[case52]聊聊flink KeyedStream的aggregation操作

序

本文主要研究一下flink KeyedStream的aggregation操作

实例

    @Test
    public void testMax() throws Exception {
        final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
        WordCount[] data = new WordCount[]{new WordCount(1,"Hello", 1), new
                WordCount(1,"World", 3), new WordCount(2,"Hello", 1)};
        env.fromElements(data)
                .keyBy("word")
                .max("frequency")
                .addSink(new SinkFunction<WordCount>() {
                    @Override
                    public void invoke(WordCount value, Context context) throws Exception {
                        LOGGER.info("value:{}",value);
                    }
                });
        env.execute("testMax");
    }

• 这里先对word字段进行keyBy操作，然后再通过KeyedStream的max方法按frequency字段取最大的WordCount

KeyedStream.aggregate

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/datastream/KeyedStream.java

    public SingleOutputStreamOperator<T> sum(int positionToSum) {
        return aggregate(new SumAggregator<>(positionToSum, getType(), getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> sum(String field) {
        return aggregate(new SumAggregator<>(field, getType(), getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> max(int positionToMax) {
        return aggregate(new ComparableAggregator<>(positionToMax, getType(), AggregationFunction.AggregationType.MAX,
                getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> max(String field) {
        return aggregate(new ComparableAggregator<>(field, getType(), AggregationFunction.AggregationType.MAX,
                false, getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> min(int positionToMin) {
        return aggregate(new ComparableAggregator<>(positionToMin, getType(), AggregationFunction.AggregationType.MIN,
                getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> min(String field) {
        return aggregate(new ComparableAggregator<>(field, getType(), AggregationFunction.AggregationType.MIN,
                false, getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> maxBy(int positionToMaxBy) {
        return this.maxBy(positionToMaxBy, true);
    }

    public SingleOutputStreamOperator<T> maxBy(String positionToMaxBy) {
        return this.maxBy(positionToMaxBy, true);
    }

    public SingleOutputStreamOperator<T> maxBy(int positionToMaxBy, boolean first) {
        return aggregate(new ComparableAggregator<>(positionToMaxBy, getType(), AggregationFunction.AggregationType.MAXBY, first,
                getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> maxBy(String field, boolean first) {
        return aggregate(new ComparableAggregator<>(field, getType(), AggregationFunction.AggregationType.MAXBY,
                first, getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> minBy(int positionToMinBy) {
        return this.minBy(positionToMinBy, true);
    }

    public SingleOutputStreamOperator<T> minBy(String positionToMinBy) {
        return this.minBy(positionToMinBy, true);
    }

    public SingleOutputStreamOperator<T> minBy(int positionToMinBy, boolean first) {
        return aggregate(new ComparableAggregator<T>(positionToMinBy, getType(), AggregationFunction.AggregationType.MINBY, first,
                getExecutionConfig()));
    }

    public SingleOutputStreamOperator<T> minBy(String field, boolean first) {
        return aggregate(new ComparableAggregator(field, getType(), AggregationFunction.AggregationType.MINBY,
                first, getExecutionConfig()));
    }

    protected SingleOutputStreamOperator<T> aggregate(AggregationFunction<T> aggregate) {
        StreamGroupedReduce<T> operator = new StreamGroupedReduce<T>(
                clean(aggregate), getType().createSerializer(getExecutionConfig()));
        return transform("Keyed Aggregation", getType(), operator);
    }

• KeyedStream的aggregation方法是protected修饰的，sum、max、min、maxBy、minBy这几个方法实际都是调用aggregate方法，只是它们创建的ComparableAggregator的AggregationType不一样，分别是SUM, MAX, MIN, MAXBY, MINBY
• 每个sum、max、min、maxBy、minBy都有两个重载方法，一个是int类型的参数，一个是String类型的参数
• maxBy、minBy比sum、max、min多了first(boolean)参数，该参数用于指定在碰到多个compare值相等时，是否取第一个返回

ComparableAggregator

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/functions/aggregation/ComparableAggregator.java

@Internal
public class ComparableAggregator<T> extends AggregationFunction<T> {

    private static final long serialVersionUID = 1L;

    private Comparator comparator;
    private boolean byAggregate;
    private boolean first;
    private final FieldAccessor<T, Object> fieldAccessor;

    private ComparableAggregator(AggregationType aggregationType, FieldAccessor<T, Object> fieldAccessor, boolean first) {
        this.comparator = Comparator.getForAggregation(aggregationType);
        this.byAggregate = (aggregationType == AggregationType.MAXBY) || (aggregationType == AggregationType.MINBY);
        this.first = first;
        this.fieldAccessor = fieldAccessor;
    }

    public ComparableAggregator(int positionToAggregate,
            TypeInformation<T> typeInfo,
            AggregationType aggregationType,
            ExecutionConfig config) {
        this(positionToAggregate, typeInfo, aggregationType, false, config);
    }

    public ComparableAggregator(int positionToAggregate,
            TypeInformation<T> typeInfo,
            AggregationType aggregationType,
            boolean first,
            ExecutionConfig config) {
        this(aggregationType, FieldAccessorFactory.getAccessor(typeInfo, positionToAggregate, config), first);
    }

    public ComparableAggregator(String field,
            TypeInformation<T> typeInfo,
            AggregationType aggregationType,
            boolean first,
            ExecutionConfig config) {
        this(aggregationType, FieldAccessorFactory.getAccessor(typeInfo, field, config), first);
    }

    @SuppressWarnings("unchecked")
    @Override
    public T reduce(T value1, T value2) throws Exception {
        Comparable<Object> o1 = (Comparable<Object>) fieldAccessor.get(value1);
        Object o2 = fieldAccessor.get(value2);

        int c = comparator.isExtremal(o1, o2);

        if (byAggregate) {
            // if they are the same we choose based on whether we want to first or last
            // element with the min/max.
            if (c == 0) {
                return first ? value1 : value2;
            }

            return c == 1 ? value1 : value2;

        } else {
            if (c == 0) {
                value1 = fieldAccessor.set(value1, o2);
            }
            return value1;
        }
    }
}

• ComparableAggregator继承了AggregationFunction，而AggregationFunction则实现了ReduceFunction接口，这里ComparableAggregator实现的reduce方法，它首先借助Comparator来比较两个对象，然后根据是否是byAggregate做不同处理，如果是byAggregate，则在比较值为0时，判断是否返回最先遇到的元素，如果是则返回value1，否则返回value2，比较值非0时，则取比较值最大的元素返回；如果不是byAggregate，则如果比较值为0(比较字段的值value1小于等于value2的情况)，则使用反射方法将value2的比较字段的值更新到value1，最后都是返回value1

AggregationFunction

@Internal
public abstract class AggregationFunction<T> implements ReduceFunction<T> {
    private static final long serialVersionUID = 1L;

    /**
     * Aggregation types that can be used on a windowed stream or keyed stream.
     */
    public enum AggregationType {
        SUM, MIN, MAX, MINBY, MAXBY,
    }
}

• AggregationFunction声明实现了ReduceFunction，同时定义了五种类型的AggregationType，分别是SUM, MIN, MAX, MINBY, MAXBY

Comparator

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/functions/aggregation/Comparator.java

@Internal
public abstract class Comparator implements Serializable {

    private static final long serialVersionUID = 1L;

    public abstract <R> int isExtremal(Comparable<R> o1, R o2);

    public static Comparator getForAggregation(AggregationType type) {
        switch (type) {
        case MAX:
            return new MaxComparator();
        case MIN:
            return new MinComparator();
        case MINBY:
            return new MinByComparator();
        case MAXBY:
            return new MaxByComparator();
        default:
            throw new IllegalArgumentException("Unsupported aggregation type.");
        }
    }

    private static class MaxComparator extends Comparator {

        private static final long serialVersionUID = 1L;

        @Override
        public <R> int isExtremal(Comparable<R> o1, R o2) {
            return o1.compareTo(o2) > 0 ? 1 : 0;
        }

    }

    private static class MaxByComparator extends Comparator {

        private static final long serialVersionUID = 1L;

        @Override
        public <R> int isExtremal(Comparable<R> o1, R o2) {
            int c = o1.compareTo(o2);
            if (c > 0) {
                return 1;
            }
            if (c == 0) {
                return 0;
            } else {
                return -1;
            }
        }

    }

    private static class MinByComparator extends Comparator {

        private static final long serialVersionUID = 1L;

        @Override
        public <R> int isExtremal(Comparable<R> o1, R o2) {
            int c = o1.compareTo(o2);
            if (c < 0) {
                return 1;
            }
            if (c == 0) {
                return 0;
            } else {
                return -1;
            }
        }

    }

    private static class MinComparator extends Comparator {

        private static final long serialVersionUID = 1L;

        @Override
        public <R> int isExtremal(Comparable<R> o1, R o2) {
            return o1.compareTo(o2) < 0 ? 1 : 0;
        }

    }
}

• Comparator则实现Serializable接口，定义了isExtremal抽象方法，同时提供了getForAggregation工厂方法，根据不同的AggregationType创建不同的Comparator
• Comparator里头定义了MaxComparator、MinComparator、MinByComparator、MaxByComparator四个子类，它们都实现了isExtremal方法
• MaxComparator直接利用Comparable接口定义的compareTo方法，不过它的返回只有0和1，compareTo大于0的时候才返回1，否则返回0，也就是大于的情况才返回1，否则返回0；MaxByComparator也先根据Comparable接口定义的compareTo方法获取值，不过它的返回值有3种，大于0的时候返回1，等于0时返回0，小于0时返回-1，也就是大于的情况返回1，相等的情况返回0，小于的情况返回-1

小结

• KeyedStream的aggregation操作主要分为sum、max、min、maxBy、minBy这几个方法，它们内部都调用了protected修饰的aggregation方法，只是它们创建的ComparableAggregator的AggregationType不一样，分别是SUM, MAX, MIN, MAXBY, MINBY
• ComparableAggregator继承了AggregationFunction，而AggregationFunction则实现了ReduceFunction接口，这里ComparableAggregator实现的reduce方法，它首先借助Comparator来比较两个对象，然后根据是否是byAggregate做不同处理，如果是byAggregate，则在比较值为0时，判断是否返回最先遇到的元素，如果是则返回最先遇到的，否则返回最后遇到的，比较值非0时，则取比较值最大的元素返回；如果不是byAggregate，则如果比较值为0，则使用反射方法将后者的值更新到value1，最后都是返回value1
• Comparator里头定义了MaxComparator、MinComparator、MinByComparator、MaxByComparator四个子类，它们都实现了isExtremal方法；MaxComparator与MaxByComparator的区别在于，MaxComparator大于返回1，小于等于返回0，而MaxByComparator返回值更精细，大于返回1，等于返回0，小于返回-1；这个区别也体现在ComparableAggregator的reduce方法中，而且maxBy、minBy比其他方法多了一个first(boolean)参数，专门用于在比较值为的0的时候选择返回哪个元素；而reduce方法对于非byAggregate操作，始终返回的是value1，在比较值小于等于的时候，使用反射更新value1，然后返回value1

doc

• DataStream Transformations