Sentinel-Go 源码系列（三）滑动时间窗口算法的工程实现

type LeapArray struct {
 bucketLengthInMs uint32 // bucket大小
 sampleCount      uint32 // bucket数量
 intervalInMs     uint32 // 窗口总大小
 array            *AtomicBucketWrapArray // bucket数组
 updateLock mutex // 更新锁
}

type AtomicBucketWrapArray struct {
 base unsafe.Pointer // 数组的起始地址
 length int // 长度，不能改变
 data   []*BucketWrap // 真正bucket的数据
}

type BucketWrap struct {
 BucketStart uint64 // bucket起始时间
 Value atomic.Value // bucket数据结构，例如 MetricBucket
}

type MetricBucket struct {
 counter        [base.MetricEventTotal]int64 // 计数数组，可放不同类型
 minRt          int64 // 最小RT
 maxConcurrency int32 // 最大并发数
}

// ①
sn.AddCount(base.MetricEventPass, int64(count))

// ②
func (bla *BucketLeapArray) AddCount(event base.MetricEvent, count int64) {
 bla.addCountWithTime(util.CurrentTimeMillis(), event, count)
}

// ③
func (bla *BucketLeapArray) addCountWithTime(now uint64, event base.MetricEvent, count int64) {
 b := bla.currentBucketWithTime(now)
 if b == nil {
  return
 }
 b.Add(event, count)
}

// ④
func (mb *MetricBucket) Add(event base.MetricEvent, count int64) {
 if event >= base.MetricEventTotal || event < 0 {
  logging.Error(errors.Errorf("Unknown metric event: %v", event), "")
  return
 }
 if event == base.MetricEventRt {
  mb.AddRt(count)
  return
 }
 mb.addCount(event, count)
}

// ⑤
func (mb *MetricBucket) addCount(event base.MetricEvent, count int64) {
 atomic.AddInt64(&mb.counter[event], count)
}

func (bla *BucketLeapArray) currentBucketWithTime(now uint64) *MetricBucket {
 // ①根据当前时间取bucket
 curBucket, err := bla.data.currentBucketOfTime(now, bla)
 ...
 b, ok := mb.(*MetricBucket)
 if !ok {
  ...
  return nil
 }
 return b
}

func (la *LeapArray) currentBucketOfTime(now uint64, bg BucketGenerator) (*BucketWrap, error) {
 ...
 // ②计算index = (now / bucketLengthInMs) % LeapArray.array.length
 idx := la.calculateTimeIdx(now)
 // ③计算bucket开始时间 = now - (now % bucketLengthInMs)
 bucketStart := calculateStartTime(now, la.bucketLengthInMs)

 for { 
  old := la.array.get(idx)
  if old == nil { // ④未使用，直接返回
   newWrap := &BucketWrap{
    BucketStart: bucketStart,
    Value:       atomic.Value{},
   }
   newWrap.Value.Store(bg.NewEmptyBucket())
   if la.array.compareAndSet(idx, nil, newWrap) {
    return newWrap, nil
   } else {
    runtime.Gosched()
   }
  } else if bucketStart == atomic.LoadUint64(&old.BucketStart) { // ⑤刚好取到是当前bucket，返回
   return old, nil
  } else if bucketStart > atomic.LoadUint64(&old.BucketStart) { // ⑥取到了旧的bucket，重置使用
   if la.updateLock.TryLock() {
    old = bg.ResetBucketTo(old, bucketStart)
    la.updateLock.Unlock()
    return old, nil
   } else {
    runtime.Gosched()
   }
  } else if bucketStart < atomic.LoadUint64(&old.BucketStart) { // ⑦取到了比当前还新的bucket，总共只有一个bucket时，并发情况可能会出现这种情况，其他情况不可能，直接报错
   if la.sampleCount == 1 {
    return old, nil
   }
   
   return nil, errors.New(fmt.Sprintf("Provided time timeMillis=%d is already behind old.BucketStart=%d.", bucketStart, old.BucketStart))
  }
 }
}

qps := stat.InboundNode().GetQPS(base.MetricEventPass)

func (m *SlidingWindowMetric) getBucketStartRange(timeMs uint64) (start, end uint64) {
 curBucketStartTime := calculateStartTime(timeMs, m.real.BucketLengthInMs())
 end = curBucketStartTime
 start = end - uint64(m.intervalInMs) + uint64(m.real.BucketLengthInMs())
 return
}