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What Happens If the Hadoop MapReduce Output Directory Already Exists?
Preparing for a Hadoop exam? Learn what happens when the MapReduce output directory already exists in HDFS, why Hadoop throws an error, and how this protects previous job results from accidental overwrite.
Question
What happens if the Output Path already exists before running a Hadoop job?
A. Hadoop overwrites the directory automatically
B. Hadoop throws an error to avoid accidental overwrites
C. Hadoop renames the output folder automatically
D. Hadoop reduces the replication factor
Answer
B. Hadoop throws an error to avoid accidental overwrites
Explanation
When a Hadoop job is started, it validates that the specified Output Path does not already exist in HDFS; if it does, the framework raises a FileAlreadyExistsException and the job fails rather than overwriting existing results. This behavior enforces the “write once, read many” philosophy and protects previous outputs from being accidentally destroyed, so if you want to reuse that path you must delete or rename the existing directory before rerunning the job.