**Why Durable:** Regular Functions are stateless—each invocation independent. Durable adds orchestration: chain steps, fan-out/fan-in, wait for external events. State persisted in Azure Storage.
**Patterns:** (1) Function chaining: A→B→C. (2) Fan-out/fan-in: parallel calls, aggregate. (3) Human interaction: wait for approval. (4) Timers: durable delay.
**Scalability:** Durable uses replay—orchestrator re-runs from history; activities run once....
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