How Does Hadoop Reducer Get Invoked?

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In Hadoop, the reducer gets invoked automatically by the framework after the shuffle and sort phase has completed. The reducer receives key-value pairs from multiple mappers, groups them by keys, and performs the appropriate aggregation or computation on the values associated with each key. The reducer function is defined by the user and is executed on the data grouped by keys, producing the final output for the job. Each reducer task is responsible for processing a subset of the keys generated by the mappers, and the number of reducer tasks can be controlled by the user. The reducer output is typically written to an output file in the Hadoop Distributed File System (HDFS) or another external storage system.

How does Hadoop manage intermediate data between mappers and reducers?

Hadoop manages intermediate data between mappers and reducers through a component called the shuffle and sort phase. In this phase, Hadoop ensures that the output from the mappers is transferred to the reducers efficiently and in an organized manner.

The intermediate data is partitioned based on the keys generated by the mappers, and each partition is sorted to ensure that all values for a given key are grouped together. The sorted data is then transferred to the reducers over the network.

Hadoop also provides features such as combiners and partitioners to optimize the shuffle and sort phase. Combiners reduce the amount of data transferred between mappers and reducers by aggregating the data before it is sent to the reducers. Partitioners control how the keys are distributed among the reducers, allowing for more efficient and parallel processing.

Overall, Hadoop's shuffle and sort phase ensures that intermediate data between mappers and reducers is managed effectively, leading to faster and more efficient processing of big data.

How does Hadoop determine the number of reducers to use?

Hadoop allows users to specify the number of reducers to use for a MapReduce job through the configuration settings. By default, the number of reducers is determined by the formula:
(number\ of\ reducers = max(min(\frac{0.95 \times \text{num_nodes} \times \text{map_tasks}}{\text{mapred.reduce.tasks.max}}, \text{max_reduces}), \text{min_reduces}))


  • num_nodes is the number of nodes in the cluster
  • map_tasks is the total number of map tasks in the job
  • mapred.reduce.tasks.max is the maximum number of reduce tasks allowed
  • max_reduces is the maximum number of reducers
  • min_reduces is the minimum number of reducers

Users can adjust the number of reducers by setting the "mapreduce.job.reduces" property in the configuration file or by explicitly calling the setNumReduceTasks() method in their MapReduce program. Additionally, users can also use the TotalOrderPartitioner to partition the output of the map tasks into a specified number of partitions, which are then used as the number of reducers.

What is the role of the OutputCollector in a Hadoop reducer?

The OutputCollector in a Hadoop reducer is responsible for collecting and organizing the key-value pairs that are output by the reducer during the processing of input data. The OutputCollector receives the intermediate key-value pairs emitted by the reducer's map function and aggregates them into a final set of key-value pairs that will be written to the output file or passed to the next stage of processing.

The OutputCollector is a crucial component in the Hadoop framework as it manages the output from the reducer and ensures that the final results are correctly formatted and organized before being written to disk or passed on for further processing. By using the OutputCollector, the reducer can efficiently handle a large amount of data and produce the desired output format for downstream processing.

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