Breaker interface {
   Name() string
   Allow() (Promise, error)
   Do(req func () error) error
   DoWithAcceptable(req func () error, acceptable Acceptable) error
   DoWithFallback(req func () error, fallback func (err error) error) error
   DoWithFallbackAcceptable(req func () error, fallback func (err error) error, acceptable Acceptable) error
}

func BreakerHandler(method, path string, metrics *stat.Metrics) func(http.Handler) http.Handler {
 brk := breaker.NewBreaker(breaker.WithName(strings.Join([]string{method, path}, breakerSeparator)))
 return func(next http.Handler) http.Handler {
     return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
         promise, err := brk.Allow()// 熔斷執(zhí)行邏輯，用戶自行控制
         if err != nil {
             metrics.AddDrop()
             logx.Errorf("[http] dropped, %s - %s - %s",
                 r.RequestURI, httpx.GetRemoteAddr(r), r.UserAgent())
             w.WriteHeader(http.StatusServiceUnavailable)
             return
         }
         cw := &response.WithCodeResponseWriter{Writer: w}
         defer func() {// 熔斷指標(biāo)采集，用戶自行控制
             if cw.Code < http.StatusInternalServerError {
                 promise.Accept()
             } else {
                 promise.Reject(fmt.Sprintf("%d %s", cw.Code, http.StatusText(cw.Code)))
             }
         }()
         next.ServeHTTP(cw, r)
     })
 }
}

doReq(req func() error, fallback func(err error) error, acceptable Acceptable) error

func (b *googleBreaker) doReq(req func() error, fallback func(err error) error, acceptable Acceptable) error {
    // 判斷熔斷器是否開啟
    if err := b.accept(); err != nil {
        if fallback != nil {
            return fallback(err)
        }
        return err
    }
    // 函數(shù)異常退出，上報(bào)失敗指標(biāo)
    defer func() {
        if e := recover(); e != nil {
            b.markFailure()
            panic(e)
        }
    }()
    // 執(zhí)行用戶函數(shù)，并通過函數(shù)執(zhí)行返回的錯(cuò)誤信息由用戶判斷是否為可接受的錯(cuò)誤
    // 上報(bào)指標(biāo)
    err := req()
    if acceptable(err) {
        b.markSuccess()
    } else {
        b.markFailure()
    }
    return err
}

func (b *googleBreaker) accept() error {
    accepts, total := b.history()
    weightedAccepts := b.k * float64(accepts)
    // https://landing.google.com/sre/sre-book/chapters/handling-overload/#eq2101
    dropRatio := math.Max(0, (float64(total-protection)-weightedAccepts)/float64(total+1))
    if dropRatio <= 0 {
        return nil
    }
    if b.proba.TrueOnProba(dropRatio) {
        return ErrServiceUnavailable
    }
    return nil
}
func (b *googleBreaker) history() (accepts, total int64) {
   b.stat.Reduce(func(b *collection.Bucket) {
      accepts += int64(b.Sum)
      total += b.Count
   })
    return
}

// 標(biāo)記成功一次
func (b *googleBreaker) markSuccess() {
    b.stat.Add(1)
}
// 標(biāo)記失敗一次
func (b *googleBreaker) markFailure() {
    b.stat.Add(0)
}