聊聊flink的Evictors

序

本文主要研究一下flink的Evictors

Evictor

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/windowing/evictors/Evictor.java

@PublicEvolving
public interface Evictor<T, W extends Window> extends Serializable {

    void evictBefore(Iterable<TimestampedValue<T>> elements, int size, W window, EvictorContext evictorContext);

    void evictAfter(Iterable<TimestampedValue<T>> elements, int size, W window, EvictorContext evictorContext);

    interface EvictorContext {

        long getCurrentProcessingTime();

        MetricGroup getMetricGroup();

        long getCurrentWatermark();
    }
}

• Evictor接收两个泛型，一个是element的类型，一个是窗口类型；它定义了evictBefore(在windowing function之前)、evictAfter(在windowing function之后)两个方法，它们都有EvictorContext参数；EvictorContext定义了getCurrentProcessingTime、getMetricGroup、getCurrentWatermark方法

CountEvictor

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/windowing/evictors/CountEvictor.java

@PublicEvolving
public class CountEvictor<W extends Window> implements Evictor<Object, W> {
    private static final long serialVersionUID = 1L;

    private final long maxCount;
    private final boolean doEvictAfter;

    private CountEvictor(long count, boolean doEvictAfter) {
        this.maxCount = count;
        this.doEvictAfter = doEvictAfter;
    }

    private CountEvictor(long count) {
        this.maxCount = count;
        this.doEvictAfter = false;
    }

    @Override
    public void evictBefore(Iterable<TimestampedValue<Object>> elements, int size, W window, EvictorContext ctx) {
        if (!doEvictAfter) {
            evict(elements, size, ctx);
        }
    }

    @Override
    public void evictAfter(Iterable<TimestampedValue<Object>> elements, int size, W window, EvictorContext ctx) {
        if (doEvictAfter) {
            evict(elements, size, ctx);
        }
    }

    private void evict(Iterable<TimestampedValue<Object>> elements, int size, EvictorContext ctx) {
        if (size <= maxCount) {
            return;
        } else {
            int evictedCount = 0;
            for (Iterator<TimestampedValue<Object>> iterator = elements.iterator(); iterator.hasNext();){
                iterator.next();
                evictedCount++;
                if (evictedCount > size - maxCount) {
                    break;
                } else {
                    iterator.remove();
                }
            }
        }
    }

    public static <W extends Window> CountEvictor<W> of(long maxCount) {
        return new CountEvictor<>(maxCount);
    }

    public static <W extends Window> CountEvictor<W> of(long maxCount, boolean doEvictAfter) {
        return new CountEvictor<>(maxCount, doEvictAfter);
    }
}

• CountEvictor实现了Evictor接口，其中element类型为Object；它有两个属性，分别是doEvictAfter、maxCount；其中doEvictAfter用于指定是使用evictBefore方法还是evictAfter方法；maxCount为窗口元素个数的阈值，超出则删掉

DeltaEvictor

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/windowing/evictors/DeltaEvictor.java

@PublicEvolving
public class DeltaEvictor<T, W extends Window> implements Evictor<T, W> {
    private static final long serialVersionUID = 1L;

    DeltaFunction<T> deltaFunction;
    private double threshold;
    private final boolean doEvictAfter;

    private DeltaEvictor(double threshold, DeltaFunction<T> deltaFunction) {
        this.deltaFunction = deltaFunction;
        this.threshold = threshold;
        this.doEvictAfter = false;
    }

    private DeltaEvictor(double threshold, DeltaFunction<T> deltaFunction, boolean doEvictAfter) {
        this.deltaFunction = deltaFunction;
        this.threshold = threshold;
        this.doEvictAfter = doEvictAfter;
    }

    @Override
    public void evictBefore(Iterable<TimestampedValue<T>> elements, int size, W window, EvictorContext ctx) {
        if (!doEvictAfter) {
            evict(elements, size, ctx);
        }
    }

    @Override
    public void evictAfter(Iterable<TimestampedValue<T>> elements, int size, W window, EvictorContext ctx) {
        if (doEvictAfter) {
            evict(elements, size, ctx);
        }
    }

    private void evict(Iterable<TimestampedValue<T>> elements, int size, EvictorContext ctx) {
        TimestampedValue<T> lastElement = Iterables.getLast(elements);
        for (Iterator<TimestampedValue<T>> iterator = elements.iterator(); iterator.hasNext();){
            TimestampedValue<T> element = iterator.next();
            if (deltaFunction.getDelta(element.getValue(), lastElement.getValue()) >= this.threshold) {
                iterator.remove();
            }
        }
    }

    @Override
    public String toString() {
        return "DeltaEvictor(" +  deltaFunction + ", " + threshold + ")";
    }

    public static <T, W extends Window> DeltaEvictor<T, W> of(double threshold, DeltaFunction<T> deltaFunction) {
        return new DeltaEvictor<>(threshold, deltaFunction);
    }

    public static <T, W extends Window> DeltaEvictor<T, W> of(double threshold, DeltaFunction<T> deltaFunction, boolean doEvictAfter) {
        return new DeltaEvictor<>(threshold, deltaFunction, doEvictAfter);
    }
}

• DeltaEvictor实现了Evictor接口，它有三个属性，分别是doEvictAfter、threshold、deltaFunction；其中doEvictAfter用于指定是使用evictBefore方法还是evictAfter方法；threshold为阈值，如果deltaFunction.getDelta方法(每个element与lastElement计算delta)算出来的值大于等于该值，则需要移除该元素

TimeEvictor

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/windowing/evictors/TimeEvictor.java

@PublicEvolving
public class TimeEvictor<W extends Window> implements Evictor<Object, W> {
    private static final long serialVersionUID = 1L;

    private final long windowSize;
    private final boolean doEvictAfter;

    public TimeEvictor(long windowSize) {
        this.windowSize = windowSize;
        this.doEvictAfter = false;
    }

    public TimeEvictor(long windowSize, boolean doEvictAfter) {
        this.windowSize = windowSize;
        this.doEvictAfter = doEvictAfter;
    }

    @Override
    public void evictBefore(Iterable<TimestampedValue<Object>> elements, int size, W window, EvictorContext ctx) {
        if (!doEvictAfter) {
            evict(elements, size, ctx);
        }
    }

    @Override
    public void evictAfter(Iterable<TimestampedValue<Object>> elements, int size, W window, EvictorContext ctx) {
        if (doEvictAfter) {
            evict(elements, size, ctx);
        }
    }

    private void evict(Iterable<TimestampedValue<Object>> elements, int size, EvictorContext ctx) {
        if (!hasTimestamp(elements)) {
            return;
        }

        long currentTime = getMaxTimestamp(elements);
        long evictCutoff = currentTime - windowSize;

        for (Iterator<TimestampedValue<Object>> iterator = elements.iterator(); iterator.hasNext(); ) {
            TimestampedValue<Object> record = iterator.next();
            if (record.getTimestamp() <= evictCutoff) {
                iterator.remove();
            }
        }
    }

    private boolean hasTimestamp(Iterable<TimestampedValue<Object>> elements) {
        Iterator<TimestampedValue<Object>> it = elements.iterator();
        if (it.hasNext()) {
            return it.next().hasTimestamp();
        }
        return false;
    }

    private long getMaxTimestamp(Iterable<TimestampedValue<Object>> elements) {
        long currentTime = Long.MIN_VALUE;
        for (Iterator<TimestampedValue<Object>> iterator = elements.iterator(); iterator.hasNext();){
            TimestampedValue<Object> record = iterator.next();
            currentTime = Math.max(currentTime, record.getTimestamp());
        }
        return currentTime;
    }

    @Override
    public String toString() {
        return "TimeEvictor(" + windowSize + ")";
    }

    @VisibleForTesting
    public long getWindowSize() {
        return windowSize;
    }

    public static <W extends Window> TimeEvictor<W> of(Time windowSize) {
        return new TimeEvictor<>(windowSize.toMilliseconds());
    }

    public static <W extends Window> TimeEvictor<W> of(Time windowSize, boolean doEvictAfter) {
        return new TimeEvictor<>(windowSize.toMilliseconds(), doEvictAfter);
    }
}

• TimeEvictor实现了Evictor接口，其中element类型为Object；它有两个属性，分别是doEvictAfter、windowSize；其中doEvictAfter用于指定是使用evictBefore方法还是evictAfter方法；windowSize用于指定窗口的时间长度，以窗口元素最大时间戳-windowSize为evictCutoff，所有timestamp小于等于evictCutoff的元素都将会被剔除

小结

• Evictor接收两个泛型，一个是element的类型，一个是窗口类型；它定义了evictBefore(在windowing function之前)、evictAfter(在windowing function之后)两个方法，它们都有EvictorContext参数；EvictorContext定义了getCurrentProcessingTime、getMetricGroup、getCurrentWatermark方法
• Evictor有几个内置的实现类，分别是CountEvictor、DeltaEvictor、TimeEvictor；其中CountEvictor是按窗口元素个数来进行剔除，TimeEvictor是按窗口长度来进行剔除，DeltaEvictor则是根据窗口元素与lastElement的delta与指定的threshold对比来进行剔除
• 如果指定了evictor(evictBefore)则会妨碍任何pre-aggregation操作，因为所有的窗口元素都会在windowing function计算之前先执行evictor操作；另外就是flink不保障窗口元素的顺序，也就是evictor如果有按窗口开头或末尾剔除元素，可能剔除的元素实际上并不是最先或最后达到的

doc

• Evictors