Flink DataStream Evictors（驱逐器）

Flink DataStream Evictors（驱逐器）"/>

Flink DataStream Evictors（驱逐器）

Flink DataStream Evictors（驱逐器）

Flink 窗口模式允许特别的算子Evictor（驱逐器），应用在WindowAssigner和trigger之间。通过evictor()方法使用。

Evictor驱逐器能够在element进入Window窗口聚合之前进行移除数据或者在进入Window窗口聚合后，Trigger触发计算操作之前移除数据。

Evictor的2个方法：

• evictBefore()：移除窗口元素，在Window Function之前调用。

• evictAfter()：移除窗口元素，在Window Function之后调用。

两个方法的参数都一样，分别是：

• Iterable<TimestampedValue> elements：当前窗口中的元素
• int size：当前窗口中的元素数量
• W window：当前窗口
• EvictorContext evictorContext：evict的上下文

Flink自带的3个Evictor：

• CountEvictor：保持窗口中用户指定数量的元素，并从窗口缓冲区的开头丢弃剩余的元素。

下面是CountEvictor类代码，我们可以直接定位到evict方法。看实现逻辑。

@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;}@Overridepublic void evictBefore(Iterable<TimestampedValue<Object>> elements, int size, W window, EvictorContext ctx) {if (!doEvictAfter) {evict(elements, size, ctx);}}@Overridepublic 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();}}}}/*** Creates a {@code CountEvictor} that keeps the given number of elements.* Eviction is done before the window function.** @param maxCount The number of elements to keep in the pane.*/public static <W extends Window> CountEvictor<W> of(long maxCount) {return new CountEvictor<>(maxCount);}/*** Creates a {@code CountEvictor} that keeps the given number of elements in the pane* Eviction is done before/after the window function based on the value of doEvictAfter.** @param maxCount The number of elements to keep in the pane.* @param doEvictAfter Whether to do eviction after the window function.*/public static <W extends Window> CountEvictor<W> of(long maxCount, boolean doEvictAfter) {return new CountEvictor<>(maxCount, doEvictAfter);}
}

• DeltaEvictor：取一个DeltaFunction和一个threshold，计算窗口缓冲区中最后一个元素与其余每个元素之间的差值，并删除Delta大于或等于阈值的值。

DeltaEvictor的evict的逻辑，需要实现一个deltaFunction对象的getDelta方法。与传入的threshold参数比较。来进行evict数据。

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();}}
}

• TimeEvictor：以interval毫秒为单位作为参数，对于给定窗口，它查找max_ts其元素的最大时间戳，并删除时间戳小于的所有元素max_ts-interval。

与CountEvictor的逻辑类似。windowSize是一个时间的毫秒值。

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();}}
}

默认情况下，Evictor在Window Function之前实现evict逻辑，通过代码应该可以看出。