HTTP-кэширование (Cache-Control, ETag): ментальная модель

Question

Accepted Answer

Two gears: **freshness** (`max-age` — serve without asking) and **validation** (`ETag` — cheap `304`). Hashed statics + `no-cache`/ETag for HTML/API = 90% solved.

## Разбор

Three cache states:

```
        max-age not expired            stale, ETag matches           stale, changed
FRESH ─────────────────────▶ serve   ─────────────▶ 304 (revalidate) ──────▶ 200 (full body)
 (0 network)                            (headers only, tiny)            (full download)
```

- **Freshness** — skip network? `Cache-Control: max-age`.
- **Validation** — stale but changed? Conditional `If-None-Match` → `304` or full `200`.

Orthogonal: `max-age` decides *whether to ask*; `ETag` decides *how cheap the ask*.

## Итог

Layer freshness for speed, validation for correctness at stable URLs.