A user in Sydney requests an image from a CDN-backed website whose origin server is in New York. The Sydney edge server does not have the image cached. What happens?
AThe request fails; the user must wait until a cache refresh propagates the image to the Sydney edge
BThe CDN's DNS redirects the user's browser directly to the New York origin for this request
CThe Sydney edge server fetches the image from the origin, caches it locally, then serves it to the user
DThe request is forwarded to the nearest edge server that does have the image cached
On a cache miss, the edge server acts as a proxy: it fetches the content from the origin server, stores a copy in its local cache, and serves it to the requesting user. Subsequent users in Sydney requesting the same image get a cache hit and are served directly from the Sydney edge, bypassing the origin. This is why the first user in a region pays the latency penalty but subsequent users do not.
Question 2 Multiple Choice
A web developer says: 'Our CDN improves performance by inspecting each HTTP request and redirecting it to the nearest server.' What is fundamentally wrong with this description?
ACDNs do not use HTTP — they operate at the IP routing level and cannot inspect request headers
BCDN geographic routing happens at the DNS resolution stage, before the HTTP connection is established — not by inspecting HTTP requests
CRedirection requires a 301 HTTP response, which introduces additional round trips that eliminate any latency savings
DCDNs cannot determine a user's geographic location, so routing decisions are made randomly
CDN routing is DNS-based, not HTTP-based. When the user's browser resolves the domain, the CDN's authoritative DNS server returns the IP of the nearest edge server — the routing decision happens before any HTTP connection is made. The browser then connects directly to that edge server. If the CDN worked by inspecting HTTP requests after connection, the user would still have connected to a distant server first, negating much of the latency benefit.
Question 3 True / False
CDN caching is most effective for dynamic, personalized content — such as a user's account dashboard — because that content generates the highest volume of requests.
TTrue
FFalse
Answer: False
CDN caching is most effective for STATIC content: images, CSS, JavaScript files, and videos that are the same for all users and change infrequently. Dynamic or personalized content (account dashboards, shopping carts, real-time feeds) is difficult to cache because it varies per user and changes frequently. Caching a personalized page would serve the wrong user's data to someone else. CDNs handle dynamic content through short TTLs, edge computing, and cache key variations — but these are workarounds for the fundamental mismatch between caching and personalization.
Question 4 True / False
When a CDN edge server serves a cache hit, the origin server receives no request and incurs no load for that content delivery.
TTrue
FFalse
Answer: True
A cache hit means the edge server has a valid cached copy and serves it directly to the user without contacting the origin. This is the primary mechanism by which CDNs reduce origin server load and absorb traffic spikes. If a video goes viral and generates millions of requests, the vast majority are served from edge caches worldwide; the origin only had to serve the content once per edge location per cache TTL period, rather than millions of times.
Question 5 Short Answer
Describe how a CDN uses DNS to route a user to the nearest edge server, starting from the moment the user's browser needs to load an image.
Think about your answer, then reveal below.
Model answer: The website's DNS configuration delegates the image subdomain to the CDN's authoritative DNS. When the browser resolves the hostname, the query reaches the CDN's authoritative DNS server. That server examines the IP address of the user's recursive DNS resolver to infer geographic location, then returns the IP address of the nearest edge server (point of presence). The browser connects to that edge server via HTTP — not to the origin. If the edge has the image cached (cache hit), it serves it immediately. If not (cache miss), it fetches from the origin, caches it, and serves it.
The key insight is that DNS is the routing mechanism — the geographic decision is made at name resolution time, before any content is transferred. This is why CDNs require DNS delegation rather than simply adding IP addresses to a load balancer: the CDN's DNS infrastructure performs the geographic intelligence. The user's browser never knows or cares that it connected to an edge server rather than the origin.