CDN Cache vs Redis Cache: Key Differences and Ideal Use Cases (Interview Insight)

Interview Focus Points

Fundamental concepts : The interviewer expects you to know that CDN (Content Delivery Network) solves network latency and origin‑server pressure by caching content at edge nodes.

Cache mechanism : You should be able to describe cache hit, expiration, and update processes, not just claim that "CDN can cache static resources".

Architecture thinking : Explain CDN’s role in the overall system, when to use it and when not to.

Core Answer

CDN (Content Delivery Network) is a distributed server network whose core idea is to cache content on edge nodes closest to users, allowing them to fetch data locally instead of always reaching the origin server.

Why can CDN cache? In one sentence: an edge node is essentially a reverse‑proxy cache server that intercepts user requests, returns the cached response if it exists and is fresh, otherwise fetches from the origin.

Deep Analysis

1. Technical Principles of CDN Caching

CDN relies on several key technologies:

DNS intelligent resolution (GSLB) : When a user requests a domain served by CDN, DNS returns the IP of the optimal edge node based on the user’s IP and node load. Example: a DNS query for static.example.com resolves to a CDN‑authoritative IP, not the origin IP.

Cache hit & miss handling : The edge node checks its local cache. If a hit occurs, it returns the content with header X-Cache: HIT. If missed, the edge node pulls the resource from the origin, caches it, and returns it to the user.

Expiration & refresh : CDN respects HTTP cache‑control headers such as Cache-Control: max-age=3600, Expires, and conditional validators like ETag / Last-Modified. When a cached object expires, the edge node validates with the origin (e.g., 304 Not Modified) or fetches a fresh copy.

A real‑world pitfall: after updating a static file, the CDN still served the old version. The solution was to add a hash fingerprint to the filename (e.g., app.a3b2c1.js) so the URL changes and the old cache is bypassed.

2. What Should Be Cached by CDN?

Suitable: static assets (JS, CSS, images, fonts), large files (installers, videos), live/on‑demand video streams, infrequently changing API responses (e.g., product detail pages).

Not suitable: high‑real‑time interfaces (stock quotes), personalized user data (personal center pages), frequently changing dynamic data, sensitive transaction requests.

In short, content that is read‑heavy, updates rarely, and does not require low latency is ideal for CDN.

3. CDN vs Browser Cache vs Nginx Cache

Cache location : Browser cache lives on the user’s device; CDN cache resides on edge nodes; Nginx cache is on the same data‑center as the origin.

Scope : Browser cache serves only the current user, CDN serves all users in the region, Nginx serves all requests passing through that Nginx instance.

Latency : Browser cache is fastest (local), CDN is low (nearby data‑center), Nginx is moderate (needs to reach the origin data‑center).

Hit rate : Browser hit depends on a single user, CDN hit is high due to many users sharing the same edge cache, Nginx hit is relatively high.

Origin protection : Browser cache provides no protection, CDN strongly protects the origin by intercepting most requests, Nginx offers moderate protection.

The full request chain is: User → Browser Cache → CDN Edge Cache → Nginx Cache → Origin Server . Each layer filters a portion of the traffic, dramatically reducing load on the origin.

High‑Frequency Follow‑Up Questions

Difference between CDN cache and browser cache : Browser cache is per‑user local storage; CDN cache is shared across users in a region. Missed browser cache hits the origin directly, while a CDN miss triggers a fetch by the edge node, invisible to the user.

How to solve stale CDN cache : Three common solutions – (a) add a hash fingerprint to filenames (recommended), (b) call the CDN provider’s cache‑refresh API, (c) shorten max-age (trades off higher origin traffic).

How CDN prevents hotlinking : Techniques include Referer validation, IP black/white lists, signed URLs with expiration tokens, and back‑origin authentication.

Memory Mnemonic

CDN Cache Four Elements : Near‑cache for fast response, DNS‑based smart dispatch, fetch from origin only on miss, and origin load naturally drops.

Remember the chain: User → Browser → CDN Edge → Nginx → Origin , the farther downstream, the fewer requests reach the origin.

Conclusion

The core value of CDN is "nearby cache + intelligent dispatch". In an interview, focus on the three main threads: GSLB scheduling, cache‑hit/back‑origin mechanism, and HTTP cache‑control headers. Combine these with practical experiences such as static‑asset hash fingerprints and anti‑hotlinking measures to deliver a solid answer.