The canonical system-design interview, built end to end: how tiny.co/aB3xK9 becomes a database lookup in single-digit milliseconds — base62, key generation, the read-heavy cache, and the scale math that makes 7 characters last a century.
Picking token bucket vs sliding window is the easy part. The real interview is what happens when the limiter runs on fifty machines: where the counter lives, why 'local counters' quietly lets 50× your limit through, and how to enforce one global cap without a round-trip on every request.
Auto-increment doesn't survive sharding, and random UUIDs wreck your database's index. How do you mint billions of unique, roughly time-sortable 64-bit IDs across a fleet with no coordinator? This is the Snowflake design — bit for bit.
The capstone that ties the whole series together: consistent hashing to place data, quorums to tune consistency, vector clocks to detect conflicts, and hinted handoff to stay writable through failure. This is Amazon's Dynamo, rebuilt from its four moving parts.
WhatsApp-scale messaging, built end to end: persistent WebSocket gateways, a user→server registry, store-and-forward for offline devices, the one-tick / two-tick / blue-tick delivery state machine, message ordering, and group fan-out. Drive the delivery protocol yourself, then take it apart.
The Twitter/Instagram timeline, worked end to end. The whole system pivots on one decision — do you build a follower's feed when someone posts (push), or when they open the app (pull)? Toggle between them, watch the write and read cost flip, and see why one celebrity breaks the naive answer.