Flink 窗口触发器(Trigger)(一)

Flink 窗口触发器(Trigger)(一)
Flink 窗口触发器(Trigger)(二)

Flink的窗口触发器（Trigger）是流处理中一个非常关键的概念，它定义了窗口何时被触发并决定触发后的行为（如进行窗口数据的计算或清理）。

一、基本概念

  • 定义：触发器决定了窗口何时被触发以及触发后的行为。在Flink中，窗口的触发是通过设置定时器来实现的。
  • 作用：控制窗口数据的聚合时机，确保数据在适当的时间点被处理和输出。
    在这里插入图片描述

二、类型

Flink提供了多种内置的触发器，以下几种为常用类型：

  1. EventTimeTrigger
  • 工作原理：基于事件时间和水印（Watermark）机制来触发窗口计算。当窗口的最大时间戳小于等于当前的水印时，立即触发窗口计算。
  • 适用场景：适用于需要基于事件时间进行处理的场景，如金融交易、日志分析等。
  1. ProcessingTimeTrigger
  • 工作原理：基于处理时间（即机器的系统时间）来触发窗口计算。当处理时间达到窗口的结束时间时，触发窗口计算。
  • 适用场景：适用于对时间精度要求不高的场景，或者当事件时间无法准确获取时。
  1. CountTrigger
  • 工作原理：根据窗口内元素的数量来触发计算。当窗口内的元素数量达到预设的阈值时，触发窗口计算。
  • 适用场景：适用于需要基于数据量进行处理的场景，如批量数据处理、流量分析等。
  1. ContinuousEventTimeTrigger和ContinuousProcessingTimeTrigger
  • 工作原理：根据间隔时间周期性触发窗口计算，或者当窗口的结束时间小于当前的时间（事件时间或处理时间）时触发计算。
  • 适用场景：适用于需要周期性处理数据的场景，如实时监控、周期性报表等。
  1. DeltaTrigger
  • 工作原理：根据接入数据计算出的Delta指标是否超过指定的阈值来触发窗口计算。
  • 适用场景：适用于需要基于数据变化量进行处理的场景，如异常检测、趋势分析等。
  1. PurgingTrigger
  • 工作原理：将其他触发器作为参数转换为Purge类型的触发器，在触发计算后清除窗口内的数据。
  • 适用场景：适用于需要在计算完成后立即清除窗口数据的场景，以节省存储空间。

三、关键方法

触发器通常包含以下几个关键方法：

  1. onElement(T element, long timestamp, W window, TriggerContext ctx)
    当元素被添加到窗口时调用，用于注册定时器或更新窗口状态。
  2. onEventTime(long time, W window, TriggerContext ctx)
    当事件时间计时器触发时调用，用于处理事件时间相关的触发逻辑。
  3. onProcessingTime(long time, W window, TriggerContext ctx)
    当处理时间计时器触发时调用，用于处理处理时间相关的触发逻辑。
  4. onMerge(W window, OnMergeContext ctx)（可选）
    当两个窗口合并时调用，用于合并窗口的状态和定时器。
  5. clear(W window, TriggerContext ctx)
    当窗口被删除时调用，用于清理窗口的状态和定时器。

四、行为

触发器在触发时会返回一个TriggerResult枚举值，以决定窗口的后续行为。常见的TriggerResult值包括：

  • CONTINUE：表示不进行任何操作，等待下一个触发条件。
  • FIRE：表示触发窗口计算并输出结果，但窗口状态保持不变。
  • PURGE：表示不触发窗口计算，只清除窗口内的数据和状态。
  • FIRE_AND_PURGE：表示触发窗口计算并输出结果，然后清除窗口内的数据和状态。

Flink的窗口触发器是流处理中非常灵活且强大的工具，它允许开发者根据实际需求定义窗口的触发条件和触发后的行为。通过选择合适的触发器和配置相应的参数，可以实现高效、准确的流数据处理。

五、Trigger

EventTimeTrigger

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.streaming.api.windowing.windows.TimeWindow;

@PublicEvolving
public class EventTimeTrigger extends Trigger<Object, TimeWindow> {
    private static final long serialVersionUID = 1L;

    private EventTimeTrigger() {
    }

    public TriggerResult onElement(Object element, long timestamp, TimeWindow window, Trigger.TriggerContext ctx) throws Exception {
        if (window.maxTimestamp() <= ctx.getCurrentWatermark()) {
            return TriggerResult.FIRE;
        } else {
            ctx.registerEventTimeTimer(window.maxTimestamp());
            return TriggerResult.CONTINUE;
        }
    }

    public TriggerResult onEventTime(long time, TimeWindow window, Trigger.TriggerContext ctx) {
        return time == window.maxTimestamp() ? TriggerResult.FIRE : TriggerResult.CONTINUE;
    }

    public TriggerResult onProcessingTime(long time, TimeWindow window, Trigger.TriggerContext ctx) throws Exception {
        return TriggerResult.CONTINUE;
    }

    public void clear(TimeWindow window, Trigger.TriggerContext ctx) throws Exception {
        ctx.deleteEventTimeTimer(window.maxTimestamp());
    }

    public boolean canMerge() {
        return true;
    }

    public void onMerge(TimeWindow window, Trigger.OnMergeContext ctx) {
        long windowMaxTimestamp = window.maxTimestamp();
        if (windowMaxTimestamp > ctx.getCurrentWatermark()) {
            ctx.registerEventTimeTimer(windowMaxTimestamp);
        }

    }

    public String toString() {
        return "EventTimeTrigger()";
    }

    public static EventTimeTrigger create() {
        return new EventTimeTrigger();
    }
}

ProcessingTimeTrigger

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.streaming.api.windowing.windows.TimeWindow;

@PublicEvolving
public class ProcessingTimeTrigger extends Trigger<Object, TimeWindow> {
    private static final long serialVersionUID = 1L;

    private ProcessingTimeTrigger() {
    }

    public TriggerResult onElement(Object element, long timestamp, TimeWindow window, Trigger.TriggerContext ctx) {
        ctx.registerProcessingTimeTimer(window.maxTimestamp());
        return TriggerResult.CONTINUE;
    }

    public TriggerResult onEventTime(long time, TimeWindow window, Trigger.TriggerContext ctx) throws Exception {
        return TriggerResult.CONTINUE;
    }

    public TriggerResult onProcessingTime(long time, TimeWindow window, Trigger.TriggerContext ctx) {
        return TriggerResult.FIRE;
    }

    public void clear(TimeWindow window, Trigger.TriggerContext ctx) throws Exception {
        ctx.deleteProcessingTimeTimer(window.maxTimestamp());
    }

    public boolean canMerge() {
        return true;
    }

    public void onMerge(TimeWindow window, Trigger.OnMergeContext ctx) {
        long windowMaxTimestamp = window.maxTimestamp();
        if (windowMaxTimestamp > ctx.getCurrentProcessingTime()) {
            ctx.registerProcessingTimeTimer(windowMaxTimestamp);
        }

    }

    public String toString() {
        return "ProcessingTimeTrigger()";
    }

    public static ProcessingTimeTrigger create() {
        return new ProcessingTimeTrigger();
    }
}

ProcessingTimeoutTrigger

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import java.time.Duration;
import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.api.common.state.ValueState;
import org.apache.flink.api.common.state.ValueStateDescriptor;
import org.apache.flink.api.common.typeutils.base.LongSerializer;
import org.apache.flink.streaming.api.windowing.windows.Window;

@PublicEvolving
public class ProcessingTimeoutTrigger<T, W extends Window> extends Trigger<T, W> {
    private static final long serialVersionUID = 1L;
    private final Trigger<T, W> nestedTrigger;
    private final long interval;
    private final boolean resetTimerOnNewRecord;
    private final boolean shouldClearOnTimeout;
    private final ValueStateDescriptor<Long> timeoutStateDesc;

    private ProcessingTimeoutTrigger(Trigger<T, W> nestedTrigger, long interval, boolean resetTimerOnNewRecord, boolean shouldClearOnTimeout) {
        this.nestedTrigger = nestedTrigger;
        this.interval = interval;
        this.resetTimerOnNewRecord = resetTimerOnNewRecord;
        this.shouldClearOnTimeout = shouldClearOnTimeout;
        this.timeoutStateDesc = new ValueStateDescriptor("timeout", LongSerializer.INSTANCE);
    }

    public TriggerResult onElement(T element, long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        TriggerResult triggerResult = this.nestedTrigger.onElement(element, timestamp, window, ctx);
        if (triggerResult.isFire()) {
            this.clear(window, ctx);
            return triggerResult;
        } else {
            ValueState<Long> timeoutState = (ValueState)ctx.getPartitionedState(this.timeoutStateDesc);
            long nextFireTimestamp = ctx.getCurrentProcessingTime() + this.interval;
            Long timeoutTimestamp = (Long)timeoutState.value();
            if (timeoutTimestamp != null && this.resetTimerOnNewRecord) {
                ctx.deleteProcessingTimeTimer(timeoutTimestamp);
                timeoutState.clear();
                timeoutTimestamp = null;
            }

            if (timeoutTimestamp == null) {
                timeoutState.update(nextFireTimestamp);
                ctx.registerProcessingTimeTimer(nextFireTimestamp);
            }

            return triggerResult;
        }
    }

    public TriggerResult onProcessingTime(long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        TriggerResult triggerResult = this.nestedTrigger.onProcessingTime(timestamp, window, ctx);
        if (this.shouldClearOnTimeout) {
            this.clear(window, ctx);
        }

        return triggerResult.isPurge() ? TriggerResult.FIRE_AND_PURGE : TriggerResult.FIRE;
    }

    public TriggerResult onEventTime(long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        TriggerResult triggerResult = this.nestedTrigger.onEventTime(timestamp, window, ctx);
        if (this.shouldClearOnTimeout) {
            this.clear(window, ctx);
        }

        return triggerResult.isPurge() ? TriggerResult.FIRE_AND_PURGE : TriggerResult.FIRE;
    }

    public void clear(W window, Trigger.TriggerContext ctx) throws Exception {
        ValueState<Long> timeoutTimestampState = (ValueState)ctx.getPartitionedState(this.timeoutStateDesc);
        Long timeoutTimestamp = (Long)timeoutTimestampState.value();
        if (timeoutTimestamp != null) {
            ctx.deleteProcessingTimeTimer(timeoutTimestamp);
            timeoutTimestampState.clear();
        }

        this.nestedTrigger.clear(window, ctx);
    }

    public String toString() {
        return "TimeoutTrigger(" + this.nestedTrigger.toString() + ")";
    }

    public static <T, W extends Window> ProcessingTimeoutTrigger<T, W> of(Trigger<T, W> nestedTrigger, Duration timeout) {
        return new ProcessingTimeoutTrigger(nestedTrigger, timeout.toMillis(), false, true);
    }

    public static <T, W extends Window> ProcessingTimeoutTrigger<T, W> of(Trigger<T, W> nestedTrigger, Duration timeout, boolean resetTimerOnNewRecord, boolean shouldClearOnTimeout) {
        return new ProcessingTimeoutTrigger(nestedTrigger, timeout.toMillis(), resetTimerOnNewRecord, shouldClearOnTimeout);
    }
}

CountTrigger

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.api.common.functions.ReduceFunction;
import org.apache.flink.api.common.state.ReducingState;
import org.apache.flink.api.common.state.ReducingStateDescriptor;
import org.apache.flink.api.common.typeutils.base.LongSerializer;
import org.apache.flink.streaming.api.windowing.windows.Window;

@PublicEvolving
public class CountTrigger<W extends Window> extends Trigger<Object, W> {
    private static final long serialVersionUID = 1L;
    private final long maxCount;
    private final ReducingStateDescriptor<Long> stateDesc;

    private CountTrigger(long maxCount) {
        this.stateDesc = new ReducingStateDescriptor("count", new Sum(), LongSerializer.INSTANCE);
        this.maxCount = maxCount;
    }

    public TriggerResult onElement(Object element, long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        ReducingState<Long> count = (ReducingState)ctx.getPartitionedState(this.stateDesc);
        count.add(1L);
        if ((Long)count.get() >= this.maxCount) {
            count.clear();
            return TriggerResult.FIRE;
        } else {
            return TriggerResult.CONTINUE;
        }
    }

    public TriggerResult onEventTime(long time, W window, Trigger.TriggerContext ctx) {
        return TriggerResult.CONTINUE;
    }

    public TriggerResult onProcessingTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        return TriggerResult.CONTINUE;
    }

    public void clear(W window, Trigger.TriggerContext ctx) throws Exception {
        ((ReducingState)ctx.getPartitionedState(this.stateDesc)).clear();
    }

    public boolean canMerge() {
        return true;
    }

    public void onMerge(W window, Trigger.OnMergeContext ctx) throws Exception {
        ctx.mergePartitionedState(this.stateDesc);
    }

    public String toString() {
        return "CountTrigger(" + this.maxCount + ")";
    }

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

    private static class Sum implements ReduceFunction<Long> {
        private static final long serialVersionUID = 1L;

        private Sum() {
        }

        public Long reduce(Long value1, Long value2) throws Exception {
            return value1 + value2;
        }
    }
}

ContinuousEventTimeTrigger和ContinuousProcessingTimeTrigger

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.annotation.VisibleForTesting;
import org.apache.flink.api.common.functions.ReduceFunction;
import org.apache.flink.api.common.state.ReducingState;
import org.apache.flink.api.common.state.ReducingStateDescriptor;
import org.apache.flink.api.common.typeutils.base.LongSerializer;
import org.apache.flink.streaming.api.windowing.time.Time;
import org.apache.flink.streaming.api.windowing.windows.Window;

@PublicEvolving
public class ContinuousEventTimeTrigger<W extends Window> extends Trigger<Object, W> {
    private static final long serialVersionUID = 1L;
    private final long interval;
    private final ReducingStateDescriptor<Long> stateDesc;

    private ContinuousEventTimeTrigger(long interval) {
        this.stateDesc = new ReducingStateDescriptor("fire-time", new Min(), LongSerializer.INSTANCE);
        this.interval = interval;
    }

    public TriggerResult onElement(Object element, long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        if (window.maxTimestamp() <= ctx.getCurrentWatermark()) {
            return TriggerResult.FIRE;
        } else {
            ctx.registerEventTimeTimer(window.maxTimestamp());
            ReducingState<Long> fireTimestampState = (ReducingState)ctx.getPartitionedState(this.stateDesc);
            if (fireTimestampState.get() == null) {
                this.registerNextFireTimestamp(timestamp - timestamp % this.interval, window, ctx, fireTimestampState);
            }

            return TriggerResult.CONTINUE;
        }
    }

    public TriggerResult onEventTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        if (time == window.maxTimestamp()) {
            return TriggerResult.FIRE;
        } else {
            ReducingState<Long> fireTimestampState = (ReducingState)ctx.getPartitionedState(this.stateDesc);
            Long fireTimestamp = (Long)fireTimestampState.get();
            if (fireTimestamp != null && fireTimestamp == time) {
                fireTimestampState.clear();
                this.registerNextFireTimestamp(time, window, ctx, fireTimestampState);
                return TriggerResult.FIRE;
            } else {
                return TriggerResult.CONTINUE;
            }
        }
    }

    public TriggerResult onProcessingTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        return TriggerResult.CONTINUE;
    }

    public void clear(W window, Trigger.TriggerContext ctx) throws Exception {
        ReducingState<Long> fireTimestamp = (ReducingState)ctx.getPartitionedState(this.stateDesc);
        Long timestamp = (Long)fireTimestamp.get();
        if (timestamp != null) {
            ctx.deleteEventTimeTimer(timestamp);
            fireTimestamp.clear();
        }

    }

    public boolean canMerge() {
        return true;
    }

    public void onMerge(W window, Trigger.OnMergeContext ctx) throws Exception {
        ctx.mergePartitionedState(this.stateDesc);
        Long nextFireTimestamp = (Long)((ReducingState)ctx.getPartitionedState(this.stateDesc)).get();
        if (nextFireTimestamp != null) {
            ctx.registerEventTimeTimer(nextFireTimestamp);
        }

    }

    public String toString() {
        return "ContinuousEventTimeTrigger(" + this.interval + ")";
    }

    @VisibleForTesting
    public long getInterval() {
        return this.interval;
    }

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

    private void registerNextFireTimestamp(long time, W window, Trigger.TriggerContext ctx, ReducingState<Long> fireTimestampState) throws Exception {
        long nextFireTimestamp = Math.min(time + this.interval, window.maxTimestamp());
        fireTimestampState.add(nextFireTimestamp);
        ctx.registerEventTimeTimer(nextFireTimestamp);
    }

    private static class Min implements ReduceFunction<Long> {
        private static final long serialVersionUID = 1L;

        private Min() {
        }

        public Long reduce(Long value1, Long value2) throws Exception {
            return Math.min(value1, value2);
        }
    }
}


//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.annotation.VisibleForTesting;
import org.apache.flink.api.common.functions.ReduceFunction;
import org.apache.flink.api.common.state.ReducingState;
import org.apache.flink.api.common.state.ReducingStateDescriptor;
import org.apache.flink.api.common.typeutils.base.LongSerializer;
import org.apache.flink.streaming.api.windowing.time.Time;
import org.apache.flink.streaming.api.windowing.windows.Window;

@PublicEvolving
public class ContinuousProcessingTimeTrigger<W extends Window> extends Trigger<Object, W> {
    private static final long serialVersionUID = 1L;
    private final long interval;
    private final ReducingStateDescriptor<Long> stateDesc;

    private ContinuousProcessingTimeTrigger(long interval) {
        this.stateDesc = new ReducingStateDescriptor("fire-time", new Min(), LongSerializer.INSTANCE);
        this.interval = interval;
    }

    public TriggerResult onElement(Object element, long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        ReducingState<Long> fireTimestampState = (ReducingState)ctx.getPartitionedState(this.stateDesc);
        timestamp = ctx.getCurrentProcessingTime();
        if (fireTimestampState.get() == null) {
            this.registerNextFireTimestamp(timestamp - timestamp % this.interval, window, ctx, fireTimestampState);
        }

        return TriggerResult.CONTINUE;
    }

    public TriggerResult onEventTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        return TriggerResult.CONTINUE;
    }

    public TriggerResult onProcessingTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        ReducingState<Long> fireTimestampState = (ReducingState)ctx.getPartitionedState(this.stateDesc);
        if (((Long)fireTimestampState.get()).equals(time)) {
            fireTimestampState.clear();
            this.registerNextFireTimestamp(time, window, ctx, fireTimestampState);
            return TriggerResult.FIRE;
        } else {
            return TriggerResult.CONTINUE;
        }
    }

    public void clear(W window, Trigger.TriggerContext ctx) throws Exception {
        ReducingState<Long> fireTimestamp = (ReducingState)ctx.getPartitionedState(this.stateDesc);
        Long timestamp = (Long)fireTimestamp.get();
        if (timestamp != null) {
            ctx.deleteProcessingTimeTimer(timestamp);
            fireTimestamp.clear();
        }

    }

    public boolean canMerge() {
        return true;
    }

    public void onMerge(W window, Trigger.OnMergeContext ctx) throws Exception {
        ctx.mergePartitionedState(this.stateDesc);
        Long nextFireTimestamp = (Long)((ReducingState)ctx.getPartitionedState(this.stateDesc)).get();
        if (nextFireTimestamp != null) {
            ctx.registerProcessingTimeTimer(nextFireTimestamp);
        }

    }

    @VisibleForTesting
    public long getInterval() {
        return this.interval;
    }

    public String toString() {
        return "ContinuousProcessingTimeTrigger(" + this.interval + ")";
    }

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

    private void registerNextFireTimestamp(long time, W window, Trigger.TriggerContext ctx, ReducingState<Long> fireTimestampState) throws Exception {
        long nextFireTimestamp = Math.min(time + this.interval, window.maxTimestamp());
        fireTimestampState.add(nextFireTimestamp);
        ctx.registerProcessingTimeTimer(nextFireTimestamp);
    }

    private static class Min implements ReduceFunction<Long> {
        private static final long serialVersionUID = 1L;

        private Min() {
        }

        public Long reduce(Long value1, Long value2) throws Exception {
            return Math.min(value1, value2);
        }
    }
}

DeltaTrigger

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.api.common.state.ValueState;
import org.apache.flink.api.common.state.ValueStateDescriptor;
import org.apache.flink.api.common.typeutils.TypeSerializer;
import org.apache.flink.streaming.api.functions.windowing.delta.DeltaFunction;
import org.apache.flink.streaming.api.windowing.windows.Window;

@PublicEvolving
public class DeltaTrigger<T, W extends Window> extends Trigger<T, W> {
    private static final long serialVersionUID = 1L;
    private final DeltaFunction<T> deltaFunction;
    private final double threshold;
    private final ValueStateDescriptor<T> stateDesc;

    private DeltaTrigger(double threshold, DeltaFunction<T> deltaFunction, TypeSerializer<T> stateSerializer) {
        this.deltaFunction = deltaFunction;
        this.threshold = threshold;
        this.stateDesc = new ValueStateDescriptor("last-element", stateSerializer);
    }

    public TriggerResult onElement(T element, long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        ValueState<T> lastElementState = (ValueState)ctx.getPartitionedState(this.stateDesc);
        if (lastElementState.value() == null) {
            lastElementState.update(element);
            return TriggerResult.CONTINUE;
        } else if (this.deltaFunction.getDelta(lastElementState.value(), element) > this.threshold) {
            lastElementState.update(element);
            return TriggerResult.FIRE;
        } else {
            return TriggerResult.CONTINUE;
        }
    }

    public TriggerResult onEventTime(long time, W window, Trigger.TriggerContext ctx) {
        return TriggerResult.CONTINUE;
    }

    public TriggerResult onProcessingTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        return TriggerResult.CONTINUE;
    }

    public void clear(W window, Trigger.TriggerContext ctx) throws Exception {
        ((ValueState)ctx.getPartitionedState(this.stateDesc)).clear();
    }

    public String toString() {
        return "DeltaTrigger(" + this.deltaFunction + ", " + this.threshold + ")";
    }

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

PurgingTrigger

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.annotation.VisibleForTesting;
import org.apache.flink.streaming.api.windowing.windows.Window;

@PublicEvolving
public class PurgingTrigger<T, W extends Window> extends Trigger<T, W> {
    private static final long serialVersionUID = 1L;
    private Trigger<T, W> nestedTrigger;

    private PurgingTrigger(Trigger<T, W> nestedTrigger) {
        this.nestedTrigger = nestedTrigger;
    }

    public TriggerResult onElement(T element, long timestamp, W window, Trigger.TriggerContext ctx) throws Exception {
        TriggerResult triggerResult = this.nestedTrigger.onElement(element, timestamp, window, ctx);
        return triggerResult.isFire() ? TriggerResult.FIRE_AND_PURGE : triggerResult;
    }

    public TriggerResult onEventTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        TriggerResult triggerResult = this.nestedTrigger.onEventTime(time, window, ctx);
        return triggerResult.isFire() ? TriggerResult.FIRE_AND_PURGE : triggerResult;
    }

    public TriggerResult onProcessingTime(long time, W window, Trigger.TriggerContext ctx) throws Exception {
        TriggerResult triggerResult = this.nestedTrigger.onProcessingTime(time, window, ctx);
        return triggerResult.isFire() ? TriggerResult.FIRE_AND_PURGE : triggerResult;
    }

    public void clear(W window, Trigger.TriggerContext ctx) throws Exception {
        this.nestedTrigger.clear(window, ctx);
    }

    public boolean canMerge() {
        return this.nestedTrigger.canMerge();
    }

    public void onMerge(W window, Trigger.OnMergeContext ctx) throws Exception {
        this.nestedTrigger.onMerge(window, ctx);
    }

    public String toString() {
        return "PurgingTrigger(" + this.nestedTrigger.toString() + ")";
    }

    public static <T, W extends Window> PurgingTrigger<T, W> of(Trigger<T, W> nestedTrigger) {
        return new PurgingTrigger(nestedTrigger);
    }

    @VisibleForTesting
    public Trigger<T, W> getNestedTrigger() {
        return this.nestedTrigger;
    }
}

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