Static Content Delivery

The Ideal Cache Citizens

 static content = bytes that don't change per user or per request:

 JS/CSS bundles, images, fonts, video segments,
 downloads, documents, generated reports (once written)

 properties that make them perfect for edge caching:
 - identical for ALL users        → maximum key sharing
 - versioned by filename          → no invalidation needed
 - high volume + high reuse       → hit rates 95-99.9%

The Immutable Pattern End-to-End

 build pipeline:
   app.js → app.d41d8cd9.js     (content hash in name)
   
 headers served (via CDN):
   /app.d41d8cd9.js
     Cache-Control: public, max-age=31536000, immutable
   /index.html  
     Cache-Control: no-cache    ← always revalidate; points to new hashes

 consequences:
 - deploys: new hashes appear; old cached files remain valid
   mid-session users finish on the old bundle seamlessly
 - ZERO purge machinery needed — filenames ARE versions
 - edge hit rate → ~100%; origin sees ~one fetch per file per region EVER

Image and Media Handling

 variants multiply content: same photo × sizes × formats:

 photo.jpg → photo_400.webp, photo_800.webp, photo_orig.jpg

 key design: encode variant in PATH (cacheable), never query hacks:
 ✓ /img/800/photo_abc.webp      clean distinct keys
 ✗ /img/photo?w=800&fmt=webp    query params need explicit key config
 
 on-the-fly transformation at edge (many CDNs offer it):
   first request for a size → generate+cache; rest are hits.
   origin stores ONE original.

Video Segments

 HLS/DASH slice video into 2-10s segments:

 /video/123/seg_047.ts
 - identical for every viewer → shared aggressively
 - popularity follows the SAME power law as pages
 - live streams: short TTLs on the sliding window;
   VOD: immutable forever

 origin math with shielding: one segment upload →
 ~1 fetch per region total. this is why streaming works at scale.

Cache-Key Hygiene for Static

 audit what enters your keys:
 - strip analytics junk (_, fbclid, utm_*) via key-normalization config
 - case-normalize paths
 - Vary header discipline: Vary: Accept-Encoding only where real;
   accidental Vary: User-Agent fragments caches into thousands
   of near-duplicate entries (classic mobile-era wound)

Numbers to Internalize

 typical static-offload outcomes:
   asset requests hitting origin:  <1%
   latency p50 far-region:         ~150ms → ~15ms
   egress cost:                    shifted to cheaper CDN pricing
   deploy propagation:             instant globally (new names)

 static delivery is SOLVED engineering — the interview question
 is whether you reach for the whole pattern reflexively.

Interview Framing

“Serve product images globally” scored answer: CDN edges + immutable/versioned URLs, path-encoded variants, key normalization against junk params, shield mention, and the deploy story (hashes make purges unnecessary). Bonus depth: edge-transform for responsive sizes keeping origin storage minimal.