高效计算spark中的top-k元素

Question

我的数据框类似于：

+---+-----+-----+
|key|thing|value|
+---+-----+-----+
| u1|  foo|    1|
| u1|  foo|    2|
| u1|  bar|   10|
| u2|  foo|   10|
| u2|  foo|    2|
| u2|  bar|   10|
+---+-----+-----+

并希望获得以下结果：

+---+-----+---------+----+
|key|thing|sum_value|rank|
+---+-----+---------+----+
| u1|  bar|       10|   1|
| u1|  foo|        3|   2|
| u2|  foo|       12|   1|
| u2|  bar|       10|   2|
+---+-----+---------+----+

目前有类似的代码：

val df = Seq(("u1", "foo", 1), ("u1", "foo", 2), ("u1", "bar", 10), ("u2", "foo", 10), ("u2", "foo", 2), ("u2", "bar", 10)).toDF("key", "thing", "value")

 // calculate sums per key and thing
 val aggregated = df.groupBy("key", "thing").agg(sum("value").alias("sum_value"))

 // get topk items per key
 val k = lit(10)
 val topk = aggregated.withColumn("rank", rank over  Window.partitionBy("key").orderBy(desc("sum_value"))).filter('rank < k)

但是，这段代码非常低效。 window 函数生成 总订单 项并导致 巨大的随机播放 。

如何更有效地计算前 k 项？也许使用近似函数，即类似于 https://datasketches.github.io/ or https://spark.apache.org/docs/latest/ml-frequent-pattern-mining.html

的草图

Answer 1

RDD 的救援

aggregated.as[(String, String, Long)].rdd.groupBy(_._1).map{ case (thing, it) => (thing, it.map(e=> (e._2, e._3)).toList.sortBy(sorter => sorter._2).take(1))}.toDF.show
+---+----------+
| _1|        _2|
+---+----------+
| u1| [[foo,3]]|
| u2|[[bar,10]]|
+---+----------+

这很可能会根据评论中的建议得到改进。 IE。不是从 aggregated 开始，而是 df。这可能类似于：

df.as[(String, String, Long)].rdd.groupBy(_._1).map{case (thing, it) => {
      val aggregatedInner = it.groupBy(e=> (e._2)).mapValues(events=> events.map(value => value._3).sum)
      val topk = aggregatedInner.toArray.sortBy(sorter=> sorter._2).take(1)
      (thing, topk)
    }}.toDF.show

Answer 2

这是推荐系统的经典算法。

case class Rating(thing: String, value: Int) extends Ordered[Rating] {
  def compare(that: Rating): Int = -this.value.compare(that.value)
}

case class Recommendation(key: Int, ratings: Seq[Rating]) {
  def keep(n: Int) = this.copy(ratings = ratings.sorted.take(n))
}

val TOPK = 10

df.groupBy('key)
  .agg(collect_list(struct('thing, 'value)) as "ratings")
  .as[Recommendation]
  .map(_.keep(TOPK))

您还可以在以下位置查看源代码：

Spotify 大数据 Rosetta 代码/TopItemsPerUser.scala，这里有几个针对 Spark 或 Scio 的解决方案
Spark MLLib / TopByKeyAggregator.scala，被认为是使用他们的推荐算法时的最佳实践，但看起来他们的示例仍然使用 RDD。

import org.apache.spark.mllib.rdd.MLPairRDDFunctions._

sc.parallelize(Array(("u1", ("foo", 1)), ("u1", ("foo", 2)), ("u1", ("bar", 10)), ("u2", ("foo", 10)),
  ("u2", ("foo", 2)), ("u2", ("bar", 10))))
  .topByKey(10)(Ordering.by(_._2))

高效计算spark中的top-k元素

Efficiently calculate top-k elements in spark

approximation

rank

window-functions

apache-spark

apache-spark-sql