System Design Interview Questions

How to design Twitter (X) for ~500M DAU with the fanout-on-write timeline pattern.

Geospatial matching at scale: how Uber matches 100M+ riders to drivers in under 5 seconds.

The most common system design phone screen. Base62 encoding, key generation, and 100:1 read/write ratio.

Token bucket vs sliding window vs fixed window: which algorithm and how to make it distributed.

The classic OOD/LLD interview question. State machine, allocation strategy, and concurrency in a 1000-spot lot.

Photo storage, feed generation, Stories TTL, and the read-heavy 100:1 workload.

End-to-end encryption, message ordering, and presence at 2B-user scale.

Adaptive bitrate streaming, global CDN, and the recommendation pipeline.

Video upload, transcoding pipeline, and serving 1B+ hours/day.

Inverted index, query understanding, and ranking 100B+ pages in under 200ms.

File sync, deduplication, and delta-block uploads across 700M+ users.

Trie-based prefix matching, real-time popularity updates, and sub-50ms response.

Politeness, dedup, and the URL frontier for crawling 1B+ pages.

Multi-channel delivery, dedup, user preferences, and at-least-once guarantees.

Consistent hashing, replication, eviction, and the thundering herd problem.

Write-once read-many, TTL expiration, and similar to URL shortener with body storage.

Order matching engine, fairness, and sub-millisecond latency requirements.

Generalized rider-driver matching system. Often a variant of the Uber question.

24-hour TTL, view ordering, and seen-state per viewer for 500M+ DAU.

Audio streaming with low buffering, offline downloads, and the discovery algorithm.