ReduceByKey with a byte array as the key

Question

I would like to work with RDD pairs of Tuple2<byte[], obj>, but byte[]s with the same contents are considered as different values because their reference values are different.

I didn't see any to pass in a custom comparer. I could convert the byte[] into a String with an explicit charset, but I'm wondering if there's a more efficient way.

Answer 1

Custom comparers are insufficient because Spark uses the hashCode of the objects to organize keys in partitions. (At least the HashPartitioner will do that, you could provide a custom partitioner that can deal with arrays)

Wrapping the array to provide proper equals and hashCode should address the issue. A lightweight wrapper should do the trick:

class SerByteArr(val bytes: Array[Byte]) extends Serializable {
    override val hashCode = bytes.deep.hashCode
    override def equals(obj:Any) = obj.isInstanceOf[SerByteArr] && obj.asInstanceOf[SerByteArr].bytes.deep == this.bytes.deep
}

A quick test:

import scala.util.Random
val data = (1 to 100000).map(_ => Random.nextInt(100).toString.getBytes("UTF-8"))
val rdd = sparkContext.parallelize(data)
val byKey = rdd.keyBy(identity)
// this won't work b/c the partitioner does not support arrays as keys
val grouped = byKey.groupByKey
// org.apache.spark.SparkException: Default partitioner cannot partition array keys.

// let's use the wrapper instead   

val keyable = rdd.map(elem =>  new SerByteArr(elem))
val bySerKey = keyable.keyBy(identity)
val grouped = bySerKey.groupByKey
grouped.count
// res14: Long = 100

Answer 2

You could create a wrapper class and defined your own equality / comparison functions. This is likely slightly faster since you don't have to do a copy of the array (although you still have an object allocation).