org.apache.flink.api.common.functions

Interface GroupReduceFunction<T,O>

Type Parameters:

T - Type of the elements that this function processes.

O - The type of the elements returned by the user-defined function.

All Superinterfaces:

Function, Serializable

All Known Implementing Classes:

CombineToGroupCombineWrapper, FirstReducer, GroupReduceIterator, HadoopReduceCombineFunction, HadoopReduceFunction, MergeOperatorStates, OperatorSubtaskStateReducer, PageRank.BuildOutgoingEdgeList, RichCombineToGroupCombineWrapper, RichGroupReduceFunction, SampleInCoordinator

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

@Public
 @FunctionalInterface
public interface GroupReduceFunction<T,O>
extends Function, Serializable

The interface for group reduce functions. GroupReduceFunctions process groups of elements. They may aggregate them to a single value, or produce multiple result values for each group. The group may be defined by sharing a common grouping key, or the group may simply be all elements of a data set.

For a reduce functions that works incrementally by combining always two elements, see ReduceFunction.

The basic syntax for using a grouped GroupReduceFunction is as follows:

 DataSet<X> input = ...;

 DataSet<X> result = input.groupBy(<key-definition>).reduceGroup(new MyGroupReduceFunction());
 

Partial computation can significantly improve the performance of a GroupReduceFunction. This technique is also known as applying a Combiner. Implement the GroupCombineFunction interface to enable partial computations, i.e., a combiner for this GroupReduceFunction.

Method Summary

Modifier and Type	Method and Description
void	reduce(Iterable<T> values, Collector<O> out) The reduce method.

Method Detail

reduce

void reduce(Iterable<T> values,
            Collector<O> out)
     throws Exception

The reduce method. The function receives one call per group of elements.

Parameters:

values - All records that belong to the given input key.

out - The collector to hand results to.

Throws:

Exception - This method may throw exceptions. Throwing an exception will cause the operation to fail and may trigger recovery.