midHadoop
What is combiner in MapReduce?
Updated May 16, 2026
Short answer
A mini-reducer that runs after mapper to reduce data transfer.
Deep explanation
It aggregates mapper output locally before sending to reducer, improving performance.
Real-world example
Word count optimization in large datasets.
Common mistakes
- Assuming combiner always runs.
Follow-up questions
- Is combiner mandatory?
- Can reducer be combiner?