Performs actual grouping and analytical operations on data. Aggregator can perform following operations: Average, Sum, Min, Max, Count, Distinct. If result after aggregation execution is null than default value of built in Aggregator for that specific type is returned. Custom aggregator, for custom data types and custom functions like Mean, Median, Mode can also be implemented.
Implements the interface to extract the meaningful attributes from given objects, Similar to Mapper in MapReduce framework.