Mid-LevelPerformance & Load Testing
What is the difference between vertical and horizontal scaling? How do you test for each?
- scaling
- horizontal
- vertical
- load balancing
Answer it first, then compare. Nothing here is saved or scored.
Model answer
Vertical: more CPU/RAM on a single machine, simpler but has a ceiling. Horizontal: more machines behind a load balancer - scales further but adds complexity (session management, data consistency). Test vertical: increase load until resources max out, note the ceiling. Test horizontal: verify load balancer distributes evenly, test with N and N+1 instances, verify auto-scaling triggers at the right threshold, and test instance startup time under sudden load.
What interviewers listen for
- Workload modellingYou design the load shape from product traffic, not from tool defaults.
- Percentile literacyYou read p50/p95/p99 separately and explain why each matters.
- Bottleneck triageYou correlate metrics across application, database, and infrastructure layers.
- Capacity instinctsYou can translate a soak test into a sizing or scaling recommendation.