Table of Contents
What does a combiner do in Hadoop?
The Hadoop framework provides a function known as Combiner that plays a key role in reducing network congestion. The primary job of Combiner a “Mini-Reducer is to process the output data from the Mapper, before passing it to Reducer. It runs after the mapper and before the Reducer. Its usage is optional.
What is the use of combiner?
A Combiner, also known as a semi-reducer, is an optional class that operates by accepting the inputs from the Map class and thereafter passing the output key-value pairs to the Reducer class. The main function of a Combiner is to summarize the map output records with the same key.
Which one is example of combiner operation?
A classic example of combiner in mapreduce is with Word Count program, where map task tokenizes each line in the input file and emits output records as (word, 1) pairs for each word in input line. The reduce() method simply sums the integer counter values associated with each map output key (word).
What is purpose of combiner in MapReduce flow?
The job of the combiner is to optimize the output of the mapper before its fed to the reducer in order to reduce the data size that is moved to the reducer.
When a combiner is used in a MapReduce job?
The combiner in MapReduce is also known as ‘Mini-reducer’. The primary job of Combiner is to process the output data from the Mapper, before passing it to Reducer. It runs after the mapper and before the Reducer and its use is optional.
Is combiner and reducer same?
Both Reducer and Combiner are conceptually the same thing. The difference is when and where they are executed. A Combiner is executed (optionally) after the Mapper phase in the same Node which runs the Mapper. So there is no Network I/O involved.
What is the role of partitioner?
A partitioner partitions the key-value pairs of intermediate Map-outputs. It partitions the data using a user-defined condition, which works like a hash function. The total number of partitions is same as the number of Reducer tasks for the job.
What is Hadoop partitioner?
What is Hadoop Partitioner? Partitioner in MapReduce job execution controls the partitioning of the keys of the intermediate map-outputs. With the help of hash function, key (or a subset of the key) derives the partition. The total number of partitions is equal to the number of reduce tasks.