**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Spark read modes: spark.read loads data (default overwrites for file sources). Write modes: (1) overwrite—replaces target; (2) append—adds data; (3) ignore—no-op if exists; (4) errorIfExists—fail if exists. Example: df.write.mode('append').parquet(path). For Delta: overwriteSchema; replaceWhere for conditional overwrite....
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