Monitoring & Troubleshooting Flashcards

A CDN-delivered API endpoint intermittently returns cached error responses (e.g., 500 errors) to clients. What configuration change would prevent this?

Answer: Configure the CDN to not cache 5xx responses (negative caching prevention)

By default some CDNs may cache error responses; explicitly configuring the CDN to bypass cache for 5xx status codes prevents error responses from being served to subsequent users.

What is the significance of monitoring the CDN's TLS handshake time separately from TTFB?

Answer: High TLS handshake time indicates edge proximity issues or misconfigured TLS session resumption

TLS handshake time measures connection establishment overhead; if high, it may indicate users are connecting to distant edge nodes or TLS session tickets/resumption is not working.

Which HTTP response header should a CDN operator monitor to detect when origin servers are setting excessively short cache TTLs that reduce offload efficiency?

Answer: Cache-Control: max-age directive value

The Cache-Control max-age value set by the origin determines how long the CDN caches objects; very low values cause frequent origin fetches and reduce offload ratio.

A CDN log analysis shows that 30% of requests include 'If-None-Match' headers but the CDN is not returning 304 responses. What does this indicate?

Answer: The CDN is not storing or honoring ETags, causing full object re-delivery instead of 304 responses

When CDN ignores ETags from conditional requests, it delivers full 200 responses instead of 304 Not Modified, wasting bandwidth that could be saved with proper ETag support.

During a traffic spike, a CDN operator notices the origin shield is being bypassed by multiple edge POPs simultaneously. What configuration addresses this?

Answer: Enable request coalescing (also called request collapsing) at the shield layer

Request coalescing at the origin shield ensures that simultaneous cache misses for the same object result in only one origin request, preventing thundering-herd overload.

What does the CDN metric 'edge error rate' measure, and why is it distinct from 'origin error rate'?

Answer: Edge error rate includes errors generated by the CDN itself (e.g., routing failures, TLS errors), while origin error rate measures errors from the upstream server

Edge errors originate within the CDN infrastructure (config errors, POP failures, TLS issues), while origin errors come from the upstream server — distinguishing them pinpoints where failures occur.

A CDN operator wants to alert on potential DDoS amplification through the CDN. Which metric combination is most indicative?

Answer: Sudden spike in request rate with high percentage of cache misses and traffic to a single URL

DDoS attacks often generate massive request spikes targeting specific URLs; high miss rates under that spike indicate the CDN is not absorbing the attack and origin is being overwhelmed.