聊聊flink的Sliding Window

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本文主要研究一下flink的Sliding Window

SlidingEventTimeWindows

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

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

    private final long size;

    private final long slide;

    private final long offset;

    protected SlidingEventTimeWindows(long size, long slide, long offset) {
        if (offset < 0 || offset >= slide || size <= 0) {
            throw new IllegalArgumentException("SlidingEventTimeWindows parameters must satisfy 0 <= offset < slide and size > 0");
        }

        this.size = size;
        this.slide = slide;
        this.offset = offset;
    }

    @Override
    public Collection<TimeWindow> assignWindows(Object element, long timestamp, WindowAssignerContext context) {
        if (timestamp > Long.MIN_VALUE) {
            List<TimeWindow> windows = new ArrayList<>((int) (size / slide));
            long lastStart = TimeWindow.getWindowStartWithOffset(timestamp, offset, slide);
            for (long start = lastStart;
                start > timestamp - size;
                start -= slide) {
                windows.add(new TimeWindow(start, start + size));
            }
            return windows;
        } else {
            throw new RuntimeException("Record has Long.MIN_VALUE timestamp (= no timestamp marker). " +
                    "Is the time characteristic set to 'ProcessingTime', or did you forget to call " +
                    "'DataStream.assignTimestampsAndWatermarks(...)'?");
        }
    }

    public long getSize() {
        return size;
    }

    public long getSlide() {
        return slide;
    }

    @Override
    public Trigger<Object, TimeWindow> getDefaultTrigger(StreamExecutionEnvironment env) {
        return EventTimeTrigger.create();
    }

    @Override
    public String toString() {
        return "SlidingEventTimeWindows(" + size + ", " + slide + ")";
    }

    public static SlidingEventTimeWindows of(Time size, Time slide) {
        return new SlidingEventTimeWindows(size.toMilliseconds(), slide.toMilliseconds(), 0);
    }

    public static SlidingEventTimeWindows of(Time size, Time slide, Time offset) {
        return new SlidingEventTimeWindows(size.toMilliseconds(), slide.toMilliseconds(),
            offset.toMilliseconds() % slide.toMilliseconds());
    }

    @Override
    public TypeSerializer<TimeWindow> getWindowSerializer(ExecutionConfig executionConfig) {
        return new TimeWindow.Serializer();
    }

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

• SlidingEventTimeWindows继承了Window，其中元素类型为Object，而窗口类型为TimeWindow；它有三个参数，一个是size，一个是slide，一个是offset，其中offset必须大于等于0，offset必须大于slide，size必须大于0
• assignWindows方法以slide作为size通过TimeWindow.getWindowStartWithOffset(timestamp, offset, slide)计算lastStart，然后以为start + size > timestamp为循环条件，每次对start减去slide，挨个计算TimeWindow(start, start + size)；getDefaultTrigger方法返回的是EventTimeTrigger；getWindowSerializer方法返回的是TimeWindow.Serializer()；isEventTime返回的为true
• SlidingEventTimeWindows提供了of静态工厂方法，可以指定size、slide及offset参数，它对于传入的offset参数转为毫秒然后与slide.toMilliseconds()取余作为最后的offset值

SlidingProcessingTimeWindows

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

public class SlidingProcessingTimeWindows extends WindowAssigner<Object, TimeWindow> {
    private static final long serialVersionUID = 1L;

    private final long size;

    private final long offset;

    private final long slide;

    private SlidingProcessingTimeWindows(long size, long slide, long offset) {
        if (offset < 0 || offset >= slide || size <= 0) {
            throw new IllegalArgumentException("SlidingProcessingTimeWindows parameters must satisfy 0 <= offset < slide and size > 0");
        }

        this.size = size;
        this.slide = slide;
        this.offset = offset;
    }

    @Override
    public Collection<TimeWindow> assignWindows(Object element, long timestamp, WindowAssignerContext context) {
        timestamp = context.getCurrentProcessingTime();
        List<TimeWindow> windows = new ArrayList<>((int) (size / slide));
        long lastStart = TimeWindow.getWindowStartWithOffset(timestamp, offset, slide);
        for (long start = lastStart;
            start > timestamp - size;
            start -= slide) {
            windows.add(new TimeWindow(start, start + size));
        }
        return windows;
    }

    public long getSize() {
        return size;
    }

    public long getSlide() {
        return slide;
    }

    @Override
    public Trigger<Object, TimeWindow> getDefaultTrigger(StreamExecutionEnvironment env) {
        return ProcessingTimeTrigger.create();
    }

    @Override
    public String toString() {
        return "SlidingProcessingTimeWindows(" + size + ", " + slide + ")";
    }

    public static SlidingProcessingTimeWindows of(Time size, Time slide) {
        return new SlidingProcessingTimeWindows(size.toMilliseconds(), slide.toMilliseconds(), 0);
    }

    public static SlidingProcessingTimeWindows of(Time size, Time slide, Time offset) {
        return new SlidingProcessingTimeWindows(size.toMilliseconds(), slide.toMilliseconds(),
            offset.toMilliseconds() % slide.toMilliseconds());
    }

    @Override
    public TypeSerializer<TimeWindow> getWindowSerializer(ExecutionConfig executionConfig) {
        return new TimeWindow.Serializer();
    }

    @Override
    public boolean isEventTime() {
        return false;
    }
}

• SlidingProcessingTimeWindows继承了Window，其中元素类型为Object，而窗口类型为TimeWindow；它有三个参数，一个是size，一个是slide，一个是offset，其中offset必须大于等于0，offset必须大于slide，size必须大于0
• assignWindows方法以slide作为size通过TimeWindow.getWindowStartWithOffset(timestamp, offset, slide)计算lastStart(与SlidingEventTimeWindows不同的是SlidingProcessingTimeWindows的这个方法里头使用context.getCurrentProcessingTime()值重置了timestamp)，然后以为start + size > timestamp为循环条件，每次对start减去slide，挨个计算TimeWindow(start, start + size)；getDefaultTrigger方法返回的是ProcessingTimeTrigger；getWindowSerializer方法返回的是TimeWindow.Serializer()；isEventTime返回的为false
• SlidingEventTimeWindows提供了of静态工厂方法，可以指定size、slide及offset参数，它对于传入的offset参数转为毫秒然后与slide.toMilliseconds()取余作为最后的offset值

小结

• flink的Sliding Window分为SlidingEventTimeWindows及SlidingProcessingTimeWindows，它们都继承了WindowAssigner，其中元素类型为Object，而窗口类型为TimeWindow；它有三个参数，一个是size，一个是slide，一个是offset，其中offset必须大于等于0，offset必须大于slide，size必须大于0
• WindowAssigner定义了assignWindows、getDefaultTrigger、getWindowSerializer、isEventTime这几个抽象方法，同时定义了抽象静态类WindowAssignerContext；它有两个泛型，其中T为元素类型，而W为窗口类型；SlidingEventTimeWindows及SlidingProcessingTimeWindows的窗口类型为TimeWindow，它有start及end属性，其中start为inclusive，而end为exclusive，maxTimestamp返回的是end-1，它还提供了mergeWindows及getWindowStartWithOffset静态方法；前者用于合并重叠的时间窗口，后者用于获取指定timestamp、offset、windowSize的window start
• SlidingEventTimeWindows及SlidingProcessingTimeWindows的不同在于assignWindows、getDefaultTrigger、isEventTime方法；前者assignWindows使用的是参数中的timestamp，而后者使用的是context.getCurrentProcessingTime()；前者的getDefaultTrigger返回的是EventTimeTrigger，而后者返回的是ProcessingTimeTrigger；前者isEventTime方法返回的为true，而后者返回的为false

doc

• Sliding Windows