/**
* Sets the Reducer class to the chain job's JobConf.
*
* <p>It has to be specified how key and values are passed from one element of the chain to the
* next, by value or by reference. If a Reducer leverages the assumed semantics that the key and
* values are not modified by the collector 'by value' must be used. If the Reducer does not
* expect this semantics, as an optimization to avoid serialization and deserialization 'by
* reference' can be used.
*
* <p>For the added Reducer the configuration given for it, <code>reducerConf</code>, have
* precedence over the job's JobConf. This precedence is in effect when the task is running.
*
* <p>IMPORTANT: There is no need to specify the output key/value classes for the ChainReducer,
* this is done by the setReducer or the addMapper for the last element in the chain.
*
* @param job job's JobConf to add the Reducer class.
* @param klass the Reducer class to add.
* @param inputKeyClass reducer input key class.
* @param inputValueClass reducer input value class.
* @param outputKeyClass reducer output key class.
* @param outputValueClass reducer output value class.
* @param byValue indicates if key/values should be passed by value to the next Mapper in the
* chain, if any.
* @param reducerConf a JobConf with the configuration for the Reducer class. It is recommended to
* use a JobConf without default values using the <code>JobConf(boolean loadDefaults)</code>
* constructor with FALSE.
*/
public static <K1, V1, K2, V2> void setReducer(
JobConf job,
Class<? extends Reducer<K1, V1, K2, V2>> klass,
Class<? extends K1> inputKeyClass,
Class<? extends V1> inputValueClass,
Class<? extends K2> outputKeyClass,
Class<? extends V2> outputValueClass,
boolean byValue,
JobConf reducerConf) {
job.setReducerClass(ChainReducer.class);
job.setOutputKeyClass(outputKeyClass);
job.setOutputValueClass(outputValueClass);
Chain.setReducer(
job,
klass,
inputKeyClass,
inputValueClass,
outputKeyClass,
outputValueClass,
byValue,
reducerConf);
}
