skywalking-go 原理剖析

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本文代码示例基于 skywalking-go v0.3.0

1. 引言

apache/skywalking-go 的内部实现比较复杂，本文针对几个核心的点进行展开，以便于大家更好的阅读和理解。

2.TracingID生成

详细参考代码 id.go

skywalking-go中的TraceID形如： "ab95670cae9d11ee87ca5626e1cdcfe2.14.39165238789840005"

一共由4部分组成:

1）uuid 使用uuid version1
2）goroutine ID，golang协程ID
3）timestamp 自1900年以来的毫秒值
4）sequence，从0 ~ 10000

3. 跨协程传递Tracing信息

在没有skywalking-go的情况下，如果想在多个协程中传递Tracing信息, 开发者必须要用到context。使用skywalking-go之后，很多细节，开发者无需关心。

下面是一个在gin中使用多协程的示例，完整代码见参考资料2

    r.GET("/ping", func(c *gin.Context) {
        g, _ := errgroup.WithContext(context.Background())

        g.Go(func() error {
            val, err := rdb.Incr(context.Background(), "helloCounter2").Result()
            zlog.Info("ping", zap.Int64("val", val), zap.Error(err))
            return nil
        })
        g.Go(func() error {
            hsetRes, err := rdb.HSet(context.Background(), "xyz", "def", 0).Result()
            zlog.Info("ping", zap.Int64("setRes", hsetRes), zap.Error(err))
            return nil
        })
        c.JSON(http.StatusOK, gin.H{
            "message": "pong",
        })
    })

Besides using g object as the in-goroutine context propagation, SkyWalking builds a mechanism to propagate context between Goroutines.

When a new goroutine is started on an existing goroutine, the runtime.newproc1 method is called to create a new goroutine based on the existing one. The agent would do context-copy from the previous goroutine to the newly created goroutine. The new context in the goroutine only shares limited information to help continues tracing.

skywalking-go 对协程G对应的数据结构进行了增强

type g struct {
    ...
    // Tracing信息保存在skywalking_tls中
    skywalking_tls interface{}
}

newproc1() 用于创建子协程

proc.go

func newproc1(fn *funcval, callergp *g, callerpc uintptr) (skywalking_result_0 *g) {
    defer func() {
            // 把父协程的tracing信息传递给子协程
        skywalking_result_0.skywalking_tls = goroutineChange(callergp.skywalking_tls)
    }()

    if fn == nil {
        fatal("go of nil func value")
    }
    ...
}

func goroutineChange(tls interface{}) interface{} {
   if tls == nil {
      return nil
   }
   if taker, ok := tls.(ContextSnapshoter); ok {
      return taker.TakeSnapShot(tls)
   }
   return tls
}

对应的skywalking-go代码位于runtime/instrument.go

4. 跨服务传递Tracing信息

跨服务传递Tracing信息都是在plugin中实现的，以net/http请求为例

Client端 plugins/http/client_intercepter.go

func (h *ClientInterceptor) BeforeInvoke(invocation operator.Invocation) error {
    request := invocation.Args()[0].(*http.Request)
    s, err := tracing.CreateExitSpan(fmt.Sprintf("%s:%s", request.Method, request.URL.Path), request.Host, 
        // 在Header中添加新的键值对，传递Tracing信息给其他微服务
    func(headerKey, headerValue string) error {
        request.Header.Add(headerKey, headerValue)
        return nil
    }, tracing.WithLayer(tracing.SpanLayerHTTP),
        tracing.WithTag(tracing.TagHTTPMethod, request.Method),
        tracing.WithTag(tracing.TagURL, request.Host+request.URL.Path),
        tracing.WithComponent(5005))
    ...
    return nil
}

Server端 plugins/http/server_intercepter.go

func (h *ServerInterceptor) BeforeInvoke(invocation operator.Invocation) error {
    request := invocation.Args()[1].(*http.Request)
    s, err := tracing.CreateEntrySpan(fmt.Sprintf("%s:%s", request.Method, request.URL.Path), 
        // 从Http Header中提取Tracing信息 
    func(headerKey string) (string, error) {
        return request.Header.Get(headerKey), nil
    }, tracing.WithLayer(tracing.SpanLayerHTTP),
        tracing.WithTag(tracing.TagHTTPMethod, request.Method),
        tracing.WithTag(tracing.TagURL, request.Host+request.URL.Path),
        tracing.WithComponent(5004))
    ...
    return nil
}

细心的读者可能留意到Skywalking-go中，

所有需要向其他服务传递Tracing信息的情况 ,使用的都是

func CreateExitSpan(operationName, peer string, injector Injector, opts ...SpanOption) (s Span, err error)

injector is the injector to inject the context into the carrier.

所有需要从请求中提取Tracing信息的情况，使用的都是

func CreateEntrySpan(operationName string, extractor Extractor, opts ...SpanOption) (s Span, err error)

extractor is the extractor to extract the context from the carrier.

server端收到的Header信息

User-Agent: go-resty/2.10.0 (https://github.com/go-resty/resty)
Sw8: 1-NjE2NTUzNGFhZjY1MTFlZWJmYzc1NjI2ZTFjZGNmZTIuMTMxLjM5MTY2MjAyNjQ2NzQwMDAz-NjE2NTUzNGFhZjY1MTFlZWJmYzc1NjI2ZTFjZGNmZTIuMTMxLjM5MTY2MjAyNjQ2NzQwMDA0-2-c3ctdGVzdA==-NjE2NTczNzBhZjY1MTFlZWJmYzc1NjI2ZTFjZGNmZTJAMTAuMi4xMzAuMTQ=-R0VUOi9zYXlIZWxsb0h0dHA=-bG9jYWxob3N0OjE4MDAx
Sw8-Correlation: 
Accept-Encoding: gzip

序列化和反序列化的代码在plugins/core/propagating.go 中

func (s *SpanContext) DecodeSW8(header string) error {
    ...
}
func (s *SpanContext) EncodeSW8() string {
    return strings.Join([]string{
        strconv.Itoa(int(s.Sample)),
        encodeBase64(s.TraceID),
        encodeBase64(s.ParentSegmentID),
        strconv.Itoa(int(s.ParentSpanID)),
        encodeBase64(s.ParentService),
        encodeBase64(s.ParentServiceInstance),
        encodeBase64(s.ParentEndpoint),
        encodeBase64(s.AddressUsedAtClient),
    }, "-")
}

下图是请求http://localhost:8000/sayHelloHttp 获取的Tracing图表