Avoid groupByKey when performing a group of multiple items by key
As already showed in [21] let's suppose we've got a RDD items like:
(3922774869,10,1)
(3922774869,11,1)
(3922774869,12,2)
(3922774869,13,2)
(1779744180,10,1)
(1779744180,11,1)
(3922774869,14,3)
(3922774869,15,2)
(1779744180,16,1)
(3922774869,12,1)
(3922774869,13,1)
(1779744180,14,1)
(1779744180,15,1)
(1779744180,16,1)
(3922774869,14,2)
(3922774869,15,1)
(1779744180,16,1)
(1779744180,17,1)
(3922774869,16,4)
...
which represent (id, age, count) and we want to group those lines to generate a dataset for which each line represent the distribution of age of each id like this ((id, age) is unique):
(1779744180, (10,1), (11,1), (12,2), (13,2) ...)
(3922774869, (10,1), (11,1), (12,3), (13,4) ...)
which is (id, (age, count), (age, count) ...)
The easiest way is first reduce by both fields and then use groupBy:
rdd
.map { case (id, age, count) => ((id, age), count) }.reduceByKey(_ + _)
.map { case ((id, age), count) => (id, (age, count)) }.groupByKey()
Which returns an `RDD[(Long, Iterable[(Int, Int)])]`, for the input above it would contain these two records:
(1779744180,CompactBuffer((16,3), (15,1), (14,1), (11,1), (10,1), (17,1)))
(3922774869,CompactBuffer((11,1), (12,3), (16,4), (13,3), (15,3), (10,1), (14,5)))
But if you have a very large dataset, in order to reduce shuffling, you should not to use `groupByKey`.
Instead you can use `aggregateByKey`:
import scala.collection.mutable
val rddById = rdd.map { case (id, age, count) => ((id, age), count) }.reduceByKey(_ + _)
val initialSet = mutable.HashSet.empty[(Int, Int)]
val addToSet = (s: mutable.HashSet[(Int, Int)], v: (Int, Int)) => s += v
val mergePartitionSets = (p1: mutable.HashSet[(Int, Int)], p2: mutable.HashSet[(Int, Int)]) => p1 ++= p2
Here are more functions to prefer over`groupByKey`:
• `combineByKey` can be used when you are combining elements but your return type differs from your input value type. You can see an example here
• `foldByKey` merges the values for each key using an associative function and a neutral "zero value".