Web Fundamentals

HTTP Caching Deep Dive: Cache-Control, ETag & Revalidation

mediumWeb Fundamentals

HTTP Caching Deep Dive: Cache-Control, ETag & Revalidation

Learn the interview-ready mental model, practical trade-offs, and production patterns for this web fundamentals topic.

Topic content

TL;DRFresh hit > stale + revalidate > full miss. Use different policies for static assets, HTML, and user data.
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HTTP caching lets browsers and intermediaries reuse prior responses. Cache-Control controls freshness and behavior. Validators like ETag and Last-Modified enable efficient revalidation via 304 responses. Good policies balance hit rate, freshness, correctness, and safety.

You put food (responses) in the fridge with an expiration date (max-age). When you want something, check if it's still fresh. If expired, quickly check with the store (revalidation) before throwing it out. Some items (immutable assets) never expire. Personalized items stay in your private mini-fridge.

First Request → Store in Cache
Subsequent Request
├── Fresh → Serve immediately
├── Stale → Revalidate (304 or 200)
└── Miss → Full fetch

1Cache-Control Directives

The primary header for controlling caching. max-age sets freshness lifetime. public/private controls shared vs private caches. no-cache requires revalidation. immutable is perfect for hashed assets.

2Validators: ETag & Last-Modified

Enable efficient revalidation. Server returns 304 Not Modified when content hasn't changed, avoiding full body download.

3stale-while-revalidate & Vary

Serve stale content while refreshing in background for better perceived speed. Vary ensures the correct representation is cached.

Key Takeaways
  • ✓Fresh cache hit is ideal — avoid network entirely
  • ✓Cache-Control is the primary control mechanism
  • ✓Use immutable + long max-age for hashed static assets
  • ✓HTML usually needs short freshness or revalidation
  • ✓Validators (ETag/Last-Modified) enable cheap 304s
  • ✓Vary and private are critical for correctness
  • ✓stale-while-revalidate improves perceived performance
  • ✓Always differentiate policies by resource type