com.hazelcast.mapreduce.aggregation

Interface Aggregation<Key,Supplied,Result>

Type Parameters:

Key - the input key type

Supplied - the value type returned from the Supplier

Result - the value type returned from the aggregation

@Beta
public interface Aggregation<Key,Supplied,Result>

The Aggregation interface combines multiple map-reduce operations steps to a single operation definition. It is a predefined set of Mapper, Combiner, Reducer and probably a Collator to execute an aggregation over a supplied set of data.

As you'll see in the following example, the Aggregations API is fully type-safe. Due to the lack of full type inference support on Java 6 and Java 7, it seems more verbose than it actually is.

 IMap<String, Employee> map = hazelcastInstance.getMap("employees");
 Supplier<String, Employee, Integer> supplier = Supplier.all((employee) -> employee.getSalaryPerMonth());
 Aggregation<String, Integer, Integer> aggregation = Aggregations.integerAvg();
 int avgSalary = map.aggregate(supplier, aggregation);
 

With Java 8 it is possible to write that all in just one line so that the API becomes very straight forward.

 IMap<String, Employee> map = hazelcastInstance.getMap("employees");
 int avgSalary = map.aggregate(Supplier.all((employee) -> employee.getSalaryPerMonth(), Aggregations.integerAvg());
 

Since:

3.3

Method Summary

Methods

Modifier and Type	Method and Description
Collator<Map.Entry,Result>	getCollator() Returns the Collator implementation used in this aggregation.
CombinerFactory	getCombinerFactory() Returns the CombinerFactory for this aggregation to pre-reduce values on mapping nodes.
Mapper	getMapper(Supplier<Key,?,Supplied> supplier) Returns the Mapper for this aggregation.
ReducerFactory	getReducerFactory() Returns the ReducerFactory for this aggregation.

Method Detail

getCollator

Collator<Map.Entry,Result> getCollator()

Returns the Collator implementation used in this aggregation.

Returns:

the aggregation defined Collator

getMapper

Mapper getMapper(Supplier<Key,?,Supplied> supplier)

Returns the Mapper for this aggregation. The Mapper implementation has to handle the Supplier to filter / transform values before emitting them to the further aggregation steps.

Parameters:

supplier - the Supplier to filter or / and transform values

Returns:

the aggregation defined Mapper

getCombinerFactory

CombinerFactory getCombinerFactory()

Returns the CombinerFactory for this aggregation to pre-reduce values on mapping nodes. Returning a CombinerFactory preserves traffic costs, but implementing a Combiner is not always possible.

Returns:

the aggregation defined CombinerFactory or null

getReducerFactory

ReducerFactory getReducerFactory()

Returns the ReducerFactory for this aggregation. If a CombinerFactory is defined, the implemented Reducer has to handle values of the returned type of the Combiner.

Returns:

the aggregation defined ReducerFactory