            scope = tracer.withSpanInScope(root);//設(shè)置 root 跨度的作用域
//開始新的跨度 s1富纸。此處使用 currentTraceContext().get() 獲取到當(dāng)前作用域中的 TraceContext
//TraceContext 中包含著鏈路中的關(guān)鍵信息，如 TraceId, parentId, spanId 等
            Span s1 = tracer.newChild(tracing.currentTraceContext().get()).name("s1").start();
            System.out.println("被跟蹤的業(yè)務(wù)代碼...");
            s1.finish();//結(jié)束跨度 s1
        } catch (Exception e) {
            root.error(e);//報錯處理
        } finally {
            scope.close();//結(jié)束作用域
        }
        root.finish();//結(jié)束跨度 root
    }
}

執(zhí)行代碼旨椒，使用的是 Brave 默認(rèn)的 LoggerReporter晓褪，輸出的日志如下，默認(rèn)使用的是V2版本综慎，比起V1版本的日志涣仿，節(jié)省了很多日志內(nèi)容，更加簡潔易懂且提升性能示惊。
可見“被跟蹤的業(yè)務(wù)代碼...”先被打印出來好港，接著打印的是跨度 s1，它的父節(jié)點是“b236c6f732bca43f”米罚，spanId是“a41bf8dd8fba3f16”钧汹，耗時 138 微秒，serviceName是在Tracing設(shè)置的“test”录择，ip地址是“192.168.1.6”
最后打印的是跨度 root拔莱，一個完整簡單的鏈路跟蹤就完成了碗降。

被跟蹤的業(yè)務(wù)代碼...
一月 24, 2019 10:20:11 下午 brave.Tracing$LoggingReporter report
信息: {"traceId":"b236c6f732bca43f","parentId":"b236c6f732bca43f","id":"a41bf8dd8fba3f16","name":"s1","timestamp":1548339611584476,"duration":138,"localEndpoint":{"serviceName":"test","ipv4":"192.168.1.6"}}
一月 24, 2019 10:20:11 下午 brave.Tracing$LoggingReporter report
信息: {"traceId":"b236c6f732bca43f","id":"b236c6f732bca43f","name":"root","timestamp":1548339611569087,"duration":53153,"localEndpoint":{"serviceName":"test","ipv4":"192.168.1.6"}}

本節(jié)先看 Tracing 的代碼節(jié)選，Tracing 主要用于初始化鏈路跟蹤所需的各組件塘秦，使用了 Builder 的模式讼渊，可根據(jù)需求自由去創(chuàng)建合適的鏈路跟蹤特性。
譬如上面所使用的默認(rèn)的 LoggingReporter嗤形，可修改為上送到ZipKin的Reporter精偿。服務(wù)名，傳播模式等都可以通過Builder定制創(chuàng)建赋兵。

public abstract class Tracing implements Closeable {
        //Tracing 內(nèi)部類 Builder見下方，許多組件都給了缺省定義
    public static final class Builder {
        //服務(wù)名與服務(wù)器Ip
        String localServiceName = "unknown", localIp;
        int localPort; // 服務(wù)端口
        // reporter搔预，用于處理（常見上報給ZipKin或打印到本地）鏈路信息
        Reporter<zipkin2.Span> spanReporter;
        Clock clock;//用于計時
       //采樣器霹期，用于定義采樣規(guī)則，默認(rèn)全樣采集
        Sampler sampler = Sampler.ALWAYS_SAMPLE; 
        //用于獲取當(dāng)前 TraceContext 拯田，默認(rèn)使用了 InheritableThreadLocal历造，支持復(fù)制到異步線程
        CurrentTraceContext currentTraceContext = CurrentTraceContext.Default.inheritable();
        //顧名思義，traceId是否128bit船庇，是否支持Join一個跨度
        boolean traceId128Bit = false, supportsJoin = true;
        //傳播工廠吭产，用于定義傳播規(guī)則，如何注入與提取等鸭轮。
        Propagation.Factory propagationFactory = B3Propagation.FACTORY;
        //錯誤處理器
        ErrorParser errorParser = new ErrorParser();
        //span結(jié)束回調(diào)器
        List<FinishedSpanHandler> finishedSpanHandlers = new ArrayList<>();

   //執(zhí)行build方法創(chuàng)建 Tracing
    public Tracing build() {
      //根據(jù)不同的jdk獲取clock（Brave支持Jdk1.6+）
      if (clock == null) clock = Platform.get().clock();
      //根據(jù)不同的jdk獲取ip
      if (localIp == null) localIp = Platform.get().linkLocalIp();
      //默認(rèn)reporter就是在此處定義了
      if (spanReporter == null) spanReporter = new LoggingReporter();
      //將 Builder 傳入創(chuàng)建 Tracing臣淤，Default 是 Tracing一個內(nèi)部類，見下方代碼
      return new Default(this);
     }
    }

static final class Default extends Tracing {
//代碼太長窃爷，見下方代碼塊邑蒋，單獨說Default
    }
}

static final class Default extends Tracing {
    final Tracer tracer;//Tracer 可以理解為鏈路對象，用于操作span
    final Propagation.Factory propagationFactory;
    final Propagation<String> stringPropagation;
    final CurrentTraceContext currentTraceContext;
    final Sampler sampler;
    final Clock clock;
    final ErrorParser errorParser;
    final AtomicBoolean noop;

    Default(Builder builder) {
      //初始化過程先默認(rèn)從builder中獲取所需對象
      this.clock = builder.clock;
      this.errorParser = builder.errorParser;
      this.propagationFactory = builder.propagationFactory;
      this.stringPropagation = builder.propagationFactory.create(Propagation.KeyFactory.STRING);
      this.currentTraceContext = builder.currentTraceContext;
      this.sampler = builder.sampler;
      this.noop = new AtomicBoolean();

      List<FinishedSpanHandler> finishedSpanHandlers = builder.finishedSpanHandlers;

      // If a Zipkin reporter is present, it is invoked after the user-supplied finished span handlers.
      FinishedSpanHandler zipkinFirehose = FinishedSpanHandler.NOOP;
      if (builder.spanReporter != Reporter.NOOP) {//若 reporter 不是空操作
        zipkinFirehose = new ZipkinFinishedSpanHandler(builder.spanReporter, errorParser,
            builder.localServiceName, builder.localIp, builder.localPort);
        finishedSpanHandlers = new ArrayList<>(finishedSpanHandlers);
        finishedSpanHandlers.add(zipkinFirehose);
      }

      // 將所有的 FinishedSpanHandler 歸集成一個 finishedSpanHandler 
      FinishedSpanHandler finishedSpanHandler =
          FinishedSpanHandlers.noopAware(FinishedSpanHandlers.compose(finishedSpanHandlers), noop);
      //創(chuàng)建一個 Tracer按厘，差不多也是將各種組件初始化到 Tracer 中
      this.tracer = new Tracer(
          builder.clock,
          builder.propagationFactory,
          finishedSpanHandler,
          new PendingSpans(clock, zipkinFirehose, noop),
          builder.sampler,
          builder.currentTraceContext,
          builder.traceId128Bit || propagationFactory.requires128BitTraceId(),
          builder.supportsJoin && propagationFactory.supportsJoin(),
          finishedSpanHandler.alwaysSampleLocal(),
          noop
      );
      maybeSetCurrent();//確保Tracing 唯一
    }
    private void maybeSetCurrent() {
      if (current != null) return;
      synchronized (Tracing.class) {
        if (current == null) current = this;
      }
    }
}

Tracing 的解讀基本完成医吊，看Tracing的定義，基本清楚知道整個鏈路跟蹤流程中會使用到的組件有哪些逮京，他們的作用也大概能得知卿堂。
下一章繼續(xù)講 Tracer。