How systems actually work.
In-depth guides paired with simulators you can experiment with, so you can read how something works and then try it for yourself. DNS, TLS, Kafka, Raft, and plenty more. The tools run entirely in your browser.
Guides
In-depth internals on the protocols and patterns underneath the web: DNS, TLS, Kafka, and OAuth, and how they actually work.
Open →Simulators
Interactive models you can push on. Toggle a partition, watch a hash ring rebalance, see CAP break in real time.
Open →Codex
Single-page curricula and deep dives across system design (with a 15-stage roadmap, five principles, and a napkin-math reference), Kubernetes, distributed systems, and databases.
Open →Handbook
When to choose what. Trade-offs for caching, sharding, the data layer, distributed mechanics.
Open →Interview prep
Time-boxed practice flows for L5, L6, and staff rounds, including a 45-minute system-design simulator and sixty distributed-systems flashcards.
Open →Tools
Single-purpose browser utilities. Format JSON, validate JWTs, calculate CIDR. Nothing leaves the page.
Open →Three to read first.
In-depth internals, illustrated and written out properly. Pick whichever one you'd open if a coworker asked.
Push the pieces.
B-Tree
Insert, split, and rebalance the data structure databases are built on.
LSM tree
Memtable, flush, and compaction: the engine behind RocksDB and Cassandra.
DNS resolution
Walk root → TLD → authoritative. Watch caches fill and TTLs tick.
CAP theorem
Toggle the partition. Watch the third guarantee fail.
Consistent hashing
A ring of nodes. Add one and only K/N keys move.
CloudFront cache
Edge → regional → origin. Watch the cascade and populate-on-return.
Prefer a guided tour to a sandbox? The new Visualize section walks through how things work, step by step: animated, pausable, and with every concept defined inline. First page up: how a computer runs a program.
Reference shelves.
Ten reference shelves: cited measurements, the maths underneath, annotated papers, the people behind the algorithms, and the outages that taught everyone better.
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 →Most-used utilities.
Single-purpose tools that load instantly and never round-trip a server. Paste in, copy out.
All tools →Most guides come with a simulator you can experiment with, and most tools come with a guide that explains what's happening underneath. Everything loads quickly and runs in your browser, so there's nothing between you and the answer.