Free system design simulator for HLD & LLD interview prep

System Design Sim is a free, interactive system design simulator built for software engineers preparing for HLD and LLD interviews. Drag load balancers, API servers, caches, queues, databases and CDNs onto a canvas, wire them together, send live traffic, and watch exactly where your design bottlenecks – right in your browser, no signup.

Practice high level design (HLD) interviews on a live simulator

Build any distributed system 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, tune cache hit ratios, or put a queue in front of your workers. The same trade-offs you discuss in FAANG system design interviews, made tangible.

HLD & LLD system design interview challenges

Work through hands-on system design challenges that mirror real HLD and LLD interviews, from junior SWE to staff engineer level. Each challenge 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.

Chaos test your system design

Inject 5×–20× traffic spikes, flush every cache at once, and kill nodes mid-run. Distributed systems fail in surprising ways – this simulator shows you exactly how and why, making it the fastest way to build intuition for system design interviews.

Who is this for?

CS students, software engineers, and anyone preparing for system design interviews at Google, Amazon, Meta, Microsoft, or any tech company. Free, browser-based, no login required.