Worked designs

Full interview prompts answered step by step, and the back-of-the-envelope estimation that sizes them.

Oct 3, 2026scalabilitymessaging
Design chat system for the interview
Design chat system for the interview. Open connections, the path of one 1:1 message, group fan-out, history, and presence.
Oct 3, 2026scalabilitycaching
Design distributed cache for the interview
Design distributed cache for the interview. Placement, eviction, a stampede, a stale write, and the node that melts on a hot key.
Oct 3, 2026scalability
Design notification system for interviews
Design notification system for interviews. Preferences, fan-out, quiet hours, and push, email, and SMS as separate workers.
Oct 3, 2026scalability
Design payment system for the interview
Design payment system for the interview. The ledger, idempotent charges, capture and refund, and what the provider keeps.
Oct 3, 2026scalability
Design search engine for the interview
Design search engine for the interview. Crawl, index, and query as three systems, plus typeahead as a smaller, hotter index.
Oct 3, 2026scalabilitycrawling
Design web crawler for the interview
Design web crawler for the interview. The frontier, politeness, the seen-set, freshness versus coverage, and a Bloom filter.
Oct 2, 2026estimationinterviews
Back-of-the-envelope estimation for system design
A step-by-step method for back-of-the-envelope estimation in system design interviews, with worked QPS and storage examples and the numbers to memorise.
Oct 2, 2026designsdistributed-systems
Design news feed for the interview
Design news feed for the interview. Fanout on write versus fanout on read, what to cache, and how ranking stays out of the storage path.
Oct 2, 2026designsestimation
Design URL shortener for the interview
Design URL shortener for the interview. The read path, the write path, how short codes are minted, and a capacity estimate you can redo.