Learn system design by building and breaking real architectures

System Design Sim is a free, interactive system design simulator and playground. Drag load balancers, API servers, caches, queues, databases and CDNs onto a canvas, wire them together, then send live traffic and watch exactly where your design bottlenecks — right in your browser, with no signup.

An interactive system design playground

Build any architecture visually and run a real-time simulation. Requests per second, p99 latency, error rate and hourly cost all update live as you scale instances, add caching layers, change cache hit ratios, or put a queue in front of your workers.

Practice with real system design interview challenges

Work through hands-on challenges that mirror real interviews, from junior to CTO level. Each asks you to hold strict SLOs through traffic spikes, cache stampedes and node failures while staying under a cost budget:

• Hello, Load Balancer — One API server is drowning under 1,800 RPS. Get error rate to zero without touching the traffic.
• Cache Money — Your database is at 95% utilization serving reads it has answered a thousand times. Cool it down below 50%.
• The Stampede — Your cache will be flushed mid-run. Survive the stampede - keep errors under 5% while it refills.
• Queue It Up — Write traffic spikes 5× partway through. Absorb the burst without dropping a single write.
• Black Friday — Traffic is 12,000 RPS and the CFO capped infra at $1,400/hr. Keep p99 under 300ms and errors under 1%.
• Chaos Monkey — A monkey will kill one of your components mid-run. Design for redundancy so users never notice.
• The CTO Budget Cut — This system works - and burns $2,900/hr doing it. Hit the same SLOs for under $1,700/hr.

Break it on purpose with chaos testing

Inject 5×–20× traffic spikes, flush every cache at once, and kill nodes mid-run to see how resilient your design really is. Failure is the fastest way to understand distributed systems.