Spark 任务无法使用滞后窗口功能进行序列化

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问题描述

我注意到，在 DataFrame 上使用 Window 函数后，如果我用函数调用 map()，Spark 会返回一个Task not serializable"；例外这是我的代码:

I've noticed that after I use a Window function over a DataFrame if I call a map() with a function, Spark returns a "Task not serializable" ExceptionThis is my code:

val hc:org.apache.spark.sql.hive.HiveContext =
    new org.apache.spark.sql.hive.HiveContext(sc)

import hc.implicits._
import org.apache.spark.sql.expressions.Window
import org.apache.spark.sql.functions._

def f() : String = "test"
case class P(name: String, surname: String)
val lag_result: org.apache.spark.sql.Column = 
    lag($"name",1).over(Window.partitionBy($"surname"))
val lista: List[P] = List(P("N1","S1"), P("N2","S2"), P("N2","S2"))
val data_frame: org.apache.spark.sql.DataFrame = 
    hc.createDataFrame(sc.parallelize(lista))

df.withColumn("lag_result", lag_result).map(x => f)

// This works
// df.withColumn("lag_result", lag_result).map{ case x =>
//     def f():String = "test";f}.collect

这是堆栈跟踪:

org.apache.spark.SparkException: 任务不可序列化在org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:304)在org.apache.spark.util.ClosureCleaner$.org$apache$spark$util$ClosureCleaner$$clean(ClosureCleaner.scala:294)在org.apache.spark.util.ClosureCleaner$.clean(ClosureCleaner.scala:122)在 org.apache.spark.SparkContext.clean(SparkContext.scala:2055) 在org.apache.spark.rdd.RDD$$anonfun$map$1.apply(RDD.scala:324) 在org.apache.spark.rdd.RDD$$anonfun$map$1.apply(RDD.scala:323) 在 ...以及更多引起:java.io.NotSerializableException:org.apache.spark.sql.Column 序列化堆栈:

对象不可序列化(类:org.apache.spark.sql.Column，值:'lag(name,1,null) windowspecdefinition(surname,UnspecifiedFrame))
字段(类:$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC,名称:lag_result，类型:class org.apache.spark.sql.Column) ...和更多

推荐答案

lag 返回不可序列化的 o.a.s.sql.Column.同样的事情也适用于 WindowSpec.在交互模式下，这些对象可以作为 map 的闭包的一部分被包含:

lag returns o.a.s.sql.Column which is not serializable. Same thing applies to WindowSpec. In interactive mode these object may be included as a part of the closure for map:

scala> import org.apache.spark.sql.expressions.Window
import org.apache.spark.sql.expressions.Window

scala> val df = Seq(("foo", 1), ("bar", 2)).toDF("x", "y")
df: org.apache.spark.sql.DataFrame = [x: string, y: int]

scala> val w = Window.partitionBy("x").orderBy("y")
w: org.apache.spark.sql.expressions.WindowSpec = org.apache.spark.sql.expressions.WindowSpec@307a0097

scala> val lag_y = lag(col("y"), 1).over(w)
lag_y: org.apache.spark.sql.Column = 'lag(y,1,null) windowspecdefinition(x,y ASC,UnspecifiedFrame)

scala> def f(x: Any) = x.toString
f: (x: Any)String

scala> df.select(lag_y).map(f _).first
org.apache.spark.SparkException: Task not serializable
    at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:304)
...
Caused by: java.io.NotSerializableException: org.apache.spark.sql.expressions.WindowSpec
Serialization stack:
    - object not serializable (class: org.apache.spark.sql.expressions.WindowSpec, value: org.apache.spark.sql.expressions.WindowSpec@307a0097)

一个简单的解决方案是将两者都标记为瞬态:

A simple solution is to mark both as transient:

scala> @transient val w = Window.partitionBy("x").orderBy("y")
w: org.apache.spark.sql.expressions.WindowSpec = org.apache.spark.sql.expressions.WindowSpec@7dda1470

scala> @transient val lag_y = lag(col("y"), 1).over(w)
lag_y: org.apache.spark.sql.Column = 'lag(y,1,null) windowspecdefinition(x,y ASC,UnspecifiedFrame)

scala> df.select(lag_y).map(f _).first
res1: String = [null]

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