Last updated: Apr 29, 2026

Incremental Static Generation

Incremental Static Generation (also called Incremental Static Regeneration, ISR) builds on Static Site Generation. Pages are still pre-rendered at build time, but each page carries a revalidation interval. When a request arrives after the interval expires, the CDN serves the stale page immediately and triggers a background regeneration. The next visitor gets the freshly built version. This is the HTTP stale-while-revalidate pattern applied to full pages.

How It Works

┌──────────┐     ┌───────┐      ┌──────────┐
│  Server  │     │  CDN  │      │ Browser  │
└────┬─────┘     └───┬───┘      └────┬─────┘
     │                │←── request ──│
     │                │ fresh?       │
     │                │ yes → cached →│
     │                │ no  → stale ─→│
     │←── rebuild ───│               │
     │── new HTML ──→│ (next request │
     │                │  gets update) │

Pages are generated at build time like SSG. Each has a revalidate time (e.g. 60 seconds). Within that window the CDN serves the cached file. After the window expires, the next request still gets the stale page (no latency penalty), but the server re-renders in the background and updates the cache.

Next.js API

Adding revalidate to getStaticProps enables ISR:

import type { GetStaticProps } from "next";

export const getStaticProps: GetStaticProps<{ product: Product }> = async ({ params }) => {
  const product = await fetchProduct(params!.id as string);
  return {
    props: { product },
    revalidate: 60,
  };
};

On-Demand Revalidation

For cases where time-based staleness is unacceptable, an API call can immediately invalidate a specific page:

import type { NextApiRequest, NextApiResponse } from "next";

export default async function handler(req: NextApiRequest, res: NextApiResponse) {
  await res.revalidate(`/products/${req.query.id}`);
  return res.json({ revalidated: true });
}

A CMS webhook can call this endpoint after a content editor publishes, so the page updates within seconds instead of waiting for the timer.

Trade-offs