System design simulations,
running in your browser.
Each one is a working model of a real system — a database index, a cache, a group of servers agreeing on one value. You press Next, it takes one step, and the screen shows what changed.
Everything is free until 1 September 2026. No sign-up needed to run one.
You move it forward, one step at a time.
Nothing plays on its own. You stop on any step and look at it for as long as you want.
- Press Next. One step happens.
- The screen shows what the system holds right now — the keys in memory, the bytes on disk, the messages between machines.
- A short line says what just happened, in plain words.
- Go back, change the settings, run it again.
Grouped by subject area.
Start anywhere. If you want an order to follow, the learning path puts them in one.
Networking basics
Concurrency vs Parallelism · The Numbers
Storage engines
B+ Tree · LSM Tree · Columnar Storage · Transaction Isolation Levels · and more
Indexes & routing
Vector Search & HNSW · Message Queue (Kafka)
Distributed systems
Kademlia DHT · Clock Synchronization · Logical & Vector Clocks · Gossip Protocol · and more
Probabilistic structures
HyperLogLog · Count-Min Sketch · Cuckoo Filter · t-digest · and more
Everything is free until 1 September 2026.
The whole site, for the launch month. These are the plans that start on 1 September.
- The foundation simulations, from a request reaching a server to a database writing to disk
- Two full interview questions — a whole interview you work through on a design board
- The learning path, the glossary, and 10 review cards a day
- Every simulation
- All 12 interview questions, every scenario
- Unlimited review cards
- Long enough to cover a full round of interview prep
- Every simulation
- All 12 interview questions, every scenario
- Unlimited review cards
- Enough for a last week of prep
Nothing is locked right now. The split above is what starts on 1 September.
See what each plan includes →Open a simulation and press Next.
All of them are free until 1 September 2026.