Combiner is an optional step between Map and Reduce. Combiner is also called Semi-Reducer. Combiner takes output from Map, creates Key-value pairs and passes these to Reducer. Combiner’s task is to summarize the outputs from Map into summary records with same key.
By using Combiner, we can reduce the data transfer between Mapper and Reducer. Combiner does the task similar to reduce but it is done on the Map machine itself.Read the full book at www.amazon.com