Problem library

The system-design interview's most famous first problem. We treat it as a scale-evolution study: four architectures for the same problem, from 10K to 1B RPS.

Store and share text snippets by URL.

Direct-to-storage uploads with pre-signed URLs.

1:1 messaging with delivery guarantees.

Enqueue and run one-shot and recurring tasks.

Deliver push/email/SMS to a targeted user.

Enforce budget on requests per client per window.

Create polls, cast votes, tally results.

Discover, fetch, and store the web in a bounded way.

Photo/video feed, uploads, stories, direct-to-S3 media, hybrid fan-out feed, ML ranking.

Timeline generation with hybrid fan-out — push for regular users, pull for celebrities.

Upload → transcode → CDN → recommendations. HLS delivery, ABR ladder, per-title encoding.

Block-level sync, deduplication, delta upload, conflict resolution.

Collaborative document storage.

Global video streaming — CDN economics, ABR, DRM, chaos engineering, Open Connect.

Uber's real architectural evolution — monolith → SOA → Ringpop → DOMA.

1:1 and group messaging at scale — WebSocket + MQTT, E2E encryption, multi-device sync.

Team messaging with presence, threads, search. Sharded WebSocket, team-affinity routing.

Concurrency control on hot seats. Hybrid Redis+DB holds, virtual queue for drops, anti-scalping.

Restaurant discovery, order, delivery orchestration.

H3 geo-indexing, DISCO batched matching, surge pricing, city-sharded state.

Catalog, cart, checkout, inventory.

Idempotent money movement with a double-entry ledger, provider integration, and reconciliation.

Multi-tenant SaaS notification platform — templates, providers, orchestration, delivery receipts.

Sub-100ms typed suggestions.

Ranked, personalized feed generation.

In-memory KV cluster with replication.

Crawl → index → serve, at web scale.

Cross-region, high-throughput push.

Approximate-consistent counters across regions.

Scale-out cron with exactly-once semantics.

Ingest, index, and search at petabyte scale.

Time-series ingest + query + alert.

Sharded inverted index + query fan-out.

Multi-region ledger with reconciliation.

Real-time bidding + serving in <100ms.

Feature store, model serving, A/B.

Streaming ingest + interactive query.

Distributed log with partitioning + replication.

Sharded, replicated, with configurable consistency.

Cross-region durable object storage.

Global-scale video with adaptive bitrate + recommendations.

Edge-heavy streaming at planetary scale.