How systems actually work.
Read how something works, then push on it until it clicks. In-depth guides paired with simulators — DNS, TLS, Kafka, Raft, and plenty more, all running in your browser.
17 study paths · 170+ deep dives · 80+ simulators — free, no sign-up
Read it. Then push on it.
Every big idea here comes in two halves: a guide that explains the machinery, and a simulator where you can break it yourself.
DNS
How a name becomes an address: root, TLD, authoritative, and every cache in between.
Raft
How five machines agree on one truth, and what happens when you kill the leader.
Database indexing
Why the B-tree might be the most successful data structure ever shipped, and what an index really costs you.
Caching
Hit ratios, eviction, invalidation. One of the two hard problems, played out in front of you.
Everything here.
Codex
The long way in. Seventeen study paths, each a single page you can actually finish, from distributed systems to Go internals.
Open →Guides
How the machinery under the web really works. How DNS finds a name, what TLS actually promises, why Kafka is fast.
Open →Simulators
Models you can push on. Toggle a partition, kill a leader, watch a hash ring rebalance under your cursor.
Open →Visualize
Guided walkthroughs, animated and pausable. Watch a database write, a TCP connection open, a B-tree split.
Open →Interview prep
Practice the way the round actually runs: time-boxed, levelled for L5 through staff, with a 45-minute design simulator.
Open →Roadmaps
Eight routes through the material, backend to DevOps to system design, so you always know what comes next.
Open →Handbook
When to choose what. The trade-offs behind caching, sharding, and the data layer, written down honestly.
Open →Compare
Postgres vs MySQL, Kafka vs RabbitMQ, Rust vs Go. What actually differs, with the marketing scraped off.
Open →Tools
Small utilities that do one thing well. Format JSON, decode a JWT, slice a CIDR. Nothing leaves the page.
Open →Also in the building The Apprenticeship ELI5 Decide Map of the codex Your progress
The libraryEleven reference shelves: cited measurements, the maths underneath, annotated papers, the people behind the algorithms, the outages that taught everyone better, and the error strings decoded.
About →Lab Notebook
Reproducible benchmarks, with every reading cited and every chart hand-drawn.
Open →Foundations
The maths and physics under it all. Information theory, complexity, queueing.
Open →Data Structures
Trees, hashes, heaps, and graphs, with operations and Big-O for each.
Open →Glossary
Every term in Semicolony, defined once and linked everywhere.
Open →Topics
A concept index: pick a topic and see every page that covers it. The sideways way through the site.
Open →Cheat Sheets
Single-page references for the syntax that won't stick.
Open →Books
A reading list, built one chapter at a time.
Open →People
The names behind the algorithms: Lamport, Codd, Karger, and Dean.
Open →Postmortems
Famous outages at Cloudflare, GitHub, and AWS: what broke, and what everyone learned.
Open →Papers
Spanner, Dynamo, and MapReduce, annotated for working engineers.
Open →Errors
CrashLoopBackOff, exit 137, ECONNRESET: the strings you search at 2 a.m., taken from symptom to fix.
Open →The junk drawer, organised.
Small tools that do one thing each and never talk to a server. Your JSON stays in the tab.
All tools →Still being written. There is more here than one visit can cover.
— ;colony