Hello Interview

A well-tuned Postgres or MySQL instance handles 10k-50k QPS. Modern cloud instances with 512 GiB RAM hold enormous working sets without touching disk. Don't shard until you've done the math. OpenAI didn't for ChatGPT.

Redis runs at ~200k ops/sec with sub-millisecond latency. A single memory-optimized instance can hold terabytes. "We need to shard the cache" is almost never the right call.

Most system design candidates are doing scale math with numbers from 2015. Modern hardware is dramatically more capable. Here's the cheat sheet.

Full Redis deep dive: hellointerview.com/learn/system-d…

The interview move: when designing a leaderboard or any Top-K feature, propose Redis Sorted Sets, then follow up with "I'd shard by region and use a separate set per time bucket." That shows you understand both the data structure and its operational limits.

At 100 users, a ranked leaderboard from a relational DB is fine. At 10 million concurrent users, querying and sorting on every request will kill you. Redis Sorted Sets exist for exactly this.

Full story: hellointerview.com/experience/sto…

The tradeoff most candidates get backwards: they optimize for interview performance instead of building the depth that makes interview performance easy.

One candidate tanked a system design round mid-loop at Microsoft. Knew it didn't go well. Got the offer anyway.