CDN Monitoring & Troubleshooting 2 — Questions and Answers
Question 1: Which metric best indicates that a CDN edge node is experiencing cache thrashing?
- High cache hit ratio above 95%
- Rapid alternation between cache HIT and MISS for the same object (Correct answer)
- Consistently low TTFB across all regions
- Increased bandwidth usage from origin
Correct answer: Rapid alternation between cache HIT and MISS for the same object
Cache thrashing occurs when objects are repeatedly evicted and re-fetched, causing rapid HIT/MISS alternation for identical cache keys.
Question 2: A CDN operator notices that certain users in the Pacific Northwest are receiving stale content even after purging. What should be investigated first?
- DNS TTL settings on the origin
- Whether the purge propagated to all edge POPs in that region (Correct answer)
- The client browser cache settings
- SSL certificate expiration dates
Correct answer: Whether the purge propagated to all edge POPs in that region
Purge propagation delays to specific edge POPs can cause localized stale content delivery even after a global purge is issued.
Question 3: What does a sudden spike in 5xx errors from a CDN edge while the origin server shows normal health indicate?
- The origin is overloaded
- A configuration error was recently deployed to the CDN edge layer (Correct answer)
- Client SSL handshake failures
- DNS misconfiguration at the registrar
Correct answer: A configuration error was recently deployed to the CDN edge layer
When origin is healthy but CDN returns 5xx errors, a recent edge configuration change (routing rules, caching directives, security policies) is the most likely cause.
Question 4: Which tool is most appropriate for diagnosing which CDN POP a specific end user is being routed to?
- ping to the CDN domain
- traceroute or MTR to the CDN hostname
- nslookup with the local resolver to identify the returned IP (Correct answer)
- curl with --resolve flag to bypass DNS
Correct answer: nslookup with the local resolver to identify the returned IP
Running nslookup or dig from the user's local resolver reveals the edge IP assigned by the CDN's Anycast or GeoDNS, identifying the serving POP.
Question 5: A CDN dashboard shows high origin shield miss rates. What is the most likely impact?
- Reduced latency for end users
- Increased load and cost on the origin server (Correct answer)
- Better cache freshness for dynamic content
- Improved SSL handshake performance
Correct answer: Increased load and cost on the origin server
High origin shield misses mean requests are frequently bypassing the shield and hitting the origin, increasing origin load and egress costs.
Question 6: When analyzing CDN access logs, which field is most useful for determining whether a request was served from cache?
- Content-Type header
- X-Cache or CF-Cache-Status response header value (Correct answer)
- User-Agent string
- Accept-Encoding header
Correct answer: X-Cache or CF-Cache-Status response header value
Headers like X-Cache (HIT/MISS/EXPIRED) or CF-Cache-Status indicate the cache disposition of each request in CDN access logs.
Question 7: A CDN-delivered video stream has high rebuffering rates in Europe but not in the US. What monitoring data would most directly explain this?
- Origin server CPU usage
- CDN edge POP bandwidth utilization and latency metrics per region (Correct answer)
- Client device screen resolution statistics
- Total CDN monthly egress volume
Correct answer: CDN edge POP bandwidth utilization and latency metrics per region
Regional edge POP bandwidth saturation or high latency specific to European nodes would directly explain geographically isolated rebuffering issues.
CDN Monitoring & Troubleshooting 2
Which metric best indicates that a CDN edge node is experiencing cache thrashing?