Fix the design
Production incidents with incomplete whiteboard architectures. Triage like you're in the war room — metrics, brief, and six interview-style questions per challenge.
Each sim maps to a practice problem on InterviewCrafted. Use the tags on each card to jump to the related system design prompt.
Flash sale spike · notifications 28 min late · duplicates · DB at 100%
During a flash sale, notifications were delayed by 20–30 minutes. · Some users received duplicate messages.
Related practice problems
Cache flush · API pool exhausted · DB 100% · p99 30s
Cache flush at peak traffic triggered 95% miss rate for 8 minutes. · API connection pools exhausted (480/500).
Related practice problems
Retry storm · double charge · 2% of checkout · support flood
2% of checkouts double-charged after PSP timeout + client retry. · Support tickets spiked 400% in 2 hours.
Related practice problems
One API key · 80% capacity · tenants starved · 429 storm
One API key consumed 80% of gateway capacity for 45 minutes. · Other tenants saw 429 storms and elevated p99.
Related practice problems
Celebrity post · one shard 100% CPU · feed p99 45s · fan-out backlog 2.1M
One timeline shard hit 100% CPU while others stayed under 30%. · Feed p99 latency 45s for affected users.
Related practice problems
Reconnect storm · duplicate messages · 12% dup rate · gateway overload
Duplicate message rate hit 12% after mobile network blip. · Reconnect storm overloaded connection gateway.
Related practice problems
Viral link · cache miss 95% · redirect p99 8s · DB read overload
Viral link caused 95% cache miss rate for one short code. · Redirect p99 latency 8s; users saw timeouts.
Related practice problems
Gateway 502 · downstream 'healthy' · thread pools saturated · retry amplification
Gateway returned 502 for 48% of requests over 22 minutes. · Downstream health checks still passed (shallow TCP).
Related practice problems
Queue lag 6h unnoticed · jobs 4h late · green dashboards · poison partition
Consumer lag grew for 6 hours before on-call noticed. · Oldest job age reached 4.2 hours.
Related practice problems