Spark : How to use mapPartition and create/close c

问题:

So, I want to do certain operations on my spark DataFrame, write them to DB and create another DataFrame at the end. It looks like this :

import sqlContext.implicits._

val newDF = myDF.mapPartitions(
  iterator => {
    val conn = new DbConnection
    iterator.map(
       row => {
         addRowToBatch(row)
         convertRowToObject(row)
     })
    conn.writeTheBatchToDB()
    conn.close()
  })
  .toDF()

This gives me an error as mapPartitions expects return type of Iterator[NotInferedR], but here it is Unit. I know this is possible with forEachPartition, but I'd like to do the mapping also. Doing it separate would be an overhead (extra spark job). What to do?

Thanks!

回答1:

The last expression in the anonymous function implementation must be the return value:

import sqlContext.implicits._

val newDF = myDF.mapPartitions(
  iterator => {
    val conn = new DbConnection
    // using toList to force eager computation - make it happen now when connection is open
    val result = iterator.map(/* the same... */).toList
    conn.writeTheBatchToDB()
    conn.close()
    result.iterator
  }
).toDF()

回答2:

On most cases, eager consuming the iterator will result to execution failure if not slow down of jobs. Thus what I've done was to check if iterator is already empty then do the cleanup routines.

rdd.mapPartitions(itr => {
    val conn = new DbConnection
    itr.map(data => {
       val yourActualResult = // do something with your data and conn here
       if(itr.isEmpty) conn.close // close the connection
       yourActualResult
    })
})

Thought this as a spark problem at first but was a scala one actually. http://www.scala-lang.org/api/2.12.0/scala/collection/Iterator.html#isEmpty:Boolean