Deep architecture.
Real tradeoffs.
Production scale.
The open-source system design library engineers actually read before interviews. 60 canonical problems, 110 concept references, real post-mortems, practice exercises, and self-scored quizzes — all free, all opinionated, all in one place.
What brings you in?
Three paths through the material. Pick the one that matches your goal right now.
Start with the 15 whiteboard exercises — each has progressive hints and a scoring rubric. Score yourself cold before reading the reference solution.
110 concept references grouped into 15 categories — caching, databases, consensus, messaging, ML systems, architecture patterns. Each with code snippets + diagrams.
12 real-world outages — AWS S3 typo, Cloudflare regex, Facebook BGP, CrowdStrike kernel panic, Knight Capital's $440M glitch. What actually breaks at scale.
Six canonical systems
If you can reason about these six, you can reason about most system design interviews. Start here.
A photo and short-video sharing platform with feeds, stories, reels, and direct messaging serving 2 billion monthly active users. The hard parts: a media upload pipeline handling 95M+ daily uploads with on-the-fly transc
YouTube / Netflix
A video streaming platform that ingests raw user-uploaded content (YouTube) or studio-licensed media (Netflix), transcodes it into adaptive renditions, and delivers billions of views globally with sub-2-second start time
Uber / Ride Sharing
Design a real-time ride-hailing platform that matches riders with nearby drivers, tracks trips live on a map, handles geographic demand spikes, and calculates ETAs — all at city-scale.
High-Throughput Stock Exchange
Design a NASDAQ-scale order-matching system that processes millions of orders per second with single-digit microsecond latency, maintains deterministic fairness guarantees, and broadcasts every state change to thousands
Search Engine
A web-scale search engine — crawl the public web, build an inverted index, serve queries in under 200 ms with high-quality ranked results. Google handles ~100K queries per second over an index of hundreds of billions of
A real-time messaging system supporting one-to-one and group chat, delivery receipts, and offline message queuing at 2 billion user scale. The core challenge: maintaining 50 million persistent WebSocket connections while
Five ways to learn
Full system designs — requirements, scale, API, architecture SVG, deep-dive, tradeoffs, failure modes, interview tips.
Fundamental references — sharding, consensus, caching, messaging. With code snippets, diagrams, real numbers.
Practice prompts with progressive hints and a scoring rubric. Reveal-gated to force you to try first.
Multi-choice quizzes across 11 clusters with instant feedback + explanations. Personal-best tracked.
Real outages — AWS, Cloudflare, Meta, GitHub, GitLab, CrowdStrike, Knight Capital. Root cause + lessons.
Track what you've read
Toggle "mark as read" on any page. State saved locally — no account needed.
Browse by category
Social & Feed
10 problemsNews Feed
A social media feed that shows posts from people you follow, ranked algorithmically, updated in near-real-time. The core challenge: one celebrity post needs to reach 10 million followers instantly — without the system me
Reddit-Style Comments
Design a nested, threaded comment system with voting, ranking algorithms, and tree pagination — where the hardest problem isn't storing data, it's retrieving a sorted, truncated tree in under 200ms.
LinkedIn MutualConnection Search
Design a system that finds shared connections between any two users on a billion-user professional network — in under 50ms, at 100K+ queries per second.
Recommendation Algorithm
Design the engine behind "Recommended for you" — a system that surfaces personalised content or products for hundreds of millions of users across a catalogue of millions of items, in real time, while continuously learnin
Top-K Leaderboard
Design a real-time ranking system that maintains sorted standings of millions of users, serving both "who's on top?" and "what's my rank?" with sub-50ms latency.
A photo and short-video sharing platform with feeds, stories, reels, and direct messaging serving 2 billion monthly active users. The hard parts: a media upload pipeline handling 95M+ daily uploads with on-the-fly transc
Twitter Trending
Surface the top-10 trending topics — hashtags, phrases, named entities — from the firehose of all tweets, refreshed every few minutes, per geo and per user. The hard parts: approximate top-K over a high-velocity stream w
YouTube Likes Counter
A distributed counter system that handles 10M+ increments per second across 10B+ videos with eventual consistency for display. The hard parts: a sharded counter architecture that absorbs bursty write traffic without hot
1.7 billion monthly visits across millions of subreddit communities. The hard parts: a Hot ranking algorithm that decays old posts and surfaces fresh content without scanning every post on every page load, nested comment
Ad Click Aggregator
10 billion clicks per day, counted in real-time for billing and analytics. Every click is money. The hard parts: a streaming aggregation pipeline that groups clicks by campaign, ad, geo, and minute with exactly-once sema
Storage & Data
6 problemsGoogle Drive
A cloud file storage and synchronisation service that lets users upload any file, access it from any device, share it with others, and keep it in sync across all their machines — even after offline edits. The hard part i
Google Docs / Live Collaborative Editor
Design a real-time collaborative document editor where multiple users type simultaneously and every screen converges to the same result — without locks, without data loss, and within milliseconds.
Distributed Logging Framework
Design a centralized logging system that ingests hundreds of thousands of events per second from thousands of services, stores them durably with compression, and enables full-text search across billions of log lines in s
Dropbox Sync
A two-way file sync engine: every file in a user's "Dropbox" folder should appear identically across all their devices within seconds of any change. The hard parts: a content-addressable block store that deduplicates chu
Amazon S3
A bucket-and-key object store with read-your-writes consistency, ~11 nines of durability, and exabyte-scale capacity. The hard parts: a key → bytes service that doesn't fall over at millions of requests per second; erasu
Key-Value Store
10 million requests per second, 100 TB of data, single-digit millisecond p99 at any scale. The hard parts: a consistent hash ring that routes every request to the right partition without a central coordinator, an LSM sto
Communication
8 problemsA real-time messaging system supporting one-to-one and group chat, delivery receipts, and offline message queuing at 2 billion user scale. The core challenge: maintaining 50 million persistent WebSocket connections while
Notification System
Design a scalable, multi-channel notification platform that reliably delivers OTPs in 2 seconds and promo blasts to 50 million users — without one starving the other.
Video Conferencing
Design a real-time video conferencing platform like Zoom or Microsoft Teams — delivering sub-300ms audio/video to millions of concurrent participants across the globe while adapting to wildly varying network conditions.
Slack / Discord
A real-time messaging platform with channels, DMs, threads, reactions, presence, and file sharing. The hard parts: a persistent WebSocket fleet holding tens of millions of long-lived connections, message fan-out into cha
Gmail
An email service for ~1.8B users handling ~1B incoming emails per day plus outgoing. The hard parts: an SMTP ingress that accepts mail from anyone on the public internet while rejecting terabytes of daily spam; a per-use
Google Calendar
A calendar and scheduling platform handling recurring events, calendar sharing, and free/busy queries for 1.5 billion users across every timezone. The hard parts: an RRULE recurrence engine that expands "every Monday for
Reminder Alert
Schedule a reminder for "Thursday 9am in America/New_York" and have it arrive on the user's phone at exactly that local time — even when DST changes, the user moves timezones, or the delivery channel is flaky. The hard p
Collaborative Whiteboard
Infinite canvas. Shapes, text, freehand drawing. Multiple users see each other's cursors in real time. Undo/redo, zoom/pan, offline edits that sync when reconnected. Like Miro or FigJam. The hard parts: a CRDT for canvas
Media & Streaming
2 problemsYouTube / Netflix
A video streaming platform that ingests raw user-uploaded content (YouTube) or studio-licensed media (Netflix), transcodes it into adaptive renditions, and delivers billions of views globally with sub-2-second start time
Live Streaming Platform
Design a Twitch-scale live streaming platform where creators broadcast live video to millions of concurrent viewers with sub-5-second latency, real-time chat, and adaptive quality.
Search & Discovery
6 problemsSearch Autocomplete
A typeahead system that returns ranked suggestions on every keystroke — at sub-100ms latency, for billions of daily searches. The challenge isn't finding matches. It's pre-organizing data so matches are already waiting.
Web Crawler
A system that automatically traverses the web — starting from seed URLs, fetching pages, extracting links, and repeating — to build a searchable index of internet content. Deceptively simple at small scale; one of the ha
Yelp / Google Places
A local business discovery platform where users search for nearby restaurants, shops, and services by location, category, and filters. The core challenge is answering "what is near me?" across 100 million listings in und
Google Maps
Design a global mapping platform that renders interactive maps, computes optimal routes in milliseconds, and ingests real-time GPS data from millions of phones to estimate live traffic and ETAs.
Search Engine
A web-scale search engine — crawl the public web, build an inverted index, serve queries in under 200 ms with high-quality ranked results. Google handles ~100K queries per second over an index of hundreds of billions of
Google News
Crawl 50K+ news sources worldwide, rank by freshness + authority + personalization, and serve a unique feed to each of 1B users. The hard parts: near-duplicate clustering so users see one card per story with "N sources"
Infrastructure
12 problemsRate Limiter
A rate limiter controls how frequently a client can make requests to an API — protecting backend services from being overwhelmed by traffic spikes, malicious abuse, or runaway retry loops. The challenge is enforcing limi
Distributed Job Scheduler
How do you reliably execute millions of scheduled tasks at precisely the right time across a fleet of unreliable machines, ensuring no job is missed and no job runs twice?
Distributed Locking
How do you guarantee that at most one process operates on a shared resource at any time, across machines that can crash, networks that can partition, and clocks that can drift?
Distributed Priority Queue
Design a horizontally scalable queue that always delivers the highest-priority message first — like a planetary-scale ER triage system where the most critical patients are always treated next, across every hospital in th
Count Unique Active Users
Design a system to count DAU, WAU, and MAU at massive scale — balancing exact accuracy for financial reporting against real-time approximate answers for dashboards, using probabilistic data structures.
CDN
A global Content Delivery Network: serve cached copies of customer content from hundreds of edge points-of-presence (PoPs), delivering to users with sub-50ms RTT anywhere on Earth. The hard parts: global routing so reque
Unique ID Generator
Generate globally unique, k-sortable, compact identifiers at over 1 million IDs per second across N machines with zero coordination. The hard parts: a bit-layout balancing timestamp, machine, and sequence that stays uniq
LeetCode
Users submit code solutions in 15+ languages. The system compiles, runs against hidden test cases with strict time/memory limits, and returns a verdict — all within seconds. The hard parts: sandboxed execution that preve
Distributed Cache
An in-memory key-value store serving ~1M ops/sec per node with sub-millisecond latency. The hard parts: hash-slot sharding across a cluster of ~100 nodes so any key routes deterministically, eviction policies (LRU/LFU) t
Metrics & Monitoring
1M hosts each running an agent that collects CPU, memory, disk, and custom application metrics every 10-15 seconds. That is ~100M data points/sec flowing into your ingest pipeline. The hard parts: a time-series database
Code Deployment Pipeline
Developer pushes code. CI builds and tests inside isolated ephemeral containers. An immutable, SHA-tagged artifact is stored in an S3-backed content-addressed store. CD does a canary deploy to 1% of traffic, runs automat
ChatGPT
User sends a prompt. API gateway routes it through a tokenizer to a GPU cluster of ~10K A100s running autoregressive inference. Tokens stream back via server-sent events — the user sees words appearing one by one. The ha
Financial & Trading
7 problemsBidding Platform
Design an eBay-scale auction system where sellers list items, buyers place real-time bids, proxy bidding resolves automatically, and auctions close with sub-second precision — all while guaranteeing financial correctness
Payment Gateway
Design a system like Stripe or Amazon Pay that orchestrates multi-party financial transactions across unreliable networks — guaranteeing money is never lost, duplicated, or stuck in limbo.
High-Throughput Stock Exchange
Design a NASDAQ-scale order-matching system that processes millions of orders per second with single-digit microsecond latency, maintains deterministic fairness guarantees, and broadcasts every state change to thousands
Stock Trading Platform
Design a Robinhood-scale platform that handles money and stock ownership with zero tolerance for inconsistency, while simultaneously streaming real-time price data to millions of users.
Crypto Exchange
A cryptocurrency exchange like Coinbase or Binance: order-book trading across hundreds of coin pairs, wallets for hundreds of coins, deposits + withdrawals to the public blockchain, 24/7 operation. The hard parts: a matc
Real-time Fraud Detection
A system that scores every incoming payment / account action in under 100 ms and decides: approve, review, or block. The hard parts: a low-latency feature store that can answer "how many card swipes has this user made in
Google Ads
Advertiser creates a campaign. Every search query or page-view triggers an ad auction in under 100 ms. The winner's ad is shown, the click is tracked, and the advertiser is billed on a CPC basis. Scale: 10M+ auctions per
Marketplace & Booking
8 problemsURL Shortener
A URL shortener maps long, unwieldy URLs to short memorable codes and redirects anyone who clicks them. The concept is trivial — a key-value lookup. The challenge is serving billions of redirects per day with sub-100ms l
E-Commerce Platform
Design a large-scale platform like Amazon or Flipkart — from product search to checkout — where millions browse concurrently but inventory must never oversell.
Ticketmaster / StubHub
Design an online ticket booking platform that handles extreme concurrency spikes — millions of users competing for thousands of seats — without ever selling the same seat twice.
Uber / Ride Sharing
Design a real-time ride-hailing platform that matches riders with nearby drivers, tracks trips live on a map, handles geographic demand spikes, and calculates ETAs — all at city-scale.
DoorDash / Uber Eats
A three-sided marketplace: eaters order, merchants prepare, Dashers deliver. The hard parts: a dispatch engine that assigns the right Dasher to the right order in real-time optimizing for ETA, efficiency, and driver earn
Airbnb
A two-sided marketplace with ~7M listings and ~100M nights booked per year. The hard parts: date-range availability search — "find homes in Lisbon for 4 guests, June 3–10, under $200/night" is a surprisingly gnarly query
Flash Sale
100K users hit "Buy" at the exact same second for 1K limited items. No overselling. The hard parts: a waiting room that absorbs the thundering herd without crashing your backend, atomic inventory decrement that never goe
Shopping Cart
500M carts updated daily across every device. Never lose a cart. The hard parts: a hybrid storage layer that keeps hot carts in Redis for sub-ms reads while persisting to DynamoDB for durability, guest-to-authenticated c
Gaming
1 problemsOther
60 problemsA real-time messaging system supporting one-to-one and group chat, delivery receipts, and offline message queuing at 2 billion user scale. The core challenge: maintaining 50 million persistent WebSocket connections while
Notification System
Design a scalable, multi-channel notification platform that reliably delivers OTPs in 2 seconds and promo blasts to 50 million users — without one starving the other.
Video Conferencing
Design a real-time video conferencing platform like Zoom or Microsoft Teams — delivering sub-300ms audio/video to millions of concurrent participants across the globe while adapting to wildly varying network conditions.
Collaborative Whiteboard
Infinite canvas. Shapes, text, freehand drawing. Multiple users see each other's cursors in real time. Undo/redo, zoom/pan, offline edits that sync when reconnected. Like Miro or FigJam. The hard parts: a CRDT for canvas
Gmail
An email service for ~1.8B users handling ~1B incoming emails per day plus outgoing. The hard parts: an SMTP ingress that accepts mail from anyone on the public internet while rejecting terabytes of daily spam; a per-use
Google Calendar
A calendar and scheduling platform handling recurring events, calendar sharing, and free/busy queries for 1.5 billion users across every timezone. The hard parts: an RRULE recurrence engine that expands "every Monday for
Reminder Alert
Schedule a reminder for "Thursday 9am in America/New_York" and have it arrive on the user's phone at exactly that local time — even when DST changes, the user moves timezones, or the delivery channel is flaky. The hard p
Slack / Discord
A real-time messaging platform with channels, DMs, threads, reactions, presence, and file sharing. The hard parts: a persistent WebSocket fleet holding tens of millions of long-lived connections, message fan-out into cha
Bidding Platform
Design an eBay-scale auction system where sellers list items, buyers place real-time bids, proxy bidding resolves automatically, and auctions close with sub-second precision — all while guaranteeing financial correctness
Payment Gateway
Design a system like Stripe or Amazon Pay that orchestrates multi-party financial transactions across unreliable networks — guaranteeing money is never lost, duplicated, or stuck in limbo.
Crypto Exchange
A cryptocurrency exchange like Coinbase or Binance: order-book trading across hundreds of coin pairs, wallets for hundreds of coins, deposits + withdrawals to the public blockchain, 24/7 operation. The hard parts: a matc
Google Ads
Advertiser creates a campaign. Every search query or page-view triggers an ad auction in under 100 ms. The winner's ad is shown, the click is tracked, and the advertiser is billed on a CPC basis. Scale: 10M+ auctions per
High-Throughput Stock Exchange
Design a NASDAQ-scale order-matching system that processes millions of orders per second with single-digit microsecond latency, maintains deterministic fairness guarantees, and broadcasts every state change to thousands
Real-time Fraud Detection
A system that scores every incoming payment / account action in under 100 ms and decides: approve, review, or block. The hard parts: a low-latency feature store that can answer "how many card swipes has this user made in
Stock Trading Platform
Design a Robinhood-scale platform that handles money and stock ownership with zero tolerance for inconsistency, while simultaneously streaming real-time price data to millions of users.
Design PUBG
Design a multiplayer battle royale system where 100 players drop onto a large map, scavenge for equipment, and fight until one player or squad survives — all synchronized in real-time over unreliable networks.
Distributed Priority Queue
Design a horizontally scalable queue that always delivers the highest-priority message first — like a planetary-scale ER triage system where the most critical patients are always treated next, across every hospital in th
Count Unique Active Users
Design a system to count DAU, WAU, and MAU at massive scale — balancing exact accuracy for financial reporting against real-time approximate answers for dashboards, using probabilistic data structures.
Distributed Job Scheduler
How do you reliably execute millions of scheduled tasks at precisely the right time across a fleet of unreliable machines, ensuring no job is missed and no job runs twice?
Distributed Locking
How do you guarantee that at most one process operates on a shared resource at any time, across machines that can crash, networks that can partition, and clocks that can drift?
CDN
A global Content Delivery Network: serve cached copies of customer content from hundreds of edge points-of-presence (PoPs), delivering to users with sub-50ms RTT anywhere on Earth. The hard parts: global routing so reque
ChatGPT
User sends a prompt. API gateway routes it through a tokenizer to a GPU cluster of ~10K A100s running autoregressive inference. Tokens stream back via server-sent events — the user sees words appearing one by one. The ha
Code Deployment Pipeline
Developer pushes code. CI builds and tests inside isolated ephemeral containers. An immutable, SHA-tagged artifact is stored in an S3-backed content-addressed store. CD does a canary deploy to 1% of traffic, runs automat
Distributed Cache
An in-memory key-value store serving ~1M ops/sec per node with sub-millisecond latency. The hard parts: hash-slot sharding across a cluster of ~100 nodes so any key routes deterministically, eviction policies (LRU/LFU) t
LeetCode
Users submit code solutions in 15+ languages. The system compiles, runs against hidden test cases with strict time/memory limits, and returns a verdict — all within seconds. The hard parts: sandboxed execution that preve
Metrics & Monitoring
1M hosts each running an agent that collects CPU, memory, disk, and custom application metrics every 10-15 seconds. That is ~100M data points/sec flowing into your ingest pipeline. The hard parts: a time-series database
Rate Limiter
A rate limiter controls how frequently a client can make requests to an API — protecting backend services from being overwhelmed by traffic spikes, malicious abuse, or runaway retry loops. The challenge is enforcing limi
Unique ID Generator
Generate globally unique, k-sortable, compact identifiers at over 1 million IDs per second across N machines with zero coordination. The hard parts: a bit-layout balancing timestamp, machine, and sequence that stays uniq
Ticketmaster / StubHub
Design an online ticket booking platform that handles extreme concurrency spikes — millions of users competing for thousands of seats — without ever selling the same seat twice.
Uber / Ride Sharing
Design a real-time ride-hailing platform that matches riders with nearby drivers, tracks trips live on a map, handles geographic demand spikes, and calculates ETAs — all at city-scale.
E-Commerce Platform
Design a large-scale platform like Amazon or Flipkart — from product search to checkout — where millions browse concurrently but inventory must never oversell.
Airbnb
A two-sided marketplace with ~7M listings and ~100M nights booked per year. The hard parts: date-range availability search — "find homes in Lisbon for 4 guests, June 3–10, under $200/night" is a surprisingly gnarly query
DoorDash / Uber Eats
A three-sided marketplace: eaters order, merchants prepare, Dashers deliver. The hard parts: a dispatch engine that assigns the right Dasher to the right order in real-time optimizing for ETA, efficiency, and driver earn
Flash Sale
100K users hit "Buy" at the exact same second for 1K limited items. No overselling. The hard parts: a waiting room that absorbs the thundering herd without crashing your backend, atomic inventory decrement that never goe
Shopping Cart
500M carts updated daily across every device. Never lose a cart. The hard parts: a hybrid storage layer that keeps hot carts in Redis for sub-ms reads while persisting to DynamoDB for durability, guest-to-authenticated c
URL Shortener
A URL shortener maps long, unwieldy URLs to short memorable codes and redirects anyone who clicks them. The concept is trivial — a key-value lookup. The challenge is serving billions of redirects per day with sub-100ms l
YouTube / Netflix
A video streaming platform that ingests raw user-uploaded content (YouTube) or studio-licensed media (Netflix), transcodes it into adaptive renditions, and delivers billions of views globally with sub-2-second start time
Live Streaming Platform
Design a Twitch-scale live streaming platform where creators broadcast live video to millions of concurrent viewers with sub-5-second latency, real-time chat, and adaptive quality.
Google Maps
Design a global mapping platform that renders interactive maps, computes optimal routes in milliseconds, and ingests real-time GPS data from millions of phones to estimate live traffic and ETAs.
Google News
Crawl 50K+ news sources worldwide, rank by freshness + authority + personalization, and serve a unique feed to each of 1B users. The hard parts: near-duplicate clustering so users see one card per story with "N sources"
Search Autocomplete
A typeahead system that returns ranked suggestions on every keystroke — at sub-100ms latency, for billions of daily searches. The challenge isn't finding matches. It's pre-organizing data so matches are already waiting.
Search Engine
A web-scale search engine — crawl the public web, build an inverted index, serve queries in under 200 ms with high-quality ranked results. Google handles ~100K queries per second over an index of hundreds of billions of
Web Crawler
A system that automatically traverses the web — starting from seed URLs, fetching pages, extracting links, and repeating — to build a searchable index of internet content. Deceptively simple at small scale; one of the ha
Yelp / Google Places
A local business discovery platform where users search for nearby restaurants, shops, and services by location, category, and filters. The core challenge is answering "what is near me?" across 100 million listings in und
Top-K Leaderboard
Design a real-time ranking system that maintains sorted standings of millions of users, serving both "who's on top?" and "what's my rank?" with sub-50ms latency.
Reddit-Style Comments
Design a nested, threaded comment system with voting, ranking algorithms, and tree pagination — where the hardest problem isn't storing data, it's retrieving a sorted, truncated tree in under 200ms.
LinkedIn MutualConnection Search
Design a system that finds shared connections between any two users on a billion-user professional network — in under 50ms, at 100K+ queries per second.
Recommendation Algorithm
Design the engine behind "Recommended for you" — a system that surfaces personalised content or products for hundreds of millions of users across a catalogue of millions of items, in real time, while continuously learnin
Ad Click Aggregator
10 billion clicks per day, counted in real-time for billing and analytics. Every click is money. The hard parts: a streaming aggregation pipeline that groups clicks by campaign, ad, geo, and minute with exactly-once sema
A photo and short-video sharing platform with feeds, stories, reels, and direct messaging serving 2 billion monthly active users. The hard parts: a media upload pipeline handling 95M+ daily uploads with on-the-fly transc
News Feed
A social media feed that shows posts from people you follow, ranked algorithmically, updated in near-real-time. The core challenge: one celebrity post needs to reach 10 million followers instantly — without the system me
1.7 billion monthly visits across millions of subreddit communities. The hard parts: a Hot ranking algorithm that decays old posts and surfaces fresh content without scanning every post on every page load, nested comment
Twitter Trending
Surface the top-10 trending topics — hashtags, phrases, named entities — from the firehose of all tweets, refreshed every few minutes, per geo and per user. The hard parts: approximate top-K over a high-velocity stream w
YouTube Likes Counter
A distributed counter system that handles 10M+ increments per second across 10B+ videos with eventual consistency for display. The hard parts: a sharded counter architecture that absorbs bursty write traffic without hot
Google Docs / Live Collaborative Editor
Design a real-time collaborative document editor where multiple users type simultaneously and every screen converges to the same result — without locks, without data loss, and within milliseconds.
Amazon S3
A bucket-and-key object store with read-your-writes consistency, ~11 nines of durability, and exabyte-scale capacity. The hard parts: a key → bytes service that doesn't fall over at millions of requests per second; erasu
Distributed Logging Framework
Design a centralized logging system that ingests hundreds of thousands of events per second from thousands of services, stores them durably with compression, and enables full-text search across billions of log lines in s
Dropbox Sync
A two-way file sync engine: every file in a user's "Dropbox" folder should appear identically across all their devices within seconds of any change. The hard parts: a content-addressable block store that deduplicates chu
Google Drive
A cloud file storage and synchronisation service that lets users upload any file, access it from any device, share it with others, and keep it in sync across all their machines — even after offline edits. The hard part i
Key-Value Store
10 million requests per second, 100 TB of data, single-digit millisecond p99 at any scale. The hard parts: a consistent hash ring that routes every request to the right partition without a central coordinator, an LSM sto