MediumExperience level: 3-5 years · 8-15 years1 min read

How does CloudFront (CDN) improve performance, and what do you cache vs not?

Reviewed by Gurusankar M.

Asked inAmazonGoogleMicrosoft
#cloudfront#cdn#caching#edge#ttl
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Short Answer

CloudFront caches content at edge locations near users, cutting latency and offloading origin traffic. Cache static assets aggressively (long TTL + cache-busting filenames); use short/no cache for dynamic/personalized responses. Control behavior with cache policies (TTLs, which headers/cookies/query strings form the cache key).

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Concept Made Simple

Coffee Chat Question

“How does CloudFront (CDN) improve performance, and what do you cache vs not?”

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Mind Map Answer

Edge cache
content near users → low latency
Static
long TTL + versioned filenames
Dynamic
short/no cache, forward needed headers
Cache key
headers/cookies/query params

Think Beyond the Expected

What If?

Users see stale content after a deploy — how do you avoid it without manual invalidations?

Use content-hashed filenames (app.a1b2c3.js) so each deploy produces new URLs the CDN treats as new objects — old cached files just age out. This 'cache-busting' avoids costly/global invalidations and eliminates stale-asset bugs.

Real World

CDN in front of S3/ALB with hashed asset filenames is standard for web performance; relying on manual invalidations instead of cache-busting is a common source of stale-content incidents.

Interviewer's Expectation

Keywords they're listening for:

  • edge caching/latency
  • static long-TTL + cache busting
  • dynamic short/no cache
  • cache key config
  • invalidation vs versioning

Common Mistakes

  • Caching personalized/dynamic responses
  • Relying on manual invalidations
  • Cache key including too many varying headers (low hit rate)

Best Practices

  • Cache-bust static assets with hashed names
  • Short/no cache for dynamic content
  • Tune the cache key for a high hit rate

Follow-up Questions

  1. How does the cache key affect hit rate?
  2. When do you invalidate vs version?
  3. Origin Access Control for private S3 origins?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: S3 (AWS)
Interview question: "How does CloudFront (CDN) improve performance, and what do you cache vs not?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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