Architecture & Topology
What is a 'Point of Presence' (PoP) in the context of a CDN, and how does it differ from a traditional data center?
What is a PoP (Point of Presence), and how does it differ from a single edge server?
Explain the concept of an 'Origin Shield' (or Tiered Caching). Why would a high-traffic site implement this between their edge and their origin?
What is the difference between the 'Control Plane' and the 'Data Plane' in a CDN?
How does a CDN perform load balancing and failover across its edge servers, and what role do health checks play?
Fundamentals
What is a CDN and what are the primary problems it solves for a modern web application?
What is an 'Edge Server' (or PoP) and how does it differ from an 'Origin Server'?
What is Time To First Byte (TTFB), and how does putting a CDN in front of your origin affect it?
In the context of a CDN, what is the 'Last Mile' vs. the 'Middle Mile' problem?
When would you NOT want to use a CDN for a specific type of data or application?
What are the primary tradeoffs of adding a CDN to your stack?
How does a CDN reduce origin bandwidth and load, and how would you quantify the offload it provides?
Content Delivery Strategies
What is the difference between a 'Push CDN' and a 'Pull CDN', and in what scenarios would you choose one over the other?
Can a CDN accelerate dynamic (non-cacheable) content? Explain 'Dynamic Site Acceleration' (DSA).
How do CDNs perform 'Image Optimization' at the edge (e.g., WebP conversion, resizing) and why is it better than doing it on the fly at the origin?
How does a CDN handle large file delivery (like 5GB video files) differently than small images?
Security & Access Control
What is 'Hotlinking' and how can a CDN prevent other sites from stealing your bandwidth?
Explain how 'Signed URLs' or 'Signed Cookies' work to protect premium/private content delivered via CDN.
What is the role of a Web Application Firewall (WAF) when deployed at the CDN edge compared to a WAF on the application server?
When would you use 'Signed URLs' versus 'Signed Cookies' for content protection?
What is rate limiting at the edge, and why is enforcing it at the CDN more effective than at the origin?
What is bot management, and how does a CDN distinguish and mitigate malicious bots at the edge?
How does a CDN provide DDoS mitigation, and what is the difference between absorbing a volumetric attack at the edge vs. at the origin?
Cache Performance & Metrics
What metrics are most important for evaluating a CDN's effectiveness?
How do you define and measure 'Cache Hit Ratio', and what are three common reasons it might be lower than expected?
How do you debug a 'Cache Miss' that you expected to be a 'Cache Hit'? Walk me through your diagnostic process.
What is 'Cache Hit Ratio' (CHR) and what are three specific ways you can improve it?
If a user reports a slow site but the origin server has low CPU/RAM usage, how would you use CDN logs to investigate?
What is the 'Thundering Herd' problem in a CDN context, and how does 'Request Collapsing' (or Cache Lock) prevent it?
Caching Mechanics
How does a CDN decide when to fetch a new version of a file from the origin?
How does the origin use Cache-Control headers to signal the CDN what is cacheable and for how long?
Explain the 'Vary' header. How does it influence how a CDN stores and serves different versions of a resource?
What is a 'Cache Key' and how do query strings or headers like User-Agent affect cache fragmentation?
What is 'Negative Caching' and why is it important for a CDN?
Explain 'Cache Key Normalization'. Why would you want to strip certain query parameters at the edge?
What is 'Cache Poisoning,' and how can misconfigured CDN cache keys lead to it?
Cache Control & Invalidation
Explain the difference between a 'Hard Purge' and a 'Soft Purge' (Invalidation vs. Revalidation).
Explain the 'stale-while-revalidate' directive and how it improves the user experience compared to a standard TTL.
What is the difference between Cache Invalidation (Purging) and Cache Expiration (TTL), and which is more expensive for a global CDN?
Explain the 'stale-if-error' directive and how a CDN uses it to keep serving content when the origin is unavailable.
Explain 'Tag-based Invalidation' (Surrogate Keys). Why is it more efficient than URL-based purging?
Routing & Request Distribution
How does a CDN handle a 'Flash Crowd' or a sudden viral traffic spike without crashing the origin?
How would you use a CDN to implement 'Geo-fencing' or localized content delivery?
How does a CDN handle 'Cold Starts' and what is 'Cache Warming'?
How does a CDN actually route a user to the 'closest' edge server? Explain the difference between Anycast and GeoDNS-based routing.
Performance Optimization
What are the performance benefits of terminating TLS/SSL at the edge rather than at the origin?
How does a CDN use 'Connection Reuse' (Keep-Alive) between the edge and the origin to improve performance?
What are the tradeoffs of enabling 'Edge Minification' or Brotli/Gzip compression at the CDN layer?
What is 'Brotli' or 'Gzip' compression at the edge, and why is it better to do it there than at the origin?
How does the transition from HTTP/2 to HTTP/3 (QUIC) at the CDN layer impact performance for users on high-latency mobile networks?
What is TLS session resumption, and how does the CDN reuse TLS sessions at the edge to reduce handshake overhead?
Edge Compute
What is 'Edge Compute' (e.g., Workers/Functions-at-the-Edge), and how does it change the traditional 'static' nature of a CDN?
What is 'Edge Compute' (or Edge Functions) and what logic is appropriate to move from the backend to the edge?
When would you use an Edge Function to modify a request/response instead of doing it at the origin?
When would you use an 'Edge Function' versus a traditional serverless function?
What are the tradeoffs of moving logic to 'Edge Functions' (Compute-at-the-Edge) vs. keeping it in the backend?
Multi Cdn & Content Protection
Why would a company adopt a 'Multi-CDN' strategy? What are the architectural complexities of doing so?