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Core
Idempotency
An idempotent operation produces the same end state whether you apply it once or five times, which is exactly what you need in a world where retries and at-least-once delivery mean every request may arrive twice. Without it, a single lost response turns one charge into two; with it, the duplicate is a no-op.
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RELATED CONCEPTS
LESSONS THAT TEACH THIS
Foundations of Forward Deployed EngineeringBuild an evidence map of an unfamiliar system28 min→Foundations of Forward Deployed EngineeringTest replay, slow dependencies and changing data before adding load30 min→Foundations of Forward Deployed EngineeringChoose retries from outcome evidence and a time budget30 min→
PRACTICE THIS IN REAL QUESTIONS
System Design & Production EngineeringWhat's the difference between at-least-once and exactly-once delivery, and why should an FDE care?→System Design & Production EngineeringWhat is a dead-letter queue, and how do you handle a poison message in production?→LLM & GenAI FundamentalsWhy do LLMs hallucinate?→System Design & Production EngineeringA Python service starts double-processing messages under load. Why does this happen, and how do you fix it?→LLM & GenAI FundamentalsGive me three concrete hallucination mitigations, and the cost of each.→System Design & Production EngineeringDesign a job queue for long-running LLM tasks, priorities, cancellation, progress, and poison messages.→