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本文主要研究一下flink的Triggers

Trigger

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

@PublicEvolving
public abstract class Trigger<T, W extends Window> implements Serializable {

    private static final long serialVersionUID = -4104633972991191369L;

    public abstract TriggerResult onElement(T element, long timestamp, W window, TriggerContext ctx) throws Exception;

    public abstract TriggerResult onProcessingTime(long time, W window, TriggerContext ctx) throws Exception;

    public abstract TriggerResult onEventTime(long time, W window, TriggerContext ctx) throws Exception;

    public boolean canMerge() {
        return false;
    }

    public void onMerge(W window, OnMergeContext ctx) throws Exception {
        throw new UnsupportedOperationException("This trigger does not support merging.");
    }

    public abstract void clear(W window, TriggerContext ctx) throws Exception;

    // ------------------------------------------------------------------------

    public interface TriggerContext {

        long getCurrentProcessingTime();

        MetricGroup getMetricGroup();

        long getCurrentWatermark();

        void registerProcessingTimeTimer(long time);

        void registerEventTimeTimer(long time);

        void deleteProcessingTimeTimer(long time);

        void deleteEventTimeTimer(long time);

        <S extends State> S getPartitionedState(StateDescriptor<S, ?> stateDescriptor);

        @Deprecated
        <S extends Serializable> ValueState<S> getKeyValueState(String name, Class<S> stateType, S defaultState);

        @Deprecated
        <S extends Serializable> ValueState<S> getKeyValueState(String name, TypeInformation<S> stateType, S defaultState);
    }

    public interface OnMergeContext extends TriggerContext {
        <S extends MergingState<?, ?>> void mergePartitionedState(StateDescriptor<S, ?> stateDescriptor);
    }
}

• Trigger接收两个泛型，一个是element类型，一个是窗口类型；它定义了onElement、onProcessingTime、onEventTime、canMerge、onMerge、clear几个方法，其中onElement、onProcessingTime、onEventTime均需要返回TriggerResult
• onElement在每个element添加到window的时候会被回调；onProcessingTime在注册的event-time timer触发时会被回调；onEventTime在注册的processing-time timer触发时会被回调
• canMerge用于标识是否支持trigger state的合并，默认返回false；onMerge在多个window合并的时候会被触发；clear用于清除TriggerContext中存储的相关state
• Trigger还定义了TriggerContext及OnMergeContext；TriggerContext定义了注册及删除EventTimeTimer、ProcessingTimeTimer方法，同时还定义了getCurrentProcessingTime、getMetricGroup、getCurrentWatermark、getPartitionedState、getKeyValueState、getKeyValueState方法
• OnMergeContext继承了TriggerContext，它多定义了mergePartitionedState方法

TriggerResult

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

public enum TriggerResult {

    CONTINUE(false, false),

    FIRE_AND_PURGE(true, true),

    FIRE(true, false),

    PURGE(false, true);

    // ------------------------------------------------------------------------

    private final boolean fire;
    private final boolean purge;

    TriggerResult(boolean fire, boolean purge) {
        this.purge = purge;
        this.fire = fire;
    }

    public boolean isFire() {
        return fire;
    }

    public boolean isPurge() {
        return purge;
    }
}

• TriggerResult用于表示trigger在onElement、onProcessingTime、onEventTime被回调时返回的action枚举，它有fire、purge两个属性，CONTINUE、FIRE_AND_PURGE、FIRE、PURGE五个枚举
• fire表示是否要触发window的computation操作；而purge表示是否要清理window的窗口数据
• CONTINUE表示不对window做任何操作；FIRE_AND_PURGE表示要触发window的computation操作然后清理window的窗口数据；FIRE表示仅仅触发window的computation操作但不清理window的窗口数据；PURGE表示不触发window的computation操作但是要清理window的窗口数据

EventTimeTrigger

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

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

    private EventTimeTrigger() {}

    @Override
    public TriggerResult onElement(Object element, long timestamp, TimeWindow window, TriggerContext ctx) throws Exception {
        if (window.maxTimestamp() <= ctx.getCurrentWatermark()) {
            // if the watermark is already past the window fire immediately
            return TriggerResult.FIRE;
        } else {
            ctx.registerEventTimeTimer(window.maxTimestamp());
            return TriggerResult.CONTINUE;
        }
    }

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

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

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

    @Override
    public boolean canMerge() {
        return true;
    }

    @Override
    public void onMerge(TimeWindow window,
            OnMergeContext ctx) {
        // only register a timer if the watermark is not yet past the end of the merged window
        // this is in line with the logic in onElement(). If the watermark is past the end of
        // the window onElement() will fire and setting a timer here would fire the window twice.
        long windowMaxTimestamp = window.maxTimestamp();
        if (windowMaxTimestamp > ctx.getCurrentWatermark()) {
            ctx.registerEventTimeTimer(windowMaxTimestamp);
        }
    }

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

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

• EventTimeTrigger继承了Trigger，element类型为Object，窗口类型为TimeWindow；SlidingEventTimeWindows、TumblingEventTimeWindows、EventTimeSessionWindows、DynamicEventTimeSessionWindows默认都使用EventTimeTrigger
• onElement在window.maxTimestamp()小于等于ctx.getCurrentWatermark()的时候，返回TriggerResult.FIRE，否则执行ctx.registerEventTimeTimer(window.maxTimestamp())，然后返回TriggerResult.CONTINUE；onEventTime在time等于window.maxTimestamp()的时候返回TriggerResult.FIRE，否则返回TriggerResult.CONTINUE；onProcessingTime则返回TriggerResult.CONTINUE
• canMerge返回true；onMerge在window.maxTimestamp()大于ctx.getCurrentWatermark()的时候会执行ctx.registerEventTimeTimer(windowMaxTimestamp)；clear则执行ctx.deleteEventTimeTimer(window.maxTimestamp())

ProcessingTimeTrigger

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

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

    private ProcessingTimeTrigger() {}

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

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

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

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

    @Override
    public boolean canMerge() {
        return true;
    }

    @Override
    public void onMerge(TimeWindow window,
            OnMergeContext ctx) {
        // only register a timer if the time is not yet past the end of the merged window
        // this is in line with the logic in onElement(). If the time is past the end of
        // the window onElement() will fire and setting a timer here would fire the window twice.
        long windowMaxTimestamp = window.maxTimestamp();
        if (windowMaxTimestamp > ctx.getCurrentProcessingTime()) {
            ctx.registerProcessingTimeTimer(windowMaxTimestamp);
        }
    }

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

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

}

• ProcessingTimeTrigger继承了Trigger，element类型为Object，窗口类型为TimeWindow；SlidingProcessingTimeWindows、TumblingProcessingTimeWindows、ProcessingTimeSessionWindows、DynamicProcessingTimeSessionWindows默认都使用ProcessingTimeTrigger
• onElement执行ctx.registerProcessingTimeTimer(window.maxTimestamp())，然后返回TriggerResult.CONTINUE；onEventTime返回TriggerResult.CONTINUE；onProcessingTime则返回TriggerResult.FIRE
• canMerge返回true；onMerge在window.maxTimestamp()大于ctx.getCurrentWatermark()的时候会执行ctx.registerProcessingTimeTimer(windowMaxTimestamp)；clear则执行ctx.deleteProcessingTimeTimer(window.maxTimestamp())

NeverTrigger

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

    @Internal
    public static class NeverTrigger extends Trigger<Object, GlobalWindow> {
        private static final long serialVersionUID = 1L;

        @Override
        public TriggerResult onElement(Object element, long timestamp, GlobalWindow window, TriggerContext ctx) {
            return TriggerResult.CONTINUE;
        }

        @Override
        public TriggerResult onEventTime(long time, GlobalWindow window, TriggerContext ctx) {
            return TriggerResult.CONTINUE;
        }

        @Override
        public TriggerResult onProcessingTime(long time, GlobalWindow window, TriggerContext ctx) {
            return TriggerResult.CONTINUE;
        }

        @Override
        public void clear(GlobalWindow window, TriggerContext ctx) throws Exception {}

        @Override
        public void onMerge(GlobalWindow window, OnMergeContext ctx) {
        }
    }

• NeverTrigger的onElement、onEventTime、onProcessingTime均返回TriggerResult.CONTINUE；GlobalWindows默认使用的是NeverTrigger

CountTrigger

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

@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 =
            new ReducingStateDescriptor<>("count", new Sum(), LongSerializer.INSTANCE);

    private CountTrigger(long maxCount) {
        this.maxCount = maxCount;
    }

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

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

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

    @Override
    public void clear(W window, TriggerContext ctx) throws Exception {
        ctx.getPartitionedState(stateDesc).clear();
    }

    @Override
    public boolean canMerge() {
        return true;
    }

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

    @Override
    public String toString() {
        return "CountTrigger(" +  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;

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

    }
}

• CountTrigger继承了Trigger，指定了element类型为Object类型；它定义了maxCount及ReducingStateDescriptor；其中ReducingStateDescriptor用于窗口计数(它使用的是自己定义的Sum函数)，在onElement方法里头，当计数大于等于maxCount时，则会清空计数，然后返回TriggerResult.FIRE，否则返回TriggerResult.CONTINUE；onEventTime、onProcessingTime均返回TriggerResult.CONTINUE；canMerge返回true；onMerge执行的是ctx.mergePartitionedState(stateDesc)；clear执行的是ctx.getPartitionedState(stateDesc).clear()

PurgingTrigger

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

@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;
    }

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

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

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

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

    @Override
    public boolean canMerge() {
        return nestedTrigger.canMerge();
    }

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

    @Override
    public String toString() {
        return "PurgingTrigger(" + 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 nestedTrigger;
    }
}

• PurgingTrigger是包装型的Trigger，它包装了nestedTrigger，其onElement、onEventTime、onProcessingTime根据nestedTrigger的返回结果，在triggerResult.isFire()为true的时候，包装返回TriggerResult.FIRE_AND_PURGE；canMerge、onMerge、clear等方法均是委托给nestedTrigger处理

小结

• Trigger接收两个泛型，一个是element类型，一个是窗口类型；它定义了onElement、onProcessingTime、onEventTime、canMerge、onMerge、clear几个方法，其中onElement、onProcessingTime、onEventTime均需要返回TriggerResult；TriggerResult用于表示trigger在onElement、onProcessingTime、onEventTime被回调时返回的action枚举，它有fire、purge两个属性(fire表示是否要触发window的computation操作；而purge表示是否要清理window的窗口数据)，CONTINUE、FIRE_AND_PURGE、FIRE、PURGE五个枚举
• SlidingEventTimeWindows、TumblingEventTimeWindows、EventTimeSessionWindows、DynamicEventTimeSessionWindows默认都使用EventTimeTrigger；SlidingProcessingTimeWindows、TumblingProcessingTimeWindows、ProcessingTimeSessionWindows、DynamicProcessingTimeSessionWindows默认都使用ProcessingTimeTrigger；GlobalWindows默认使用的是NeverTrigger
• CountTrigger主要用于计数的窗口类型，它使用ReducingStateDescriptor来进行窗口计数，在onElement方法里头，当计数大于等于maxCount时，则会清空计数，然后返回TriggerResult.FIRE，否则返回TriggerResult.CONTINUE；PurgingTrigger是包装型的Trigger，它包装了nestedTrigger，其onElement、onEventTime、onProcessingTime根据nestedTrigger的返回结果，在triggerResult.isFire()为true的时候，包装返回TriggerResult.FIRE_AND_PURGE；canMerge、onMerge、clear等方法均是委托给nestedTrigger处理

doc

• Triggers