#01Ready
URL Shortener
Shorten a long URL. Read-heavy. Don't collide.
Beginner~25 min9 stages
base62id generationread-heavy caching+1
Foundations
The index · 81 systems
Pick a system. Defend every decision.
Don't skip to the diagram — that's the trap that makes designs collapse at scale. Start with clarifications, defend each choice against an AI staff engineer, then watch the simulator execute the diagram you land on.
81problems
50ready to solve
49design products
32build infra
Compose products from existing primitives — the classic interview loop: clarifications, requirements, capacity, API, data model, HLD, deep dives, trade-offs.
/
Topics
Broad systems
architecture & decomposition#02Ready
Pastebin
Store text/code blobs with TTL and access control.
Beginner~45 min9 stages
object store + metadata DB splitedge cache key design (Vary, Cache-Control: private)TTL: lazy on-read + active sweeper + S3 lifecycle+4
Foundations
#06Ready
Web Crawler
Politely traverse the web at scale. Don't crawl yourself in circles.
Advanced~45 min9 stages
Mercator back-queue schedulingper-host / per-IP / per-AS politenessBloom-filter + RocksDB seen-set+7
Search & Indexing
#07Ready
Twitter / X Timeline
Push or pull? Both. The canonical fanout problem.
Intermediate~45 min9 stages
fanout-on-writefanout-on-readcelebrity problem+1
Feed & Timeline
#08Ready
Instagram News Feed
Ranked feed with cursor pagination. No `OFFSET`.
Intermediate~45 min9 stages
rankingtwo-stage retrievalcursor pagination+3
Feed & Timeline
#09Ready
Reddit / Hacker News
Vote-driven ranking with hot/top/new at scale.
Intermediate~45 min9 stages
vote-driven rankingprecomputed listingscomment-tree precompute+4
Feed & Timeline
#11Ready
WhatsApp / Messenger
Hundreds of millions of long-lived sockets, sub-second 1:1 + group delivery, E2E-encrypted, multi-device, multi-region active-active.
Advanced~50 min9 stages
long-lived TLS socketsfanout-on-write vs readoutbox pattern+7
Real-time messaging
#12Ready
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.
Advanced~60 min9 stages
single-writer doc-actor (Figma model)OT vs CRDT vs server-authoritative hybridcausal ordering / monotonic op IDs+9
Real-time collaboration
#12Ready
Slack / Discord
Channels, presence, history. Push or pull — and how a hot-channel fanout doesn't melt the gateway.
Intermediate~50 min9 stages
long-lived WebSocket gatewaychannel server (consistent-hash by channel_id)mega-channel relay tier (Discord Maxjourney)+7
Real-time messaging
#13Ready
Notification System
Push, email, SMS. Idempotent. Failover.
Intermediate~60 min9 stages
end-to-end idempotencyper-channel bulkheadingactive-region fencing tokens+4
Real-time messaging
#14Ready
Live Comments / Score Updates
Pub/sub at scale. WebSocket vs SSE vs long polling. Approximate by design — mega-rooms drop comments on purpose.
Intermediate~55 min9 stages
pub/sub fanoutwebsocket vs sse vs long-pollapproximate-by-design sampling+7
Real-time messaging
#15Ready
Uber / Lyft — Match Drivers and Riders
Match a rider to the closest acceptable driver in under 3 s. Geohash, S2, surge.
Advanced~60 min9 stages
H3 hexagonal hierarchical indexkRing candidate searchgeo-sharded dispatch (RingPop / DISCO)+5
Geospatial
#17Preview
Google Maps / Routing
Tiles, road graphs, contraction hierarchies, ETA.
Advanced~45 min9 stages
map tilesroad graphscontraction hierarchies+1
Geospatial
#18Ready
Dropbox / Google Drive
Chunking, dedup, sync, conflict resolution.
Advanced~55 min9 stages
content-defined chunkingcontent-addressed dedupfilesystem journal + cursors+2
Storage & Files
#19Ready
YouTube / Netflix Streaming
ABR, CDN, transcoding, hot/cold — the egress, fan-out, and popularity Pareto.
Advanced~60 min9 stages
ABR (HLS / DASH / CMAF)multi-tier CDNper-title / per-shot encoding+3
Storage & Files
#21Ready
Ad Click Aggregator
Stream processing with watermarks and exactly-once.
Advanced~45 min9 stages
stream processingexactly-oncewatermarks+1
Streaming & Analytics
#24Preview
Recommendation System
Candidate generation + ranking. Online + batch.
Advanced~45 min9 stages
candidate generationrankingembeddings+1
Streaming & Analytics
#25Ready
Payment / Wallet System
Idempotent, double-entry, sagas vs 2PC.
Advanced~45 min9 stages
idempotencydouble-entrysagas+3
Concurrency & Consistency
#26Ready
Ticketmaster / Hotel Booking
Don't oversell. Hold-then-confirm.
Intermediate~55 min9 stages
inventory lockinghold-then-confirmvirtual waiting room+4
Concurrency & Consistency
#27Ready
Stock Exchange / Order Matching
Single-writer matching. Microsecond budgets.
Advanced~45 min9 stages
single-writer principledeterministic state machineAeron Cluster / Raft sequencer+11
Concurrency & Consistency
#29Preview
Distributed Job Scheduler
Cron at scale: leader, exactly-once, DAGs.
Intermediate~45 min9 stages
leader electionexactly-onceDAGs
Coordination
#56Ready
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.
Advanced~45 min9 stages
durable execution (Temporal-style event-sourced workflow)brain / hands / state separationFirecracker microVM sandboxing+12
AI & Agentic Systems
Focused features
small surface, deep mechanics#03Ready
Distributed Unique ID Generator
Generate globally unique, monotonic-ish IDs at scale.
Intermediate~45 min9 stages
Snowflake bit budget (timestamp / worker / sequence)UUIDv7 (RFC 9562) — index-locality vs randomnessworker-id leasing via ZooKeeper / etcd ephemeral nodes+7
Foundations
#04Preview
Rate Limiter
Cap request rates per user/key across a fleet.
Intermediate~30 min9 stages
token bucketsliding windowRedis Lua+1
Foundations
#16Preview
Yelp / Nearby Places
Search by location with relevance ranking.
Intermediate~45 min9 stages
geo searchrelevance
Geospatial
#31Ready
Concurrent Hotel Viewers
"X users viewing this right now." Hot keys, HLL.
Intermediate~45 min9 stages
presenceTTLhot keys+6
Ephemeral state & Live counters
#32Ready
Online Indicator
Green dot for contacts. Mind the N² watch problem. Approximate by design — never quote presence more precisely than reality.
Intermediate~45 min9 stages
viewport-bounded subscription (the N² solver)lazy fanout vs eager broadcastheartbeat TTL economics (set EX > heartbeat × 2 + jitter)+6
Ephemeral state & Live counters
#33Preview
Typing Indicators
High write rate, ephemeral, do not persist.
Beginner~45 min9 stages
debouncingephemerality
Ephemeral state & Live counters
#34Preview
Active-on-Document Avatars
Cursor + presence broadcast. Awareness protocols.
Intermediate~45 min9 stages
presencecursor broadcastawareness
Ephemeral state & Live counters
#35Ready
Live Viewer Count (YouTube/Twitch)
Millions of viewers on one entity. Approximate by design.
Advanced~75 min9 stages
sampled heartbeats vs explicit presenceHyperLogLog (p=14, ±0.81%) and PFMERGE across regionstwo-stage aggregation against hot streams+7
Ephemeral state & Live counters
#36Ready
Like Button at Scale
Eventual consistency, but the liker sees their own write. Counts are approximate by design; hot keys are the real enemy.
Intermediate~45 min9 stages
counter shardinghot key absorptionoutbox + event-sourced fanout+4
Counters & Likes
#37Ready
View Count on a Video/Post
Dedup, bot-filter, batched aggregation.
Intermediate~45 min9 stages
fire-and-forget event logidempotent counter rollupHyperLogLog uniques+4
Counters & Likes
#38Ready
Trending Topics
Sliding windows + Count-Min Sketch + top-K.
Advanced~75 min9 stages
multi-resolution sliding windowsCount-Min Sketch (Cormode-Muthukrishnan)HeavyKeeper / Space-Saving top-K+6
Counters & Likes
#39Preview
Read Receipts
Per-message per-recipient. Group chat amplifies.
Intermediate~45 min9 stages
per-recipient statefanoutamplification
Counters & Likes
#40Ready
Live Sports Scores to Millions
Goal scored → 30M phones updated in <2s. Pub/sub fanout, edge delivery, thundering herd.
Intermediate~55 min9 stages
pub/sub fanoutedge websocket terminationfan-out tree+6
Real-time delivery
#41Preview
Stock Ticker / Price Feed
Conflate stale, prioritize, ordered, backpressure.
Intermediate~45 min9 stages
conflationprioritybackpressure
Real-time delivery
#42Preview
Live Auction Bidding
Order, fairness, anti-sniping, optimistic UI.
Intermediate~45 min9 stages
orderingfairnessanti-sniping
Real-time delivery
#43Preview
Push Notification Fanout (1 → 100M)
APNs/FCM rate limits, dedup, DND.
Advanced~45 min9 stages
fanout pipelinevendor limitsDND
Real-time delivery
#44Preview
Submit Order (Prevent Double-Charge)
Idempotency keys, dedup window, retry storms.
Intermediate~45 min9 stages
idempotency keysdedup windowretry storms
Idempotency & Dedup
#46Preview
Webhook Delivery
At-least-once, backoff, poison pills, signing.
Intermediate~45 min9 stages
at-least-onceexponential backoffsigning
Idempotency & Dedup
#47Ready
Distributed Cron — Mass Scheduled Email
Single trigger, 50M recipient idempotency, catch-up.
Advanced~75 min9 stages
Borgcron-style at-least-once schedulerper-recipient idempotency at fan-out tailoutbox (durable trigger log + publish)+7
Idempotency & Dedup
#48Preview
Autocomplete / Typeahead
Trie/FST, prefix sharding, personalization.
Intermediate~35 min9 stages
trieFSTpersonalization
Search-as-you-type
#49Preview
Did You Mean / Spell Correction
Edit distance at scale + LMs + query log mining.
Intermediate~45 min9 stages
edit distancelanguage modelsquery log
Search-as-you-type
#50Preview
Recent Searches / People Also Searched
Per-user storage + privacy + cold start.
Intermediate~45 min9 stages
per-user storageprivacycold start
Search-as-you-type
#51Preview
Cache Stampede / Thundering Herd
Coalesce, probabilistic early expiration, SWR.
Intermediate~45 min9 stages
request coalescingPERstale-while-revalidate
Caching corner cases
#52Ready
Cache Invalidation Across a Fleet
Write-through vs write-behind. Two generals.
Advanced~45 min9 stages
write-throughwrite-behindpub/sub invalidation+4
Caching corner cases
#53Preview
Negative Caching
Cache misses for things that never existed.
Intermediate~45 min9 stages
negative cacheBloom filters as cache front
Caching corner cases
#54Ready
Distributed Lock
Redlock controversy, fencing tokens, lease vs lock.
Advanced~45 min9 stages
fencing tokenslease vs lockRedlock
Coordination at small scope
#55Preview
Leader Election for a Singleton Job
ZK/etcd primitives, split-brain, lease renewal.
Advanced~45 min9 stages
leader electionsplit-brainlease renewal
Coordination at small scope