A CombinerAggregator is used to combine a set of tuples into a single field. It has the following signature:

public interface CombinerAggregator {
   T init (TridentTuple tuple);
   T combine(T val1, T val2);
   T zero();
}

Storm calls the init() method with each tuple, and then repeatedly calls the combine() method until the partition is processed. The values passed into the combine() method are partial aggregations, the result of combining the values returned by calls to init(). Partitions are discussed more in the following sessions, but a partition is effectively a subset of a stream of tuples that resides on the same host. After combing the values from processing the tuples, Storm emits the...