eu.stratosphere.arraymodel.functions.ReduceFunction Maven / Gradle / Ivy
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* Copyright (C) 2010-2013 by the Stratosphere project (http://stratosphere.eu)
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
* an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
* specific language governing permissions and limitations under the License.
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package eu.stratosphere.arraymodel.functions;
import java.lang.reflect.Method;
import java.util.Iterator;
import eu.stratosphere.api.common.functions.GenericReducer;
import eu.stratosphere.types.Value;
import eu.stratosphere.util.Collector;
public abstract class ReduceFunction extends AbstractArrayModelFunction implements GenericReducer {
/**
* The central function to be implemented for a reducer. The function receives per call one
* key and all the values that belong to that key. Each key is guaranteed to be processed by exactly
* one function call across all involved instances across all computing nodes.
*
* @param records All records that belong to the given input key.
* @param out The collector to hand results to.
*
* @throws Exception Implementations may forward exceptions, which are caught by the runtime. When the
* runtime catches an exception, it aborts the reduce task and lets the fail-over logic
* decide whether to retry the reduce execution.
*/
@Override
public abstract void reduce(Iterator records, Collector out) throws Exception;
/**
* No default implementation provided.
* This method must be overridden by reduce stubs that want to make use of the combining feature.
* In addition, the ReduceFunction extending class must be annotated as Combinable.
* Note that this function must be implemented, if the reducer is annotated as combinable.
*
* The use of the combiner is typically a pre-reduction of the data. It works similar as the reducer, only that is
* is not guaranteed to see all values with the same key in one call to the combine function. Since it is called
* prior to the reduce() method, input and output types of the combine method are the input types of
* the reduce() method.
*
* @see eu.stratosphere.api.java.record.operators.ReduceOperator.Combinable
* @param records
* The records to be combined. Unlike in the reduce method, these are not necessarily all records
* belonging to the given key.
* @param out The collector to write the result to.
*
* @throws Exception Implementations may forward exceptions, which are caught by the runtime. When the
* runtime catches an exception, it aborts the combine task and lets the fail-over logic
* decide whether to retry the combiner execution.
*/
@Override
public void combine(Iterator records, Collector out) throws Exception {
// to be implemented, if the reducer should use a combiner. Note that the combining method
// is only used, if the stub class is further annotated with the annotation
// @ReduceOperator.Combinable
reduce(records, out);
}
@Override
public Method getUDFMethod() {
try {
return getClass().getMethod("reduce", Iterator.class, Collector.class);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
}