Developing Apache Storm Applications
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The ReducerAggregator interface has the following interface definition:

public interface ReducerAggregator<T> extends Serializable {
    T init();
    T reduce(T curr, TridentTuple tuple);

When applying a ReducerAggregator to a partition, Storm first calls the init() method to obtain an initial value. It then calls the reduce() method repeatedly, to process each tuple in the partition. The first argument to the reduce() method is the current cumulative aggregation, which the method returns after applying the tuple to the aggregation. When all tuples in the partition have been processed, Storm emits the last value returned by reduce().