## Combine - Combine PerKey

After creating a keyed PCollection (for example, by using a GroupByKey transform), a common pattern is to combine the collection of values associated with each key into a single, merged value. This pattern of a GroupByKey followed by merging the collection of values is equivalent to Combine PerKey transform. The combine function you supply to Combine PerKey must be an associative reduction function or a subclass of CombineFn.

Kata: Implement the sum of scores per player using Combine.perKey.

Use Combine.perKey(GlobalCombineFn).
Extend the Combine.BinaryCombineFn class that counts the sum of the number.
Refer to the Beam Programming Guide "Combining values in a keyed PCollection" section for more information.