Developing Apache Storm Applications
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The CombinerAggregator interface is used to combine a set of tuples into a single field. In the word count example the Count class is an example of a CombinerAggregator that summed field values across a partition. The CombinerAggregator interface is as follows:

public interface CombinerAggregator<T> extends Serializable {
    T init(TridentTuple tuple);
    T combine(T val1, T val2);
    T zero();

When executing Aggregator, Storm calls init() for each tuple, and calls combine() repeatedly to process each tuple in the partition.

When complete, the last value returned by combine() is emitted. If the partition is empty, the value of zero() will be emitted.