middle⏱ 25 min
PostgreSQL Connection Pool Exhaustion
Response time degraded from 150ms to 8000ms. Error rate jumped to 22%. Can you find the bottleneck before the on-call engineer does?
PostgreSQLHikariCPJavaSpring Boot
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Featured Cases
Start with These Cases
Handpicked cases that cover the most common interview topics
senior⏱ 35 min
JVM Memory Leak Under Load
The app runs fine at 200 RPS but crashes at 600 RPS. Heap dumps collected. Old Gen filling suspiciously. What's leaking?
JVMHeap DumpJavaGC
junior⏱ 20 min
Nginx Load Balancer Misconfiguration
One backend node is receiving 80% of traffic. Others are mostly idle. Classic or weighted? Let's investigate.
NginxLoad Balancingupstreamkeepalive
Case Categories
From database bottlenecks to infrastructure misconfiguration
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Database
Connection pools, slow queries, deadlocks, replication lag
14 cases →⚙️
Application
Memory leaks, thread starvation, GC pauses, CPU saturation
18 cases →🏗️
Infrastructure
Network bottlenecks, disk I/O, load balancer issues
9 cases →📊
Monitoring
Alert tuning, metrics interpretation, SLO/SLA analysis
6 cases →How It Works
The same process as a real performance debugging session
01
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Choose a Case
Pick by difficulty (Junior → Senior) or technology stack. Filter by time available.
02
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Investigate
Reveal clues one by one — graphs, logs, configs, query results. Think like an on-call engineer.
03
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Submit Hypothesis
Write your root cause analysis and proposed solution. No time limit.
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Compare & Learn
See the expert solution, common mistakes, and what interviewers actually look for.