How do you test the performance of a CDN or caching layer?
- CDN
- caching
- edge performance
Answer it first, then compare. Nothing here is saved or scored.
Model answer
Tests: (1) Cache hit ratio: request the same resource multiple times, verify subsequent requests are faster and served from cache (check X-Cache headers). (2) Cache invalidation: update content, verify the cache serves fresh content after TTL or purge. (3) Geographic distribution: test from multiple regions to verify edge server performance. (4) Cache stampede: expire a popular resource and send concurrent requests, verify the origin is not overwhelmed. (5) Bypass testing: verify cache-busting parameters work. (6) Compare response times with and without CDN.
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.