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Reducer is the 2nd phase of processing the data in Hadoop. Reducer takes the intermiedate (key,value pairs) output which stored in local disk from the mapper as input.Several reducers can run parallely since they are independent of each other.In Reducer we do aggregation or summation computation anlaysis.
Reducer has 3 phases
Shuffle: Output from the mapper is shuffled from all the mappers.
Sort: Sorting is done parallely with shuffle phase where the input from different mappers is sorted
Reduce: Reducer task aggerates the key value pair and gives the required output based on the business logic implemented.The output of reducer is written on HDFS and is not sorted.By default, no of reducers is set to 1.The user can set no of reducers for the job to run.
Right no of reducers =0.95 or 1.75 * no of nodes * no of the maximum container per node.Follow the link to learn more about Reducer in Hadoop
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