Understand how something works
Read a guide
Illustrated, end-to-end walkthroughs of the systems under the web.
New here?
Start with what you came to do.
Semicolony is a study site for how real systems work — guides, interactive simulators, roadmaps, deep dives, and a reference library. Pick the door that fits.
Learn by doing
Push on a simulator
Hand-built, interactive models you can step through and break.
Follow a path
Pick a roadmap
Structured routes through a discipline, each topic linked to a deep dive.
Go deep
Open the Codex
Single-page deep dives across distributed systems, databases, networking, and more.
Prepare for interviews
Practice a round
Time-boxed flows: a system-design simulator, flashcards, and coding patterns.
Reach for a reference
Browse the library
Data structures, foundations, annotated papers, the people behind the ideas.
Two things worth knowing
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