Why this exists

Most system design material falls into one of two traps. It either lists components (load balancer, cache, queue) without saying why any of them is there, or it shows a finished architecture for a famous company and leaves you to guess how anyone got there.

Real systems don't arrive finished. They start small and break in specific ways as they grow, and each piece exists because something went wrong without it. System Design Road teaches it in that order. You follow one made-up app, Chirp, from two founders and one rented server to fifty million users, and every lesson starts with the thing that broke.

What's here

• The course: 37 lessons in eight parts, from what happens when you open an app to consensus, multi-region and payments. Every lesson has real numbers and diagrams you can interact with.
• The simulation: run Chirp yourself. Make the design decisions at each stage, then watch real traffic test them. Early choices come back later.
• Interview practice: ten classic system design questions, practised stage by stage the way a real 45-minute round runs, each with a full walkthrough.
• Low-level design: object-oriented design, SOLID, the classic design patterns and concurrency, then ten LLD interview problems. The parking lot, LRU cache, Splitwise, chess and more, with Java code that compiles and outputs that come from real runs.
• Engineering blogs: well-known outages retold in plain words, with what the engineers learned and the lessons behind them.

How it's made

Three rules run through everything here:

1. Explain why, not just what. Every component, pattern and class appears because a requirement or a failure needs it.
2. Use real numbers. Estimates are worked through, latencies are realistic, and every code output on the site comes from actually running the code.
3. Plain words. If a technical term is needed, it's explained the first time it appears.

Chirp, its founders and its users are fictional. The technology, the numbers and the outages in the blog are real, and each blog post links to the original report it retells.

It's free

Everything you can read here is free, and your progress, notes and whiteboards are saved in your own browser. Nothing is stored about you on a server. The privacy page has the details.

Found a mistake?

Corrections make the site better for everyone. If a number looks wrong, a diagram is unclear, or an explanation skips a step, please let us know.

What stays on your device

Your progress through lessons, your practice answers, whiteboards, notes and reading settings are saved in your browser's local storage. They never leave your device and we can't see them. Clearing your browser's site data deletes them.

Visitor statistics

We use Cloudflare Web Analytics to count visits: which pages are read, roughly where visitors come from (for example a search engine or a link on another site), the country, the type of device, and how fast pages load. It doesn't use cookies, doesn't build a profile of you, and doesn't track you across other websites. The numbers are only ever seen in aggregate.

Services the site relies on

• Hosting. The site is served by Netlify, which, like any web host, processes your IP address to deliver pages and may keep short-lived server logs for security and reliability.
• Fonts. Typefaces are loaded from Google Fonts, so your browser contacts Google's servers to download them.
• Statistics. Cloudflare, as described above.

Each provider handles that data under its own privacy policy.

What we don't do

• No accounts, no sign-in, no passwords.
• No advertising, no ad trackers, no selling of data.
• No email collection on the site at the moment. If that changes, this page will say what's collected and why before it starts.

Changes

If this policy changes, the updated version will appear on this page with a new date.

Last updated: 12 November 2026.