Invalidation and the CDN

Key points

• TTL is the invalidation you never have to remember. Choose the staleness you can live with and let entries expire.
• Eviction on write (@CacheEvict, Redis DEL) is precise but fragile: every write path must remember to evict, forever.
• Versioned keys dodge the problem. Read product:42:v7, bump to v8 on write, and let the old entry expire alone.
• Stampede: a hot key expires and a thousand threads hit the database together. Fix it with per-key loading or jittered TTLs.
• A CDN is a cache you rent near the user: static assets, images, and cacheable GET responses served from an edge node instead of your origin.
• Fingerprint asset names and cache them forever; keep HTML on no-cache so a deploy is visible immediately.

Example

# Fingerprinted asset: the name changes when the bytes change, so cache it forever.
curl -sI https://cdn.example.com/static/app.7f3a91c2.js | grep -Ei 'cache-control|age|x-cache'
# cache-control: public, max-age=31536000, immutable
# age: 82311          <- seconds this copy has sat at the edge
# x-cache: HIT        <- never reached the origin

# HTML must never be stale: store it, but revalidate with the ETag every time.
curl -sI https://example.com/ | grep -i cache-control
# cache-control: no-cache

# Purge only as an escape hatch - it is slow and provider-specific.
curl -X POST https://api.cdn.example.com/v1/purge \
     -H "Authorization: Bearer $CDN_TOKEN" \
     -d '{"paths":["/index.html"]}'

Immutable names for assets, short TTLs for data, purge only when something is on fire.