Why does custom DefaultSource give java.io.NotSerializableException? - scala

Question

this is my first post on SO and my apology if the improper format is being used.
I'm working with Apache Spark to create a new source (via DefaultSource), BaseRelations, etc... and run into a problem with serialization that I would like to understand better. Consider below a class that extends BaseRelation and implements the scan builder.
class RootTableScan(path: String, treeName: String)(#transient val sqlContext: SQLContext) extends BaseRelation with PrunedFilteredScan{
private val att: core.SRType =
{
val reader = new RootFileReader(new java.io.File(Seq(path) head))
val tmp =
if (treeName==null)
buildATT(findTree(reader.getTopDir), arrangeStreamers(reader), null)
else
buildATT(reader.getKey(treeName).getObject.asInstanceOf[TTree],
arrangeStreamers(reader), null)
tmp
}
// define the schema from the AST
def schema: StructType = {
val s = buildSparkSchema(att)
s
}
// builds a scan
def buildScan(requiredColumns: Array[String], filters: Array[Filter]): RDD[Row] = {
// parallelize over all the files
val r = sqlContext.sparkContext.parallelize(Seq(path), 1).
flatMap({fileName =>
val reader = new RootFileReader(new java.io.File(fileName))
// get the TTree
/* PROBLEM !!! */
val rootTree =
// findTree(reader)
if (treeName == null) findTree(reader)
else reader.getKey(treeName).getObject.asInstanceOf[TTree]
new RootTreeIterator(rootTree, arrangeStreamers(reader),
requiredColumns, filters)
})
println("Done building Scan")
r
}
}
}
PROBLEM identifies where the issue happens. treeName is a val that gets injected into the class thru the constructor. The lambda that uses it is supposed to be executed on the slave and I do need to send the treeName - serialize it. I would like to understand why exactly the code snippet below causes this NotSerializableException. I know for sure that without treeName in it, it works just fine
val rootTree =
// findTree(reader)
if (treeName == null) findTree(reader)
else reader.getKey(treeName).getObject.asInstanceOf[TTree]
Below is the Stack trace
org.apache.spark.SparkException: Task not serializable
at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:298)
at org.apache.spark.util.ClosureCleaner$.org$apache$spark$util$ClosureCleaner$$clean(ClosureCleaner.scala:288)
at org.apache.spark.util.ClosureCleaner$.clean(ClosureCleaner.scala:108)
at org.apache.spark.SparkContext.clean(SparkContext.scala:2056)
at org.apache.spark.rdd.RDD$$anonfun$flatMap$1.apply(RDD.scala:375)
at org.apache.spark.rdd.RDD$$anonfun$flatMap$1.apply(RDD.scala:374)
at org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:151)
at org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:112)
at org.apache.spark.rdd.RDD.withScope(RDD.scala:358)
at org.apache.spark.rdd.RDD.flatMap(RDD.scala:374)
at org.dianahep.sparkroot.package$RootTableScan.buildScan(sparkroot.scala:95)
at org.apache.spark.sql.execution.datasources.DataSourceStrategy$$anonfun$8.apply(DataSourceStrategy.scala:260)
at org.apache.spark.sql.execution.datasources.DataSourceStrategy$$anonfun$8.apply(DataSourceStrategy.scala:260)
at org.apache.spark.sql.execution.datasources.DataSourceStrategy$$anonfun$pruneFilterProject$1.apply(DataSourceStrategy.scala:303)
at org.apache.spark.sql.execution.datasources.DataSourceStrategy$$anonfun$pruneFilterProject$1.apply(DataSourceStrategy.scala:302)
at org.apache.spark.sql.execution.datasources.DataSourceStrategy$.pruneFilterProjectRaw(DataSourceStrategy.scala:379)
at org.apache.spark.sql.execution.datasources.DataSourceStrategy$.pruneFilterProject(DataSourceStrategy.scala:298)
at org.apache.spark.sql.execution.datasources.DataSourceStrategy$.apply(DataSourceStrategy.scala:256)
at org.apache.spark.sql.catalyst.planning.QueryPlanner$$anonfun$1.apply(QueryPlanner.scala:60)
at org.apache.spark.sql.catalyst.planning.QueryPlanner$$anonfun$1.apply(QueryPlanner.scala:60)
at scala.collection.Iterator$$anon$12.nextCur(Iterator.scala:434)
at scala.collection.Iterator$$anon$12.hasNext(Iterator.scala:440)
at org.apache.spark.sql.catalyst.planning.QueryPlanner.plan(QueryPlanner.scala:61)
at org.apache.spark.sql.execution.SparkPlanner.plan(SparkPlanner.scala:47)
at org.apache.spark.sql.execution.SparkPlanner$$anonfun$plan$1$$anonfun$apply$1.applyOrElse(SparkPlanner.scala:51)
at org.apache.spark.sql.execution.SparkPlanner$$anonfun$plan$1$$anonfun$apply$1.applyOrElse(SparkPlanner.scala:48)
at org.apache.spark.sql.catalyst.trees.TreeNode$$anonfun$transformUp$1.apply(TreeNode.scala:301)
at org.apache.spark.sql.catalyst.trees.TreeNode$$anonfun$transformUp$1.apply(TreeNode.scala:301)
at org.apache.spark.sql.catalyst.trees.CurrentOrigin$.withOrigin(TreeNode.scala:69)
at org.apache.spark.sql.catalyst.trees.TreeNode.transformUp(TreeNode.scala:300)
at org.apache.spark.sql.catalyst.trees.TreeNode$$anonfun$4.apply(TreeNode.scala:298)
at org.apache.spark.sql.catalyst.trees.TreeNode$$anonfun$4.apply(TreeNode.scala:298)
at org.apache.spark.sql.catalyst.trees.TreeNode$$anonfun$5.apply(TreeNode.scala:321)
at org.apache.spark.sql.catalyst.trees.TreeNode.mapProductIterator(TreeNode.scala:179)
at org.apache.spark.sql.catalyst.trees.TreeNode.transformChildren(TreeNode.scala:319)
at org.apache.spark.sql.catalyst.trees.TreeNode.transformUp(TreeNode.scala:298)
at org.apache.spark.sql.execution.SparkPlanner$$anonfun$plan$1.apply(SparkPlanner.scala:48)
at org.apache.spark.sql.execution.SparkPlanner$$anonfun$plan$1.apply(SparkPlanner.scala:48)
at scala.collection.Iterator$$anon$11.next(Iterator.scala:409)
at org.apache.spark.sql.execution.QueryExecution.sparkPlan$lzycompute(QueryExecution.scala:78)
at org.apache.spark.sql.execution.QueryExecution.sparkPlan(QueryExecution.scala:76)
at org.apache.spark.sql.execution.QueryExecution.executedPlan$lzycompute(QueryExecution.scala:83)
at org.apache.spark.sql.execution.QueryExecution.executedPlan(QueryExecution.scala:83)
at org.apache.spark.sql.Dataset.withTypedCallback(Dataset.scala:2572)
at org.apache.spark.sql.Dataset.head(Dataset.scala:1934)
at org.apache.spark.sql.Dataset.take(Dataset.scala:2149)
at org.apache.spark.sql.Dataset.showString(Dataset.scala:239)
at org.apache.spark.sql.Dataset.show(Dataset.scala:526)
at org.apache.spark.sql.Dataset.show(Dataset.scala:486)
at org.apache.spark.sql.Dataset.show(Dataset.scala:495)
... 50 elided
Caused by: java.io.NotSerializableException: org.dianahep.sparkroot.package$RootTableScan
Serialization stack:
- object not serializable (class: org.dianahep.sparkroot.package$RootTableScan, value: org.dianahep.sparkroot.package$RootTableScan#6421e9e7)
- field (class: org.dianahep.sparkroot.package$RootTableScan$$anonfun$1, name: $outer, type: class org.dianahep.sparkroot.package$RootTableScan)
- object (class org.dianahep.sparkroot.package$RootTableScan$$anonfun$1, <function1>)
at org.apache.spark.serializer.SerializationDebugger$.improveException(SerializationDebugger.scala:40)
at org.apache.spark.serializer.JavaSerializationStream.writeObject(JavaSerializer.scala:46)
at org.apache.spark.serializer.JavaSerializerInstance.serialize(JavaSerializer.scala:100)
at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:295)
From the stack I think I can deduce that it tries to serialize my lambda and can not. this lambda should be a closure as we have a val in there that is defined outside of the lambda scope. But I don't understand why this can not be serialized.
Any help would be really appreciated!!!
Thanks a lot!

Answer 1

Any time a scala closure references a class variable, like treeName, then the JVM serializes the parent class along with the closure. Your class RootTableScan is not serializable, though! The solution is to create a local string variable:
// builds a scan
def buildScan(requiredColumns: Array[String], filters: Array[Filter]): RDD[Row] = {
val localTreeName = treeName // this is safe to serialize
// parallelize over all the files
val r = sqlContext.sparkContext.parallelize(Seq(path), 1).
flatMap({fileName =>
val reader = new RootFileReader(new java.io.File(fileName))
// get the TTree
/* PROBLEM !!! */
val rootTree =
// findTree(reader)
if (localTreeName == null) findTree(reader)
else reader.getKey(localTreeName).getObject.asInstanceOf[TTree]
new RootTreeIterator(rootTree, arrangeStreamers(reader),
requiredColumns, filters)
})