flink dataset join笔记

1、dataset的join连接，通过key进行关联，一般情况下的join都是inner join，类似sql里的inner join

key包括以下几种情况：
a key expression
a key-selector function
one or more field position keys (Tuple DataSet only).
Case Class Fields

2、inner join的几种情况

2.1 缺省的join，jion到一个Tuple2元组里

public static class User { public String name; public int zip; }
public static class Store { public Manager mgr; public int zip; }
DataSet<User> input1 = // [...]
DataSet<Store> input2 = // [...]
// result dataset is typed as Tuple2
DataSet<Tuple2<User, Store>>
            result = input1.join(input2)
                           .where("zip")       // key of the first input (users)
                           .equalTo("zip");    // key of the second input (stores)

2.2 用户自定义JoinFuncation，使用with语句

// some POJO
public class Rating {
  public String name;
  public String category;
  public int points;
}

// Join function that joins a custom POJO with a Tuple
public class PointWeighter
         implements JoinFunction<Rating, Tuple2<String, Double>, Tuple2<String, Double>> {

  @Override
  public Tuple2<String, Double> join(Rating rating, Tuple2<String, Double> weight) {
    // multiply the points and rating and construct a new output tuple
    return new Tuple2<String, Double>(rating.name, rating.points * weight.f1);
  }
}

DataSet<Rating> ratings = // [...]
DataSet<Tuple2<String, Double>> weights = // [...]
DataSet<Tuple2<String, Double>>
            weightedRatings =
            ratings.join(weights)

                   // key of the first input
                   .where("category")

                   // key of the second input
                   .equalTo("f0")

                   // applying the JoinFunction on joining pairs
                   .with(new PointWeighter());

2.3 使用Flat-Join Function，这种JoinFuncation和FlatJoinFuncation与MapFuncation和FlatMapFuncation的关系类似

public class PointWeighter
         implements FlatJoinFunction<Rating, Tuple2<String, Double>, Tuple2<String, Double>> {
  @Override
  public void join(Rating rating, Tuple2<String, Double> weight,
      Collector<Tuple2<String, Double>> out) {
    if (weight.f1 > 0.1) {
        out.collect(new Tuple2<String, Double>(rating.name, rating.points * weight.f1));
    }
  }
}

DataSet<Tuple2<String, Double>>
            weightedRatings =
            ratings.join(weights) // [...]

2.4 join的投影构造，生成自定义的结果集

DataSet<Tuple3<Integer, Byte, String>> input1 = // [...]
DataSet<Tuple2<Integer, Double>> input2 = // [...]
DataSet<Tuple4<Integer, String, Double, Byte>>
            result =
            input1.join(input2)
                  // key definition on first DataSet using a field position key
                  .where(0)
                  // key definition of second DataSet using a field position key
                  .equalTo(0)
                  // select and reorder fields of matching tuples
                  .projectFirst(0,2).projectSecond(1).projectFirst(1);

projectFirst(int...) and projectSecond(int...)
选择应组合成输出元组的第一个和第二个连接输入的字段。索引的顺序定义了输出元组中字段的顺序。
连接投影也适用于非元组数据集，在这种情况下,必须在不带参数的情况下调用projectFirst（）或projectSecond（），以将连接元素添加到输出元组。

2.5 加入join数据集大小提示，这是为了优化join的效率，引导优化器选择正确的执行策略。

DataSet<Tuple2<Integer, String>> input1 = // [...]
DataSet<Tuple2<Integer, String>> input2 = // [...]

DataSet<Tuple2<Tuple2<Integer, String>, Tuple2<Integer, String>>>
            result1 =
            // hint that the second DataSet is very small
            input1.joinWithTiny(input2)
                  .where(0)
                  .equalTo(0);

DataSet<Tuple2<Tuple2<Integer, String>, Tuple2<Integer, String>>>
            result2 =
            // hint that the second DataSet is very large
            input1.joinWithHuge(input2)
                  .where(0)
                  .equalTo(0);

2.6 join的算法提示，Flink运行时可以以各种方式执行连接。在不同情况下，每种可能的方式都优于其他方式。系统会尝试自动选择合理的方式，但允许您手动选择策略，以防您想要强制执行连接的特定方式。

DataSet<SomeType> input1 = // [...]
DataSet<AnotherType> input2 = // [...]

DataSet<Tuple2<SomeType, AnotherType> result =
      input1.join(input2, JoinHint.BROADCAST_HASH_FIRST)
            .where("id").equalTo("key");

    OPTIMIZER_CHOOSES：相当于不提供任何提示，将选择留给系统。

    BROADCAST_HASH_FIRST：广播第一个输入并从中构建哈希表，由第二个输入探测。如果第一个输入非常小，这是一个很好的策略。

    BROADCAST_HASH_SECOND：广播第二个输入并从中构建一个哈希表，由第一个输入探测。如果第二个输入非常小，这是一个好策略。

    REPARTITION_HASH_FIRST：系统对每个输入进行分区（shuffle）（除非输入已经分区）并从第一个输入构建哈希表。如果第一个输入小于第二个输入，则此策略很好，但两个输入仍然很大。
    注意：如果不能进行大小估算，并且不能重新使用预先存在的分区和排序顺序，则这是系统使用的默认回退策略。

    REPARTITION_HASH_SECOND：系统对每个输入进行分区（shuffle）（除非输入已经被分区）并从第二个输入构建哈希表。如果第二个输入小于第一个输入，则此策略很好，但两个输入仍然很大。

    REPARTITION_SORT_MERGE：系统对每个输入进行分区（shuffle）（除非输入已经被分区）并对每个输入进行排序（除非它已经排序）。输入通过已排序输入的流合并来连接。如果已经对一个或两个输入进行了排序，则此策略很好。

3、FlatJoinFunction与FlatMapFunction的区别（JoinFuncation和MapFuncation的情况类似）

1、实际上两者可以干相同的事情
2、使用的区别是FlatJoinFunction有两个输入（就是join的两个数据集）一个输出，
    而FlatMapFunction只有一个输入，但是这个输入参数里可以直接包括多个输入结构（即join的两个数据集都可以放入到一个输入参数里），
    所以最终实现的结果实际是一致的。

3.1 FlatMapFunction应用join的例子

      DataSet<Long> pagesInput = // [...]
      DataSet<Tuple2<Long, Long>> linksInput = // [...]

      DataSet<Tuple2<Long, Double>> pagesWithRanks = // [...]

      DataSet<Tuple2<Long, Long[]>> adjacencyListInput =// [...]

      IterativeDataSet<Tuple2<Long, Double>> iteration = // [...]

       DataSet<Tuple2<Long, Double>> newRanks = iteration.join(adjacencyListInput)
                .where(0).equalTo(0)
                .flatMap(new JoinVertexWithEdgesMatch())
                //下面的不用关注
                .groupBy(0)
                .aggregate(Aggregations.SUM, 1)

                .map(new Dampener(PageRank.DAMPENING_FACTOR, numPages));

    public static final class JoinVertexWithEdgesMatch implements FlatMapFunction<Tuple2<Tuple2<Long, Double>, Tuple2<Long, Long[]>>, Tuple2<Long, Double>> {

        @Override
        public void flatMap(Tuple2<Tuple2<Long, Double>, Tuple2<Long, Long[]>> value, Collector<Tuple2<Long, Double>> out) {
            Long[] neighbors = value.f1.f1;
            double rank = value.f0.f1;
            double rankToDistribute = rank / ((double) neighbors.length);

            for (Long neighbor : neighbors) {
                out.collect(new Tuple2<Long, Double>(neighbor, rankToDistribute));
            }
        }
    }

从上面的例子可以看到FlatMapFunction虽然只有一个输入，但是输入参数Tuple2里包含两个Tuple2，这被包含的两个Tuple2就是join的两个数据集。

3.2 FlatJoinFunction和JoinFuncation例子，它们使用with语句来实现

DataSet<Tuple2<Long, Long>> changes =   iteration.getWorkset().join(edges)
                .where(0).equalTo(0)
                .with(new NeighborWithComponentIDJoin())
                .groupBy(0).aggregate(Aggregations.MIN, 1)
                .join(iteration.getSolutionSet()).where(0).equalTo(0)
                .with(new ComponentIdFilter());

    public static final class NeighborWithComponentIDJoin implements JoinFunction<Tuple2<Long, Long>, Tuple2<Long, Long>, Tuple2<Long, Long>> {

        @Override
        public Tuple2<Long, Long> join(Tuple2<Long, Long> vertexWithComponent, Tuple2<Long, Long> edge) {
            return new Tuple2<>(edge.f1, vertexWithComponent.f1);
        }
    }

    public static final class ComponentIdFilter implements FlatJoinFunction<Tuple2<Long, Long>, Tuple2<Long, Long>, Tuple2<Long, Long>> {

        @Override
        public void join(Tuple2<Long, Long> candidate, Tuple2<Long, Long> old, Collector<Tuple2<Long, Long>> out) {
            if (candidate.f1 < old.f1) {
                out.collect(candidate);
            }
        }
    }

从上述例子可以看到FlatJoinFunction或者JoinFunction是两个输入参数，也就是join的两个数据集

3.3 从源码上看，FlatJoinFunction与FlatMapFunction两者实际没太大区别

@Public
@FunctionalInterface
public interface FlatJoinFunction<IN1, IN2, OUT> extends Function, Serializable {

    /**
     * The join method, called once per joined pair of elements.
     *
     * @param first The element from first input.
     * @param second The element from second input.
     * @param out The collector used to return zero, one, or more elements.
     *
     * @throws Exception This method may throw exceptions. Throwing an exception will cause the operation
     *                   to fail and may trigger recovery.
     */
    void join (IN1 first, IN2 second, Collector<OUT> out) throws Exception;
}

@Public
@FunctionalInterface
public interface FlatMapFunction<T, O> extends Function, Serializable {

    /**
     * The core method of the FlatMapFunction. Takes an element from the input data set and transforms
     * it into zero, one, or more elements.
     *
     * @param value The input value.
     * @param out The collector for returning result values.
     *
     * @throws Exception This method may throw exceptions. Throwing an exception will cause the operation
     *                   to fail and may trigger recovery.
     */
    void flatMap(T value, Collector<O> out) throws Exception;
}

4、outer join，外连接，类似sql的left join,right join,full join的情况

OuterJoin在两个数据集上执行左，右或全外连接。外连接类似于常规的（inner join）连接，并创建在其键上相等的所有元素对。
此外，如果在另一侧没有找到匹配的key，则保留“外部”侧（左侧，右侧或两者）的记录。
匹配元素对（或一个元素和另一个输入的空值）被赋予JoinFunction以将元素对转换为单个元素，
或者给予FlatJoinFunction以将元素对转换为任意多个（包括none）元素。

4.1 外连接OuterJoin

OuterJoin调用用户定义的连接函数来处理连接元组。连接函数接收第一个输入DataSet的一个元素和第二个输入DataSet的一个元素，并返回一个元素。根据外连接的类型（left，right，full），join函数的两个输入元素之一可以为null。

以下代码使用键选择器函数执行DataSet与自定义java对象和Tuple DataSet的左外连接，并显示如何使用用户定义的连接函数：

// some POJO
public class Rating {
  public String name;
  public String category;
  public int points;
}

// Join function that joins a custom POJO with a Tuple
public class PointAssigner
         implements JoinFunction<Tuple2<String, String>, Rating, Tuple2<String, Integer>> {

  @Override
  public Tuple2<String, Integer> join(Tuple2<String, String> movie, Rating rating) {
    // Assigns the rating points to the movie.
    // NOTE: rating might be null
    return new Tuple2<String, Double>(movie.f0, rating == null ? -1 : rating.points;
  }
}

DataSet<Tuple2<String, String>> movies = // [...]
DataSet<Rating> ratings = // [...]
DataSet<Tuple2<String, Integer>>
            moviesWithPoints =
            movies.leftOuterJoin(ratings)

                   // key of the first input
                   .where("f0")

                   // key of the second input
                   .equalTo("name")

                   // applying the JoinFunction on joining pairs
                   .with(new PointAssigner());

4.2 FlatJoinFuncation实现OuterJoin

public class PointAssigner
         implements FlatJoinFunction<Tuple2<String, String>, Rating, Tuple2<String, Integer>> {
  @Override
  public void join(Tuple2<String, String> movie, Rating rating
    Collector<Tuple2<String, Integer>> out) {
  if (rating == null ) {
    out.collect(new Tuple2<String, Integer>(movie.f0, -1));
  } else if (rating.points < 10) {
    out.collect(new Tuple2<String, Integer>(movie.f0, rating.points));
  } else {
    // do not emit
  }
}

DataSet<Tuple2<String, Integer>>
            moviesWithPoints =
            movies.leftOuterJoin(ratings) // [...]