System Design Sim
Design a system

System design challenges, from junior to CTO

13 broken systems, each with a target to hit: keep p99 latency, error rate and cost inside the limits while traffic spikes, caches flush and nodes die. You fix the architecture on a live simulator and it tells you whether your design held.

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  1. 01

    Hello, Load Balancer

    Junior    One API server is drowning under 1,500 RPS. Get error rate to zero without touching the traffic.p99 < 500ms · errors < 0.5% across 1 stage
  2. 02

    Cache Money

    Junior    Your database is at 95% utilization serving reads it has answered a thousand times. Cool it down below 50%.p99 < 150ms · errors < 0.5% across 1 stage
  3. 03

    Design a URL Shortener

    Mid    Redirect traffic grows from 1,000 to 20,000 requests per second. Keep every redirect fast through a cache restart and a server failure at peak, within budget.p99 < 150ms · errors < 0.5% · cost < $5,000/month across 5 stages
  4. 04

    The Stampede

    Mid    Your cache will be flushed mid-run. Survive the stampede - keep errors under 5% while it refills.p99 < 800ms · errors < 0.5% across 2 stages
  5. 05

    Queue It Up

    Mid    Write traffic spikes 5× partway through. Absorb the burst without dropping a single write.p99 < 2,000ms · errors < 0.5% across 3 stages
  6. 06

    Stop the Scraper Flood

    Mid    A scraper is about to hit your public API with 10× normal traffic. Keep real users' requests fast while the flood lasts, and be healthy again the moment it stops.p99 < 200ms · errors < 0.5% across 3 stages
  7. 07

    Design a Chat System

    Senior    Messages and history reads grow to 10,000 a second, then New Year's midnight triples it. Keep sends fast and every message delivered within 10 seconds, within budget.p99 < 300ms · errors < 0.5% · cost < $15,000/month across 4 stages
  8. 08

    Design a Notification System

    Senior    Your app sends push notifications through a provider that accepts 2,000 a second. Breaking news is about to multiply events 20×. Deliver every notification within a minute without the provider rejecting any.p99 < 300ms · errors < 0.5% · cost < $6,000/month across 3 stages
  9. 09

    Design a News Feed

    Senior    Timeline reads and new posts grow to 30,000 requests a second at peak. Keep timelines fast, get every post into followers' feeds and search within 10 seconds, within budget.p99 < 200ms · errors < 0.5% · cost < $15,000/month across 3 stages
  10. 10

    The Retry Storm

    Senior    Your database is about to slow down by 400 ms. Your clients retry failures up to three times. Keep the API standing.p99 < 1,000ms · errors < 0.5% across 2 stages
  11. 11

    Black Friday

    Senior    Traffic is 12,000 RPS and the CFO capped infra at $3,000/month. Keep p99 under 300ms and errors under 1%.p99 < 300ms · errors < 1% · cost < $3,000/month across 2 stages
  12. 12

    Chaos Monkey

    Staff    A monkey will kill your busiest API server mid-run. Design for redundancy so users never notice.p99 < 500ms · errors < 2% across 2 stages
  13. 13

    The CTO Budget Cut

    CTO    This system works - and burns about $15,000/month doing it. Hit the same SLOs for under $1,500/month.p99 < 250ms · errors < 0.5% · cost < $1,500/month across 1 stage
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Read the guide, then break the system

Each guide walks through requirements, estimates and the design interviewers expect.