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How many combiners run in MapReduce?

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How many combiners run in MapReduce?



@Pritam Pal , Combiner is used to summarize map output records with same key. This process reduces the data transfer across network between mapper and reducer.

Hadoop doesn’t guarantee on how many times a combiner function will be called for each map output key. At times, it may not be executed at all, while at times it may be used once, twice, or more times depending on the size and number of output files generated by the mapper for each reducer.