**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
DAGs in Spark: directed acyclic graph of RDD/DataFrame lineage. Role: (1) Optimization—pipeline narrow transformations, prune; (2) Fault tolerance—recompute lost partitions; (3) Scheduling—define stages and tasks. No cycles; each node is deterministic. Example: read, filter, map, groupBy....
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 Freecharge. The answer also includes follow-up discussion points that interviewers commonly explore.
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