**Why Priority Queues in Data Pipelines:** Task scheduling (Airflow, Celery), event processing (process by timestamp/priority), and merge of K sorted streams (heapq.merge).
**Implementations:** (1) heapq: min-heap, O(log n) push/pop. (2) queue.PriorityQueue: thread-safe wrapper. (3) For dynamic priority updates: need decrease_key—heapq doesn't support; use lazy deletion (mark stale, ignore when popped) or custom Fibonacci heap.
**Scalability:** Single-process heap = millions of items....
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 PayPal. The answer also includes follow-up discussion points that interviewers commonly explore.
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