Memory card and where the analogy breaks

The memory card (tear this page out)

#	Question	Memory code
26.1	Why a CDN?	Stock it near the shop, and the factory can rest
26.2	Edge locations?	A warehouse in every city; the road takes you to the nearest
26.3	Pull vs push?	Restock when asked, or ship before launch day
26.4	Cache keys and headers?	One label per product, and a use-by date on every carton
26.5	Purging?	Recall by product or by batch, but better to change the label than chase the cartons
26.6	Origin shield?	Warehouses call the depot; only the depot calls the factory
26.7	Dynamic content?	Cache the shared part for seconds, and do the small jobs near the customer
26.8	Video segments?	Cut the film into reels, in several qualities, and let the player pick reel by reel
26.9	CDN failure?	If the warehouses strike, have a second chain, and keep old stock you can still sell

The whole chapter in one line: Serve copies from a nearby edge, define "the same file" carefully, label freshness with Cache-Control, version instead of purging, shield the origin, cut video into segments, and plan for the CDN itself failing.

Where the analogy breaks

A biscuit carton is the same for every shop. Web responses can be personal, and a CDN that treats a personal page like a carton hands one user's data to another.
A recall reaches every warehouse but not the biscuits already in people's kitchens. A purge can't reach browsers' caches either, which is why browser TTLs need care.
Opening a new warehouse takes months. A CDN can route you to another city in milliseconds, which is why a CDN's mistakes also spread worldwide in seconds.

Quick check

Five questions on this chapter. Pick an answer, then check it.

quiz

1[
2 {
3  "q": "What's the main win of a CDN?",
4  "options": [
5   "Cheaper databases",
6   "Content served from nearby edge servers: faster for users, less load on the origin",
7   "Better SEO only",
8   "Encryption"
9  ],
10  "answer": 1,
11  "explain": "Stock it near the shop, and the factory can rest."
12 },
13 {
14  "q": "You redeploy app.js with a bug fix. What's the most reliable way to make users get the new file?",
15  "options": [
16   "Purge every edge and hope",
17   "Change the file name (e.g. app.3f9a.js) so it's a new cache key",
18   "Lower the TTL afterwards",
19   "Ask users to clear caches"
20  ],
21  "answer": 1,
22  "explain": "Better to change the label than chase the cartons."
23 },
24 {
25  "q": "What does an origin shield do?",
26  "options": [
27   "Blocks attacks only",
28   "Edges ask one shield layer on a miss, so the origin sees one request instead of hundreds",
29   "Encrypts content",
30   "Stores user data"
31  ],
32  "answer": 1,
33  "explain": "Warehouses call the depot; only the depot calls the factory."
34 },
35 {
36  "q": "How does adaptive video streaming work with a CDN?",
37  "options": [
38   "One huge file per video",
39   "Video is cut into short segments in several qualities; the player picks a quality per segment",
40   "The CDN transcodes live per user",
41   "Only audio is cached"
42  ],
43  "answer": 1,
44  "explain": "Cut the film into reels, in several qualities, and let the player pick reel by reel."
45 },
46 {
47  "q": "Pull vs push CDN: pull means...",
48  "options": [
49   "You upload everything in advance",
50   "The edge fetches from the origin on the first miss, then caches it",
51   "Users push content",
52   "No caching"
53  ],
54  "answer": 1,
55  "explain": "Restock when asked, or ship before launch day."
56 }
57]

26. CDNs

The memory card (tear this page out)

Where the analogy breaks

Quick check

Platform

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