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Each district is a unit. Lessons unlock as they're published, and free lessons are open to everyone.

1. District 0 · Open

   Start here

   What system design is, how to approach a design question, and the numbers every engineer should know.

   • What system design is
   • The 6-step approach
   • The client–server model
   • Latency numbers every engineer should know
   • Estimation basics

2. District 1 · Open

   How the web talks

   DNS, HTTP, TCP and UDP, API styles, and real-time connections.

   • Network layers and IP addresses
   • DNS
   • HTTP and HTTPS
   • TCP vs UDP
   • REST vs RPC (gRPC) vs GraphQL
   • Polling, long polling, WebSockets and SSE
   • Webhooks

3. District 2 · Open

   Scale basics

   Scalability, latency and throughput, horizontal scaling, availability.

   • Performance vs scalability
   • Latency vs throughput
   • Vertical vs horizontal scaling
   • Concurrency vs parallelism
   • Stateless services
   • Availability and the nines
   • Single points of failure

4. District 3 · Open

   Traffic management

   Load balancers, reverse proxies, CDNs, API gateways and rate limiting.

   • Load balancers
   • Load balancing algorithms
   • Reverse proxies
   • Content delivery networks
   • API gateways
   • Rate limiting algorithms
   • Case study: design a rate limiter

5. District 4 · Open

   Caching

   Where to cache, caching strategies, eviction and invalidation.

   • Why cache, and where
   • Cache-aside
   • Write-through, write-behind and refresh-ahead
   • Eviction policies
   • Invalidation and cache stampedes
   • Distributed caching
   • Redis vs Memcached

6. District 5 · Open

   Data storage

   SQL and NoSQL, indexes, replication, sharding, blob storage and IDs.

   • SQL vs NoSQL
   • Database indexes
   • B-trees vs LSM trees
   • ACID transactions and isolation
   • Replication
   • Sharding
   • Consistent hashing
   • The NoSQL families
   • Denormalization and federation
   • Blob storage
   • Generating unique IDs
   • Case study: design a URL shortener
   • Case study: design Pastebin

7. District 6 · Coming soon

   Async & messaging

   Queues, pub/sub, logs, delivery guarantees and back pressure.

   Coming soon

8. District 7 · Coming soon

   Distributed systems

   CAP, consistency models, quorums, consensus and distributed transactions.

   Coming soon

9. District 8 · Coming soon

   Architecture styles

   Monoliths, microservices, event-driven systems, CQRS and serverless.

   Coming soon

10. District 9 · Coming soon

    Search & special data structures

    Inverted indexes, tries, Bloom filters and geospatial indexes.

    Coming soon

11. District 10 · Coming soon

    Reliability & operations

    Observability, retries, circuit breakers, antipatterns and disaster recovery.

    Coming soon

12. District 11 · Coming soon

    Security essentials

    Authentication, authorization, tokens, encryption and secrets.

    Coming soon

13. District 12 · Coming soon

    Cloud design patterns

    Reusable patterns for messaging, data, migration and resilience.

    Coming soon