Next.js: ISR (Incremental Static Regeneration) — типичные gotcha на собеседовании

Question

Accepted Answer

Кто платит regeneration, minimum revalidate, dynamic API opts out, `revalidate = 0`, serverless per-instance cache — частые gotchas ISR.

## Разбор

- **«Who pays regeneration cost?»** Not triggering user — stale instantly. Background render для next visitor. «User waits rebuild» = SSR, не ISR.
- **Route window = minimum of all `revalidate`.** One fetch `revalidate: 10` в page `revalidate: 3600` → whole route 10s.
- **Dynamic API opts out entirely.** `cookies()`, `headers()`, `searchParams` → fully dynamic, Full Route Cache не applies — «ISR не кэширует» = accidental dynamic API.
- **`revalidate = 0` is not ISR** — `force-dynamic`.
- **Serverless cache per-instance** без shared storage — each container regenerates independently; self-hosted нужен shared cache handler.

## Итог

ISR interview = stale-while-revalidate semantics + minimum window rule + dynamic API poison pill.