CDN и edge-кэширование: как это работает на практике

Question

Accepted Answer

CDN cache key defines «same object»; `s-maxage` for edge TTL; purge vs versioned URLs; edge compute for dynamic personalization.

## Разбор

**Cache key** — method + host + path + query + `Vary`. `?utm_source=...` in key → hit rate collapse. Ignore param that changes response → wrong body. Strip tracking params — highest-leverage knob.

**Edge freshness:** `Cache-Control: s-maxage` (CDN TTL, overrides `max-age` for shared cache), `public`. `private`/`no-store` — CDN must not cache per-user.

**Invalidation:** **purge** (evict now — slow, rate-limited) vs **versioned URLs** (new path, old never requested — preferred). Content-hashed filenames.

**Modern CDN:** `stale-while-revalidate`, **edge functions** (Workers, Lambda@Edge) personalize at edge without full origin trip.

## Итог

CDN = shared HTTP cache at edge; cache key tuning and hashed assets matter more than raw PoP count.