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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Practise this in questions
System Design & Production EngineeringWhat's the difference between at-least-once and exactly-once delivery, and why should an FDE care? →easy · FreeSystem Design & Production EngineeringA Python service starts double-processing messages under load. Why does this happen, and how do you fix it? →medium · FreeRAG & Agent System DesignYour agent fails 5% of the time mid-workflow, after executing real side effects. Make it safe. →hard · PremiumCoding & DSABuild a client for a flaky API: retries with exponential backoff and jitter, timeouts, and idempotency →hard · PremiumRAG & Agent System DesignAn agent fleet has executed thousands of real side effects, then a bad deploy corrupts state. Design the recovery. →hard · PremiumSystem Design & Production EngineeringDistribute a 10GB file from one bandwidth-constrained source to thousands of interconnected hosts, minimizing time to completion. →hard · Premium