**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Troubleshooting Spark performance: (1) Identify bottleneck—check Spark UI for long-running stages, skew, or spill; (2) Skew—use salting for hot keys, increase partitions; (3) Spill—increase executor memory or reduce partition size; (4) Slow shuffles—use broadcast for small joins, coalesce after filter; (5) Small files—use repartition/coalesce before write,...
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 American Express. The answer also includes follow-up discussion points that interviewers commonly explore.
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