**Why XCom matters**: Cross-task communication for DAG orchestration; misuse = performance and size issues. **Mechanism**: Push via return value or `xcom_push`; pull via `xcom_pull`. Stored in Airflow metadata DB by default. **Scalability trade-offs**: Default backend has ~1MB limit; large payloads = DB bloat. **Cost implications**: XCom in DB = latency for large data....
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