Interview Q&A / System Design
System Design interview questions
Caching, scaling, databases and classic design problems for beginners.
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- Why do engineers track p95 and p99 latency instead of the average?
Averages hide slow outliers. p99 means 99% of requests were faster than this value, so it shows what your unluckiest users feel, which is what drives complaints and timeouts.
BeginnerQuick answer1 min read - Why does my useEffect run twice in development?
In Strict Mode, React 18 and later deliberately mounts, unmounts and remounts each component once in development to check that effects clean up properly. It does not happen in production.
BeginnerQuick answer1 min read