Default Partitioning Scheme in Spark

Question

When I execute below command:

scala> val rdd = sc.parallelize(List((1,2),(3,4),(3,6)),4).partitionBy(new HashPartitioner(10)).persist()
rdd: org.apache.spark.rdd.RDD[(Int, Int)] = ShuffledRDD[10] at partitionBy at <console>:22

scala> rdd.partitions.size
res9: Int = 10

scala> rdd.partitioner.isDefined
res10: Boolean = true


scala> rdd.partitioner.get
res11: org.apache.spark.Partitioner = org.apache.spark.HashPartitioner@a

It says that there are 10 partitions and partitioning is done using HashPartitioner. But When I execute below command:

scala> val rdd = sc.parallelize(List((1,2),(3,4),(3,6)),4)
...
scala> rdd.partitions.size
res6: Int = 4
scala> rdd.partitioner.isDefined
res8: Boolean = false

It says that there are 4 partitions and partitioner is not defined. So, What is default Partitioning Scheme in Spark ? / How data is partitioned in second case?

Innsbruck

Answer 1

You have to distinguish between two different things:

• partitioning as distributing data between partitions depending on a value of the key which is limited only to the PairwiseRDDs (RDD[(T, U)]). This creates a relationship between partition and the set of keys which can be found on a given partition.

• partitioning as splitting input into multiple partitions where data is simply divided into chunks containing consecutive records to enable distributed computation. Exact logic depends on a specific source but it is either number of records or size of a chunk.

  In case of parallelize data is evenly distributed between partitions using indices. In case of HadoopInputFormats (like textFile) it depends on properties like mapreduce.input.fileinputformat.split.minsize / mapreduce.input.fileinputformat.split.maxsize.

So default partitioning scheme is simply none because partitioning is not applicable to all RDDs. For operations which require partitioning on a PairwiseRDD (aggregateByKey, reduceByKey etc.) default method is use hash partitioning.

Enschede

Thanks for great answer. Does it mean that when we group data for key-value pair RDDs using paritionBy, then they are partitioned by hash paritioner and are co-located in same worker nodes but with different partitions. – Chrestomathy