What is a CDN and do you need one?
How content delivery networks work, when they pay off, and when they are overkill.
A CDN (content delivery network) is a set of servers around the world that cache your content and serve it from a location near each visitor.
How it works
A visitor requests
https://cdn.example.com/app.js.The nearest CDN edge answers from cache if it has the file.
On a miss, the edge fetches from your origin (your server), caches it, and serves it.
Your origin handles far fewer requests; visitors get lower latency.
What a CDN gives you
Speed — content served from nearby, not across an ocean.
Offload — static and media traffic stops hitting your server and your bandwidth allowance.
Spike absorption — a traffic surge or attack is spread across the edge.
TLS at the edge — certificates and termination handled for you.
When you do NOT need one
Your audience is in one region, near your server.
Your site is small and light.
You have not yet optimised the basics (caching headers, compression, image sizes). A CDN in front of a slow, unoptimised site is a band-aid on a bigger problem.
Fix the basics first; a CDN multiplies what is already there, for better or worse.
The parts worth understanding
Cache TTL — how long the edge keeps an object. Long for fingerprinted assets; short/none for dynamic pages.
Cache hit ratio — the share of requests served from cache. Low ratio = little benefit; usually a caching misconfiguration.
Purge/invalidation — forcing the edge to drop a cached object after a change.
Origin shield — a single mid-tier cache that reduces origin load further.
WAF — many CDNs include a web application firewall; useful for bot and attack filtering.
The gotcha: two TLS legs
With a CDN there are two connections: visitor → CDN, and CDN → origin. Both need valid certificates. Set them up deliberately or you get a redirect loop or a security warning.
A sensible default
Small/regional site — skip the CDN; optimise caching and images.
Media-heavy or global — use a CDN for static assets and media first, dynamic pages later.
Under attack or expecting a spike — a CDN/WAF in front is the best single defence you can add.
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