Concepts
The same handful of ideas appear again and again across the designs. Each page explains one with a diagram you can step through — and links to the systems that rely on it.
Data & storage
Caching
Keep copies of frequently read data in fast memory so most requests never reach the database.
Seen in YouTube, Netflix, Instagram, Twitter / X, Amazon, URL shortener, API rate limiter, Search autocomplete, Pastebin, Dropbox, Google Maps, BookMyShow, MakeMyTrip, Airbnb, Meeting scheduler, Car rental, Zomato / food delivery, Quora, LinkedIn, Live sports scores, Redis Cluster
Change data capture
Stream every database change as an event so other systems stay in sync without dual writes.
Seen in YouTube, Amazon
Double-entry ledger
Record every money movement as balanced debits and credits so money can never appear or vanish.
InteractiveSeen in Payment gateway, AstroTalk, Buy now, pay later, Stock brokerage
Read replicas
Copy the primary database to extra servers that serve read queries.
Seen in Instagram, Redis Cluster
Search indexing
Maintain an inverted index so text queries find matching documents in milliseconds.
Seen in YouTube, Twitter / X, Amazon, Google Maps, BookMyShow, MakeMyTrip, Airbnb, NoBroker, Zomato / food delivery, Practo / teleconsultation, Quora, LinkedIn
Sharding (partitioning)
Split one big dataset across many machines by a key so each holds and serves only part of it.
Seen in YouTube, Netflix, Instagram, WhatsApp / Messenger, Payment gateway, URL shortener, Search autocomplete, Web crawler, Pastebin, Dropbox, Google Maps, Driver booking & fleet management, Quora, LinkedIn, Redis Cluster
Scale & distribution
Bloom filters
A tiny probabilistic set that answers 'definitely not seen' or 'probably seen' using a few bits per item.
InteractiveSeen in Web crawler
Consistent hashing
Map keys to servers on a hash ring so adding or removing a server moves only a small share of keys.
InteractiveSeen in Twitter / X, WhatsApp / Messenger, API rate limiter, Web crawler, WebSockets at scale, Redis Cluster
Geospatial indexing
Bucket locations into cells so 'what's near me' becomes a quick lookup.
InteractiveSeen in Uber, Google Maps, Airbnb, NoBroker, Zomato / food delivery, Urban Company, Porter / goods delivery, Driver booking & fleet management
Horizontal scaling
Handle more load by adding more machines rather than bigger ones.
Seen in Amazon, WebSockets at scale, Video calling (WebRTC)
Multi-region deployment
Run the system in several geographic regions for lower latency and to survive a regional outage.
Seen in Netflix, WhatsApp / Messenger, Uber, Video calling (WebRTC)
Tries & prefix search
Store strings by their prefixes so 'top suggestions for what the user has typed so far' is one lookup.
InteractiveSeen in Search autocomplete
Unique ID generation
Create unique, often time-sortable IDs across many machines without a central bottleneck.
InteractiveSeen in Instagram, Twitter / X, URL shortener, Pastebin
Messaging & flow
Adaptive bitrate streaming
Encode video at several qualities in small segments so the player can switch quality as bandwidth changes.
Seen in YouTube, Netflix
Back-pressure & queues
When producers outpace consumers, buffer and slow down instead of falling over.
Seen in YouTube, Web crawler, WebSockets at scale, BookMyShow, MakeMyTrip, Stock brokerage, Live sports scores
Event-driven architecture
Services publish events about what happened; others react asynchronously.
Seen in Netflix, Instagram, Twitter / X, Uber, Payment gateway, Search autocomplete, Dropbox, Google Maps, WebSockets at scale, Airline reservations, NoBroker, Meeting scheduler, Car rental, Zomato / food delivery, Urban Company, Porter / goods delivery, Driver booking & fleet management, AstroTalk, Quora, LinkedIn, Buy now, pay later, Stock brokerage, Live sports scores
Fan-out (push vs pull feeds)
Deliver one item to many recipients — either copied at write time or gathered at read time.
Seen in Instagram, Twitter / X, Quora, LinkedIn, Live sports scores
Pre-signed uploads
Let clients upload large files straight to object storage using a short-lived signed URL.
Seen in YouTube, Instagram, WhatsApp / Messenger, Dropbox, Airbnb, NoBroker
Saga pattern
Run a multi-service transaction as a series of local steps, undoing completed steps if a later one fails.
Seen in Amazon, MakeMyTrip, Buy now, pay later
WebRTC calling
Real-time audio and video between browsers and apps, with servers only for signalling and relaying when needed.
Seen in Video calling (WebRTC), Practo / teleconsultation, AstroTalk
WebSockets & real-time push
Keep a persistent connection open so the server can push updates instantly.
Seen in WhatsApp / Messenger, Uber, Dropbox, WebSockets at scale, Video calling (WebRTC), Airline reservations, NoBroker, Zomato / food delivery, Porter / goods delivery, Driver booking & fleet management, Practo / teleconsultation, AstroTalk, Stock brokerage, Live sports scores
Reliability & correctness
Distributed locks & holds
Stop two processes from claiming the same thing at once — a seat, a driver, a time slot.
Seen in Uber, Amazon, BookMyShow, Airline reservations, Airbnb, Meeting scheduler, Car rental, Urban Company, Practo / teleconsultation
Idempotency
Make an operation safe to repeat, so retries never cause double effects.
Seen in WhatsApp / Messenger, Uber, Amazon, Payment gateway, Dropbox, BookMyShow, MakeMyTrip, Airline reservations, Airbnb, Meeting scheduler, Car rental, Zomato / food delivery, Urban Company, Porter / goods delivery, Practo / teleconsultation, AstroTalk, Buy now, pay later, Stock brokerage
Leader election & coordination
Agree on which node is in charge or who owns which shard, using a consistent coordination service.
Seen in URL shortener, API rate limiter, Search autocomplete, Web crawler, Redis Cluster
Microservices
Split a system into small services, each owning one domain and its own data.
Seen in Netflix, Amazon
Rate limiting
Cap how many requests a client can make in a time window to prevent abuse and protect capacity.
InteractiveSeen in URL shortener, API rate limiter, Pastebin