Databases

A URL shortener turns https://example.com/a/very/long/path?campaign=autumn into something like sho.rt/6ZY3LCH, and sends anyone who opens the short link to the …

PostgreSQL never overwrites a row in place. An UPDATE writes a new version of the row and marks the old one as replaced; a DELETE only marks the row. That desig…

A multi-tenant analytics platform partitions incoming events by tenant_id, hashed onto 16 shards — a design that looked perfectly balanced in every load test. T…

A URL shortener's redirect endpoint, GET /r/{code}, looks purely read-heavy — billions of redirects against a handful of short codes created per second. But eve…

A shopping cart write to cart-77 needs to land on enough replicas that a later read is guaranteed to see it — but "enough" is a number you choose, and the numbe…

A user changes their display name, refreshes the page a second later from a phone on a different network, and sees the old name. Nothing crashed. No request fai…

A shopping cart must remember items, so the product cannot be literally stateless. The useful design question is where that state lives. Keep the cart inside on…

An order API can return responses all day and still charge the wrong amount. It can be temporarily unreachable while every stored order remains safe. It can fai…

Two support agents both open the same account balance, both see $100, both independently process a $50 withdrawal, and the balance ends up at $50 instead of $0 …