聊聊flink的Tumbling Window

序

本文主要研究一下flink的Tumbling Window

WindowAssigner

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

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

    /**
     * Returns a {@code Collection} of windows that should be assigned to the element.
     *
     * @param element The element to which windows should be assigned.
     * @param timestamp The timestamp of the element.
     * @param context The {@link WindowAssignerContext} in which the assigner operates.
     */
    public abstract Collection<W> assignWindows(T element, long timestamp, WindowAssignerContext context);

    /**
     * Returns the default trigger associated with this {@code WindowAssigner}.
     */
    public abstract Trigger<T, W> getDefaultTrigger(StreamExecutionEnvironment env);

    /**
     * Returns a {@link TypeSerializer} for serializing windows that are assigned by
     * this {@code WindowAssigner}.
     */
    public abstract TypeSerializer<W> getWindowSerializer(ExecutionConfig executionConfig);

    /**
     * Returns {@code true} if elements are assigned to windows based on event time,
     * {@code false} otherwise.
     */
    public abstract boolean isEventTime();

    /**
     * A context provided to the {@link WindowAssigner} that allows it to query the
     * current processing time.
     *
     * <p>This is provided to the assigner by its containing
     * {@link org.apache.flink.streaming.runtime.operators.windowing.WindowOperator},
     * which, in turn, gets it from the containing
     * {@link org.apache.flink.streaming.runtime.tasks.StreamTask}.
     */
    public abstract static class WindowAssignerContext {

        /**
         * Returns the current processing time.
         */
        public abstract long getCurrentProcessingTime();

    }
}

• 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

Window

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

@PublicEvolving
public abstract class Window {

    /**
     * Gets the largest timestamp that still belongs to this window.
     *
     * @return The largest timestamp that still belongs to this window.
     */
    public abstract long maxTimestamp();
}

• Window对象代表把无限流数据划分为有限buckets的集合，它有一个maxTimestamp，代表该窗口数据在该时间点内到达；它有两个子类，一个是GlobalWindow，一个是TimeWindow

TimeWindow

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

@PublicEvolving
public class TimeWindow extends Window {

    private final long start;
    private final long end;

    public TimeWindow(long start, long end) {
        this.start = start;
        this.end = end;
    }

    /**
     * Gets the starting timestamp of the window. This is the first timestamp that belongs
     * to this window.
     *
     * @return The starting timestamp of this window.
     */
    public long getStart() {
        return start;
    }

    /**
     * Gets the end timestamp of this window. The end timestamp is exclusive, meaning it
     * is the first timestamp that does not belong to this window any more.
     *
     * @return The exclusive end timestamp of this window.
     */
    public long getEnd() {
        return end;
    }

    /**
     * Gets the largest timestamp that still belongs to this window.
     *
     * <p>This timestamp is identical to {@code getEnd() - 1}.
     *
     * @return The largest timestamp that still belongs to this window.
     *
     * @see #getEnd()
     */
    @Override
    public long maxTimestamp() {
        return end - 1;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }

        TimeWindow window = (TimeWindow) o;

        return end == window.end && start == window.start;
    }

    @Override
    public int hashCode() {
        return MathUtils.longToIntWithBitMixing(start + end);
    }

    @Override
    public String toString() {
        return "TimeWindow{" +
                "start=" + start +
                ", end=" + end +
                '}';
    }

    /**
     * Returns {@code true} if this window intersects the given window.
     */
    public boolean intersects(TimeWindow other) {
        return this.start <= other.end && this.end >= other.start;
    }

    /**
     * Returns the minimal window covers both this window and the given window.
     */
    public TimeWindow cover(TimeWindow other) {
        return new TimeWindow(Math.min(start, other.start), Math.max(end, other.end));
    }

    // ------------------------------------------------------------------------
    // Serializer
    // ------------------------------------------------------------------------

    //......

    // ------------------------------------------------------------------------
    //  Utilities
    // ------------------------------------------------------------------------

    /**
     * Merge overlapping {@link TimeWindow}s. For use by merging
     * {@link org.apache.flink.streaming.api.windowing.assigners.WindowAssigner WindowAssigners}.
     */
    public static void mergeWindows(Collection<TimeWindow> windows, MergingWindowAssigner.MergeCallback<TimeWindow> c) {

        // sort the windows by the start time and then merge overlapping windows

        List<TimeWindow> sortedWindows = new ArrayList<>(windows);

        Collections.sort(sortedWindows, new Comparator<TimeWindow>() {
            @Override
            public int compare(TimeWindow o1, TimeWindow o2) {
                return Long.compare(o1.getStart(), o2.getStart());
            }
        });

        List<Tuple2<TimeWindow, Set<TimeWindow>>> merged = new ArrayList<>();
        Tuple2<TimeWindow, Set<TimeWindow>> currentMerge = null;

        for (TimeWindow candidate: sortedWindows) {
            if (currentMerge == null) {
                currentMerge = new Tuple2<>();
                currentMerge.f0 = candidate;
                currentMerge.f1 = new HashSet<>();
                currentMerge.f1.add(candidate);
            } else if (currentMerge.f0.intersects(candidate)) {
                currentMerge.f0 = currentMerge.f0.cover(candidate);
                currentMerge.f1.add(candidate);
            } else {
                merged.add(currentMerge);
                currentMerge = new Tuple2<>();
                currentMerge.f0 = candidate;
                currentMerge.f1 = new HashSet<>();
                currentMerge.f1.add(candidate);
            }
        }

        if (currentMerge != null) {
            merged.add(currentMerge);
        }

        for (Tuple2<TimeWindow, Set<TimeWindow>> m: merged) {
            if (m.f1.size() > 1) {
                c.merge(m.f1, m.f0);
            }
        }
    }

    /**
     * Method to get the window start for a timestamp.
     *
     * @param timestamp epoch millisecond to get the window start.
     * @param offset The offset which window start would be shifted by.
     * @param windowSize The size of the generated windows.
     * @return window start
     */
    public static long getWindowStartWithOffset(long timestamp, long offset, long windowSize) {
        return timestamp - (timestamp - offset + windowSize) % windowSize;
    }
}

• TimeWindow有start及end属性，其中start为inclusive，而end为exclusive，所以maxTimestamp返回的是end-1；这里重写了equals及hashcode方法
• TimeWindow提供了intersects方法用于表示本窗口与指定窗口是否有交叉；而cover方法用于返回本窗口与指定窗口的重叠窗口
• TimeWindow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

TumblingEventTimeWindows

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

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

    private final long size;

    private final long offset;

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

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

    @Override
    public Collection<TimeWindow> assignWindows(Object element, long timestamp, WindowAssignerContext context) {
        if (timestamp > Long.MIN_VALUE) {
            // Long.MIN_VALUE is currently assigned when no timestamp is present
            long start = TimeWindow.getWindowStartWithOffset(timestamp, offset, size);
            return Collections.singletonList(new TimeWindow(start, start + size));
        } 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(...)'?");
        }
    }

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

    @Override
    public String toString() {
        return "TumblingEventTimeWindows(" + size + ")";
    }

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

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

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

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

• TumblingEventTimeWindows继承了Window，其中元素类型为Object，而窗口类型为TimeWindow；它有两个参数，一个是size，一个是offset，其中offset必须大于等于0，size必须大于offset
• assignWindows方法获取的窗口为start及start+size，而start=TimeWindow.getWindowStartWithOffset(timestamp, offset, size)；getDefaultTrigger方法返回的是EventTimeTrigger；getWindowSerializer方法返回的是TimeWindow.Serializer()；isEventTime返回true
• TumblingEventTimeWindows提供了of静态工厂方法，可以指定size及offset参数

TumblingProcessingTimeWindows

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

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

    private final long size;

    private final long offset;

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

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

    @Override
    public Collection<TimeWindow> assignWindows(Object element, long timestamp, WindowAssignerContext context) {
        final long now = context.getCurrentProcessingTime();
        long start = TimeWindow.getWindowStartWithOffset(now, offset, size);
        return Collections.singletonList(new TimeWindow(start, start + size));
    }

    public long getSize() {
        return size;
    }

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

    @Override
    public String toString() {
        return "TumblingProcessingTimeWindows(" + size + ")";
    }

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

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

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

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

• TumblingProcessingTimeWindows继承了WindowAssigner，其中元素类型为Object，而窗口类型为TimeWindow；它有两个参数，一个是size，一个是offset，其中offset必须大于等于0，size必须大于offset
• assignWindows方法获取的窗口为start及start+size，而start=TimeWindow.getWindowStartWithOffset(now, offset, size)，而now值则为context.getCurrentProcessingTime()，则是与TumblingEventTimeWindows的不同之处，TumblingProcessingTimeWindows不使用timestamp参数来计算，它使用now值替代；getDefaultTrigger方法返回的是ProcessingTimeTrigger，而isEventTime方法返回的为false
• TumblingProcessingTimeWindows也提供了of静态工厂方法，可以指定size及offset参数

小结

• flink的Tumbling Window分为TumblingEventTimeWindows及TumblingProcessingTimeWindows，它们都继承了WindowAssigner，其中元素类型为Object，而窗口类型为TimeWindow；它有两个参数，一个是size，一个是offset，其中offset必须大于等于0，size必须大于offset
• 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；TumblingEventTimeWindows及TumblingProcessingTimeWindows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
• TumblingEventTimeWindows及TumblingProcessingTimeWindows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now值；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

• Tumbling Windows