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16.2. The MapReduce API

16.2.1. The MapReduce API
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In JBoss Data Grid, each MapReduce task has four main components:
  • Mapper
  • Reducer
  • Collator
  • MapReduceTask
The Mapper class implementation is a component of MapReduceTask, which is invoked once per input cache entry key/value pair. Map is a the process of applying a given function to each element of a list, returning a list of results
Each node in the JBoss Data Grid executes the Mapper on a given cache entry key/value input pair. It then transforms this cache entry key/value pair into an intermediate key/value pair, which is emitted into the provided Collator instance. 
				
public interface Mapper<KIn, VIn, KOut, VOut> extends Serializable {
 
   /**
    * Invoked once for each input cache entry KIn,VOut pair.
    */
   void map(KIn key, VIn value, Collector<KOut, VOut> collector);
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At this stage, for each output key there may be multiple output values. The multiple values must be reduced to a single value, and this is the task of the Reducer. JBoss Data Grid's distributed execution environment creates one instance of Reducer per execution node. 
			
public interface Reducer<KOut, VOut> extends Serializable {
 
   /**
    * Combines/reduces all intermediate values for a particular intermediate key to a single value.
    * <p>
    *
    */
   VOut reduce(KOut reducedKey, Iterator<VOut> iter);
 
}
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The same Reducer interface is used for Combiners. A Combiner is similar to a Reducer, except that it must be able to work on partial results. The Combiner is executed on the results of the Mapper, on the same node, without considering the other nodes that might have generated values for the same intermediate key.
As Combiners only see a part of the intermediate values, they cannot be used in all scenarios, however when used they can reduce network traffic significantly.
The Collator coordinates results from Reducers that have been executed on JBoss Data Grid, and assembles a final result that is delivered to the initiator of the MapReduceTask. The Collator is applied to the final map key/value result of MapReduceTask. 
		
public interface Reducer<KOut, VOut> extends Serializable {
 
   /**
    * Combines/reduces all intermediate values for a particular intermediate key to a single value.
    * <p>
    *
    */
   VOut reduce(KOut reducedKey, Iterator<VOut> iter);
 
}
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