Next.js allows you to create or update static pages after you’ve built your site. Incremental Static Regeneration (ISR) enables you to use static-generation on a per-page basis, without needing to rebuild the entire site. With ISR, you can retain the benefits of static while scaling to millions of pages.

To use ISR add the revalidate prop to getStaticProps:

function Blog({ posts }) {
  return (
    <ul>
      {posts.map((post) => (
        <li>{post.title}</li>
      ))}
    </ul>
  )
}

// This function gets called at build time on server-side.
// It may be called again, on a serverless function, if
// revalidation is enabled and a new request comes in
export async function getStaticProps() {
  const res = await fetch('https://.../posts')
  const posts = await res.json()

  return {
    props: {
      posts,
    },
    // Next.js will attempt to re-generate the page:
    // - When a request comes in
    // - At most once every 10 seconds
    revalidate: 10, // In seconds
  }
}

// This function gets called at build time on server-side.
// It may be called again, on a serverless function, if
// the path has not been generated.
export async function getStaticPaths() {
  const res = await fetch('https://.../posts')
  const posts = await res.json()

  // Get the paths we want to pre-render based on posts
  const paths = posts.map((post) => ({
    params: { id: post.id },
  }))

  // We'll pre-render only these paths at build time.
  // { fallback: blocking } will server-render pages
  // on-demand if the path doesn't exist.
  return { paths, fallback: 'blocking' }
}

export default Blog

When a request is made to a page that was pre-rendered at build time, it will initially show the cached page.

• Any requests to the page after the initial request and before 10 seconds are also cached and instantaneous.
• After the 10-second window, the next request will still show the cached (stale) page
• Next.js triggers a regeneration of the page in the background.
• Once the page has been successfully generated, Next.js will invalidate the cache and show the updated page. If the background regeneration fails, the old page would still be unaltered.

When a request is made to a path that hasn’t been generated, Next.js will server-render the page on the first request. Future requests will serve the static file from the cache.

Incremental Static Regeneration covers how to persist the cache globally and handle rollbacks.