Interactive simulator · Two curricula
Drag a database into your design. Trace a request. See what breaks at p99.
Defend every choice. The AI staff engineer pushes back when you wave your hands.
Recommended first problem
Start solvingURL Shortener
~25 min · beginner · 9 stages
Why this one: Small surface, classic mechanics, clean introduction to the toolset.
34 problems
Compose products from existing primitives. The classic interview loop: clarifications, requirements, capacity, API, data model, HLD, deep dives, trade-offs.
Broad systems
18 problems · architecture and decomposition~25 minbeginner9 stages
URL Shortener
Shorten a long URL. Read-heavy. Don't collide.
base62
id generation
+2
Foundations
~45 minbeginner9 stages
Pastebin
Store text/code blobs with TTL and access control.
object store + metadata DB split
edge cache key design (Vary, Cache-Control: private)
+5
Foundations
~45 minintermediate9 stages
Distributed Unique ID Generator
Generate globally unique, monotonic-ish IDs at scale.
Snowflake bit budget (timestamp / worker / sequence)
UUIDv7 (RFC 9562) — index-locality vs randomness
+8
Foundations
~45 minadvanced9 stages
Web Crawler
Politely traverse the web at scale. Don't crawl yourself in circles.
Mercator back-queue scheduling
per-host / per-IP / per-AS politeness
+8
Search & Indexing
~45 minintermediate9 stages
Twitter / X Timeline
Push or pull? Both. The canonical fanout problem.
fanout-on-write
fanout-on-read
+2
Feed & Timeline
~45 minintermediate9 stages
Instagram News Feed
Ranked feed with cursor pagination. No `OFFSET`.
ranking
two-stage retrieval
+4
Feed & Timeline
~45 minintermediate9 stages
Reddit / Hacker News
Vote-driven ranking with hot/top/new at scale.
vote-driven ranking
precomputed listings
+5
Feed & Timeline
~50 minadvanced9 stages
WhatsApp / Messenger
Hundreds of millions of long-lived sockets, sub-second 1:1 + group delivery, E2E-encrypted, multi-device, multi-region active-active.
long-lived TLS sockets
fanout-on-write vs read
+8
Real-time messaging
~60 minadvanced9 stages
Collaborative Editor (Google Docs)
OT vs CRDT. Causal ordering. Real conflict-freedom. Server-authoritative single-writer doc-actor with WAL-before-ack — per-doc serialization, persisted before broadcast, pinned to a home region.
single-writer doc-actor (Figma model)
OT vs CRDT vs server-authoritative hybrid
+10
Real-time collaboration
~50 minintermediate9 stages
Slack / Discord
Channels, presence, history. Push or pull — and how a hot-channel fanout doesn't melt the gateway.
long-lived WebSocket gateway
channel server (consistent-hash by channel_id)
+8
Real-time messaging
~60 minintermediate9 stages
Notification System
Push, email, SMS. Idempotent. Failover.
end-to-end idempotency
per-channel bulkheading
+5
Real-time messaging
~60 minadvanced9 stages
Uber / Lyft — Match Drivers and Riders
Match a rider to the closest acceptable driver in under 3 s. Geohash, S2, surge.
H3 hexagonal hierarchical index
kRing candidate search
+6
Geospatial
~60 minadvanced9 stages
YouTube / Netflix Streaming
ABR, CDN, transcoding, hot/cold — the egress, fan-out, and popularity Pareto.
ABR (HLS / DASH / CMAF)
multi-tier CDN
+4
Storage & Files
~45 minadvanced9 stages
Ad Click Aggregator
Stream processing with watermarks and exactly-once.
stream processing
exactly-once
+2
Streaming & Analytics
~45 minadvanced9 stages
Payment / Wallet System
Idempotent, double-entry, sagas vs 2PC.
idempotency
double-entry
+4
Concurrency & Consistency
~55 minintermediate9 stages
Ticketmaster / Hotel Booking
Don't oversell. Hold-then-confirm.
inventory locking
hold-then-confirm
+5
Concurrency & Consistency
~45 minadvanced9 stages
Stock Exchange / Order Matching
Single-writer matching. Microsecond budgets.
single-writer principle
deterministic state machine
+12
Concurrency & Consistency
~45 minadvanced9 stages
AI Agent Platform
Long-running multi-step LLM agents — durable workflow + sandboxed execution + LLM gateway. Brain / hands / state are independently replaceable. The agent run is a workflow, not a request.
durable execution (Temporal-style event-sourced workflow)
brain / hands / state separation
+13
AI & Agentic Systems