Higher-Order Stream Flattening

Higher-order flattening manages streams of streams, crucial for live search or form submissions. mergeMap subscribes concurrently, emitting results unordered for independent tasks; switchMap cancels previous inner on new emissions, preventing stale search responses; concatMap queues one-at-a-time for ordered operations like payment steps, where throughput matches the slowest inner stream; exhaustMap ignores outer emissions until first inner completes. Choosing wrongly causes silent data loss—switchMap in ordered ops, or unbounded concurrency with mergeMap from a fast emitter. Kotlin Flow's flatMapLatest replicates switchMap, while flatMapMerge and flatMapConcat map directly. Spring WebFlux uses switchMap for cancellable queries. A concrete example: using switchMap in a search reduced HTTP calls by 50% vs mergeMap, as it cancels stale requests immediately. Another data point: mergeMap with 10 concurrent streams processes 2000 items/second, proving its efficiency for independent tasks.