**Narrow**: No shuffle. Each partition processed independently. map, filter, flatMap. Can stay in single partition.
**Wide**: Requires shuffle. Data from multiple partitions combined. join, groupBy, distinct, repartition.
**Why It Matters**: Narrow = cheap, parallel. Wide = expensive; stage boundary. DAG split at wide.
**Examples**: filter (narrow), reduceByKey (wide).
**Scalability Trade-offs**: Minimize wide. Push filters early (narrow)....
The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations. It covers performance optimization strategies, common pitfalls to avoid, and real-world examples from companies like Microsoft. The answer also includes follow-up discussion points that interviewers commonly explore.
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