Interview questions

Interview questions (with the counter-questions you'll get)

Q1. What is a CDN, and why use one?

They're testing: fundamentals.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** A network of edge servers in many locations that cache and serve content close to users. It cuts latency (shorter round trips), offloads the origin, reduces bandwidth cost, absorbs spikes and DDoS, and adds edge features like TLS termination, compression and WAF.
3
4**Counter-questions:**
5- *"Does it help an uncacheable API?"* → Somewhat: warm, optimised connections to the origin and TLS at the edge; but the work still happens at the origin.
6
7**Trap:** "it makes everything faster".

Q2. How do you decide what the CDN caches and for how long?

They're testing: HTTP caching knowledge.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** Through Cache-Control from the origin: public or private, max-age for browsers and CDNs, s-maxage for shared caches only, no-store for sensitive data, immutable for versioned assets, plus stale-while-revalidate and stale-if-error. Validators (ETag) allow cheap 304 revalidation. Versioned URLs let static assets be cached for a year.
3
4**Counter-questions:**
5- *"Why s-maxage separately?"* → So the CDN can keep a page longer (and be purged) while browsers revalidate sooner, since you can't purge browsers.
6
7**Trap:** one TTL for everything.

Q3. What's a cache key, and how can it go wrong?

They're testing: practical hit-ratio thinking.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** The identity of a cached object: usually host, path and query, plus headers named in Vary. It goes wrong when noise like tracking parameters or unsorted queries fragments it, lowering the hit ratio, or when Vary includes high-cardinality headers like Cookie. Normalise the key and include only what changes the response.
3
4**Counter-questions:**
5- *"And the opposite mistake?"* → Leaving something out of the key that does change the response (language, device type), so users get the wrong variant.
6
7**Trap:** caching per cookie.

Q4. How do you update content that's cached on a CDN?

They're testing: invalidation strategies.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** For static assets, versioned file names, so no purge is needed. For pages, short TTLs with stale-while-revalidate, plus purges by URL or by surrogate-key tags when something must change immediately. Avoid purge-all, which stampedes the origin. Remember browser caches can't be purged.
3
4**Counter-questions:**
5- *"A legal takedown must disappear now?"* → Purge by URL and tag, return 410 from the origin, and keep browser TTLs short for such content.
6
7**Trap:** relying on purge-everything.

Q5. What is an origin shield?

They're testing: CDN architecture depth.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** A middle cache tier that all edges go through before the origin. It collapses many edge misses for the same object into one origin request, increases the overall hit ratio and protects the origin from stampedes, at the cost of an extra hop on misses.
3
4**Counter-questions:**
5- *"Where do you place it?"* → Near the origin, with good connectivity to it.
6
7**Trap:** confusing it with a WAF.

Q6. How do you serve personalised pages with a CDN?

They're testing: nuance.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** Cache the shared parts and fetch personal parts separately (client-side calls or edge includes); micro-cache shared dynamic pages for seconds; never cache personal responses in shared caches (private or no-store); use edge functions for light personalisation such as geography or A/B buckets.
3
4**Counter-questions:**
5- *"What's the danger?"* → Caching a personal page publicly, so one user sees another's data.
6
7**Trap:** caching everything HTML with a public TTL.

Q7. How does video streaming use CDNs?

They're testing: segmenting and ABR.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** Video is encoded into a ladder of bitrates and cut into short segments (around 6 seconds for HLS VOD), listed in playlists (HLS .m3u8 or DASH .mpd). Players fetch segments over HTTP and pick a bitrate per segment based on measured throughput. Segments are immutable files that CDNs cache like any other; live playlists use short TTLs.
3
4**Counter-questions:**
5- *"Shorter segments for live?"* → Lower latency but more requests and overhead; low-latency HLS uses partial segments.
6
7**Trap:** "the CDN streams one big file".

Q8. Your CDN goes down. What happens, and how do you prepare?

They're testing: resilience.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** Every site behind it fails, even with healthy origins, as Fastly's 2021 outage showed. Prepare with multi-CDN and health-based traffic steering, an origin that can survive direct traffic briefly behind rate limits, stale-if-error for origin problems, CDN config treated as code with gradual rollout, and monitoring from users' locations.
3
4**Counter-questions:**
5- *"Downside of multi-CDN?"* → Cost, duplicated config, lower hit ratio per provider and more complexity.
6
7**Trap:** assuming the CDN can't fail.

Q9. Pull or push CDN?

They're testing: trade-offs.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** Pull fetches from the origin on a miss and caches by headers: easy, and caches only popular content, but first requests per PoP are slow. Push uploads content to CDN storage in advance: no cold origin misses, but you manage uploads and deletions. Most sites pull, often from object storage, and pre-warm for big launches.
3
4**Counter-questions:**
5- *"When would you push?"* → Large, predictable releases like a game patch or a big video premiere.
6
7**Trap:** not knowing the difference.

Q10. How would you measure whether your CDN is working well?

They're testing: operational metrics.

Think about your answer first, then open it.

reveal

1Show the strong answer, counter-questions and trap
2**Strong answer:** Cache hit ratio (requests and bytes), per PoP and per content type; origin request rate and egress; edge latency and time to first byte from real users; error rates (4xx and 5xx at edge vs origin); and purge latency. Investigate low hit ratios through cache-key fragmentation and TTLs.
3
4**Counter-questions:**
5- *"Hit ratio is 70%. Where do you look first?"* → Query-string noise, Vary headers, very short TTLs, and uncacheable responses mixed into the numbers.
6
7**Trap:** only watching the origin's health.
8
9---
10
11## Practice coverage
12
13Every section in this chapter is tested at least once.
14
15| Section | Tested by |
16|---|---|
17| 26.1 Why a CDN | Level 1B.1, Level 2.1, Level 2.2, Level 2.3, Q1 |
18| 26.2 Edge locations | Level 2.1, Q1 |
19| 26.3 Pull vs push | Q9 |
20| 26.4 Cache keys and headers | Level 1A, Level 1B.2, Level 3.1, Level 3.2, Level 5.1, Level 5.2, Q2, Q3 |
21| 26.5 Purging | Level 1B.3, Level 3.3, Level 5.3, Q4 |
22| 26.6 Origin shield | Level 1B.4, Level 2.5, Q5 |
23| 26.7 Dynamic content | Level 1A.5, Level 5.4, Q6 |
24| 26.8 Video segments | Level 1B.5, Level 2.4, Level 3.4, Q7 |
25| 26.9 CDN failure | Level 3.5, Level 4, Level 5.5, Q8, Q10 |
26
27---

26. CDNs