CDN и edge-кэширование: ментальная модель

Question

Accepted Answer

CDN — caches near users via anycast; hit = single-digit ms; miss occasional origin trip. Physics: move response to user.

## Разбор

Sydney → Virginia = ~200ms before server work. CDN **moves response to user**. PoPs hold copies; **anycast** routes to nearest.

```
Without CDN:  User (Sydney) ───── 200ms+ ─────▶ Origin (Virginia)   every request

With CDN:     User (Sydney) ─ 5ms ─▶ Edge (Sydney)  ── HIT: done
                                        │
                                        └── MISS ── 200ms ──▶ Origin  (once, then cached)
```

Mental model: **shared HTTP cache you don't run** in front of origin. Same `Cache-Control` rules + `s-maxage` + purge/versioning.

## Итог

CDN defeats distance latency; correctness = cache keys + invalidation strategy.