滑动时间算法与sentinel实践

滑动时间算法

图示

在上图中，整个红色的矩形框表示一个时间窗口，在我们的例子中，一个时间窗口就是一分钟。然后我们将时间窗口进
行划分，比如图中，我们就将滑动窗口划成了6格，所以每格代表的是10秒钟。每过10秒钟，我们的时间窗口就会往右
滑动一格。每一个格子都有自己独立的计数器counter，比如当一个请求 在0:35秒的时候到达，那么0:30~0:39对应的
counter就会加1。
计数器算法其实就是滑动窗口算法。只是它没有对时间窗口做进一步地划分，所以只有1格。
由此可见，当滑动窗口的格子划分的越多，那么滑动窗口的滚动就越平滑，限流的统计就会越精确

• 简单代码举例

sentinel的滑动时间窗口

sentinel 统计源码

//主要是这个方法
node.addPassRequest(count);
    
@Override
    public void addPassRequest(int count) {
        super.addPassRequest(count);
        this.clusterNode.addPassRequest(count);
    }

    @Override
    public void addPassRequest(int count) {
        rollingCounterInSecond.addPass(count);
        rollingCounterInMinute.addPass(count);
    }

这两个是一样的，拿rollingCounterInSecond.addPass(count);举例
    @Override
    public void addPass(int count) {
//获取当前的时间窗口
        WindowWrap wrap = data.currentWindow();
//添加pass的值
        wrap.value().addPass(count);
    }

    public WindowWrap currentWindow(long timeMillis) {
        if (timeMillis < 0) {
            return null;
        }

        //计算当前的时间是在哪一块区域里
        int idx = calculateTimeIdx(timeMillis);
        // 计算当前的时间开始值(时间区间数组的下标开始值)
        long windowStart = calculateWindowStart(timeMillis);

        /*
         * Get bucket item at given time from the array.从时间区间array里面取bucket
         *
         * (1) Bucket is absent, then just create a new bucket and CAS update to circular array.
              //如果该时间区间取不到，则用cas创建新的时间区间
         * (2) Bucket is up-to-date, then just return the bucket.
              //如果该时间区间正常准备更新，则返回
         * (3) Bucket is deprecated, then reset current bucket and clean all deprecated buckets.
              //如果该时间废弃了，则重置bucket并且清空所有bucket里面的内容
         */
        while (true) {
            WindowWrap old = array.get(idx);
            if (old == null) {
                /*
                 *     B0       B1      B2    NULL      B4
                 * ||_______|_______|_______|_______|_______||___
                 * 200     400     600     800     1000    1200  timestamp
                 *                             ^
                 *                          time=888
                 *        bucket 为NULL则重新创建 ，   bucket is empty, so create new and update
                 * 如果旧bucket不存在，则重新创建
                 * If the old bucket is absent, then we create a new bucket at {@code windowStart},
                 * 然后cas来循环修改，只允许一个线程修改成功，其他线程都yield 掉
                 * then try to update circular array via a CAS operation. Only one thread can
                 * succeed to update, while other threads yield its time slice.
                 */
//新建一个windowrap(下面有图解)
                WindowWrap window = new WindowWrap(windowLengthInMs, windowStart, newEmptyBucket(timeMillis));
                if (array.compareAndSet(idx, null, window)) {
                    // Successfully updated, return the created bucket.
                    return window;
                } else {
                    // Contention failed, the thread will yield its time slice to wait for bucket available.
                    Thread.yield();
                }
            } else if (windowStart == old.windowStart()) {
                /*
                 *     B0       B1      B2     B3      B4
                 * ||_______|_______|_______|_______|_______||___
                 * 200     400     600     800     1000    1200  timestamp
                 *                             ^
                 *                          time=888
                 *            startTime of Bucket 3: 800, so it's up-to-date
                 *
                 * If current {@code windowStart} is equal to the start timestamp of old bucket,
                 * that means the time is within the bucket, so directly return the bucket.
                 */
                return old;
            } else if (windowStart > old.windowStart()) {
                /*
                 *   (old)
                 *             B0       B1      B2    NULL      B4
                 * |_______||_______|_______|_______|_______|_______||___
                 * ...    1200     1400    1600    1800    2000    2200  timestamp
                 *                              ^
                 *                           time=1676
                 *          startTime of Bucket 2: 400, deprecated, should be reset
                 *
                 * If the start timestamp of old bucket is behind provided time, that means
                 * the bucket is deprecated. We have to reset the bucket to current {@code windowStart}.
                 * Note that the reset and clean-up operations are hard to be atomic,
                 * so we need a update lock to guarantee the correctness of bucket update.
                 *
                 * The update lock is conditional (tiny scope) and will take effect only when
                 * bucket is deprecated, so in most cases it won't lead to performance loss.
                 */
                if (updateLock.tryLock()) {
                    try {
                        // Successfully get the update lock, now we reset the bucket.
                        return resetWindowTo(old, windowStart);
                    } finally {
                        updateLock.unlock();
                    }
                } else {
                    // Contention failed, the thread will yield its time slice to wait for bucket available.
                    Thread.yield();
                }
            } else if (windowStart < old.windowStart()) {
                // Should not go through here, as the provided time is already behind.
                return new WindowWrap(windowLengthInMs, windowStart, newEmptyBucket(timeMillis));
            }
        }
    }

生成的wrap包含PASS，Block等是否通过等字段

• 限流统计时的计算方法

    @Override
    public long pass() {
        //首先要计算当前时间区间(该替换的替换，该新建的新建)
        data.currentWindow();
        long pass = 0;
        List list = data.values();
        //然后再将每个通过数相加
        for (MetricBucket window : list) {
            pass += window.pass();
        }
        return pass;
    }