**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Spark execution: (1) Action triggers job; (2) DAG Scheduler splits the DAG into stages at wide transformation boundaries (shuffle dependency); (3) Each stage has tasks (one per partition); (4) Task Scheduler launches tasks on executors. Stage boundary occurs when a shuffle is required—e.g., before a reduceByKey or join....
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