Incremental Static Regeneration (ISR) is a feature in Next.js

Incremental Static Regeneration (ISR) is a feature in Next.js that enables you to update static pages after they've been built, essentially allowing static pages to be rendered and revalidated in the background at a specified interval. This means you can benefit from the fast performance of static generation along with the up-to-date benefits of server-side rendering.

How ISR Works

With ISR, you can define a revalidation time for each page at build time. This tells Next.js how often it should regenerate the page to refresh the data. Here's how the process works:

1. **Build Time**: During the build process, Next.js pre-renders the page into static HTML, based on the data fetched at build time.

2. **User Request**: When a user requests the page, the statically generated page is immediately served.

3. **Background Revalidation**: If the page was configured to revalidate after a certain period and that time has elapsed, the next request to the page triggers a regeneration of the page in the background. The user still receives the statically cached page immediately.

4. **Cache Update**: Once the page is regenerated, the static HTML is updated in the cache without causing any downtime or delays to the user experience. Subsequent requests serve the updated page.

Implementing ISR in Next.js

To implement ISR in Next.js, you use the `getStaticProps` function along with a `revalidate` property. Here is a basic example:

// pages/posts/[id].js

export async function getStaticProps(context) {

  const { params } = context;

  const res = await fetch(`https://api.example.com/posts/${params.id}`);

  const post = await res.json();

  return {

    props: {

      post,

    },

    // Next.js will attempt to re-generate the page:

    // - When a request comes in

    // - At most once every 10 seconds

    revalidate: 10, // In seconds

  };

}

export async function getStaticPaths() {

  return {

    paths: [],

    fallback: 'blocking' // or true, depending on your needs

  };

}

Advantages of ISR

• **Performance**: ISR combines the benefits of static generation (fast page loads) with the flexibility of server-side rendering (fresh data).

• **Scalability**: Since the pages are pre-built and only regenerated periodically, this approach can scale to very large numbers of pages or high traffic with less strain on your backend compared to SSR.

• **SEO Friendly**: Static pages are generally better for SEO because they can be indexed more easily and load faster.

Use Cases for ISR

• **Blogs and News Sites**: Websites that update content regularly but don't require instant updates for each change can benefit greatly from ISR.

• **E-commerce**: Product pages that do not change minute-by-minute can be built as static pages but revalidate periodically to ensure prices and stock levels are up to date.

• **Event Listings**: Pages that aggregate upcoming events, where new events are added or existing ones are updated periodically.

Considerations

• **Stale Data**: There's a trade-off between revalidation frequency and data freshness. You need to balance these based on your application's needs.

• **Resource Usage**: While revalidation runs in the background, it still consumes server resources. Plan your infrastructure to handle this load, especially during peak traffic.

ISR provides a powerful middle ground between fully static and fully dynamic rendering, making it suitable for a wide range of applications where balance between performance and data freshness is required.