Incremental Static Regeneration (ISR) in Next.js

Incremental Static Regeneration (ISR) is a powerful feature introduced in Next.js that allows you to update static content after the site has been built and deployed. With ISR, Next.js can regenerate static pages in the background at runtime, ensuring that content remains fresh without rebuilding the entire site.

This feature is particularly useful when you need to update pages frequently or have large websites where rebuilding the entire site would be inefficient. ISR allows you to maintain the performance benefits of static site generation (SSG) while ensuring that your pages are always up to date.

1. What is ISR?

Incremental Static Regeneration (ISR) allows you to:

Pre-render static pages at build time (SSG).
Update or regenerate specific pages on-demand after the site has been deployed, without needing to rebuild the entire site.
Control how frequently the static pages are updated based on your specific needs.

ISR is configured by setting a revalidate property in the getStaticProps function. This property tells Next.js how often a page should be re-generated after the specified interval.

2. How ISR Works

Build Time (Initial Rendering):
- When you first build your Next.js app, it will pre-render the static pages just like traditional static site generation.
Serving Static Content:
- These pre-rendered pages are served by a CDN (Content Delivery Network) for fast delivery.
Background Regeneration:
- If a page is accessed after the specified revalidation time (revalidate), Next.js will regenerate the page in the background while still serving the static content.
New Content Served:
- Once the regeneration is complete, the new content is served to subsequent users.

3. Using ISR in Next.js

To enable ISR in Next.js, you need to use getStaticProps in conjunction with the revalidate key. Here’s an example:

import React from 'react';

const BlogPost = ({ post }) => {
  return (
    <div>
      <h1>{post.title}</h1>
      <p>{post.body}</p>
    </div>
  );
};

export async function getStaticProps() {
  const res = await fetch('https://jsonplaceholder.typicode.com/posts/1');
  const post = await res.json();

  return {
    props: { post },
    revalidate: 60,  // Regenerate the page every 60 seconds
  };
}

export default BlogPost;

Key Points in the Example:

revalidate: 60: This tells Next.js to regenerate the page every 60 seconds in the background when a request comes in. If a page has been served for 60 seconds, Next.js will trigger a regeneration when a new request arrives.
The page is still statically generated on the first request, and then incrementally updated as necessary.

4. How to Configure ISR

You can set the revalidate option in getStaticProps to determine how often the page should be regenerated:

revalidate: false: The page will never be regenerated and will be served statically indefinitely (like traditional static sites).
revalidate: n: The page will be regenerated after n seconds. For example, revalidate: 60 means that the page will be updated every 60 seconds.

You can also use getStaticPaths in combination with ISR to generate static pages for dynamic routes, similar to how getStaticProps works.

5. Example: ISR with Dynamic Routes

When dealing with dynamic routes, such as product pages or blog posts, you can use getStaticPaths with getStaticProps and ISR. Here’s an example:

import React from 'react';

const Product = ({ product }) => {
  return (
    <div>
      <h1>{product.name}</h1>
      <p>{product.description}</p>
    </div>
  );
};

// Generate the paths at build time
export async function getStaticPaths() {
  const res = await fetch('https://jsonplaceholder.typicode.com/posts');
  const products = await res.json();

  const paths = products.map((product) => ({
    params: { id: product.id.toString() },  // Dynamic route based on product id
  }));

  return {
    paths,
    fallback: true,  // Show a fallback while the page is being generated
  };
}

// Fetch data for each product page and set revalidation interval
export async function getStaticProps({ params }) {
  const res = await fetch(`https://jsonplaceholder.typicode.com/posts/${params.id}`);
  const product = await res.json();

  return {
    props: { product },
    revalidate: 60,  // Regenerate the page every 60 seconds
  };
}

export default Product;

How this Example Works:

getStaticPaths generates the paths for all the product pages (e.g., /products/[id]).
getStaticProps fetches the data for each product at build time.
The page is regenerated every 60 seconds based on the revalidate value.

6. Benefits of ISR

Performance: ISR allows for static page generation, which results in fast loading times for users. Even when the data changes, only the necessary pages are regenerated, rather than the entire site.
Fresh Content: You can control how often static pages are updated to ensure users always get the latest content without waiting for the whole site to rebuild.
Scalability: ISR reduces the need for dynamic rendering or SSR, lowering server load and enabling highly scalable architectures.
SEO Benefits: Since the pages are served statically, search engines can crawl and index the fully rendered HTML.

7. Fallbacks in ISR

The getStaticPaths function has a fallback option that determines what happens when a user requests a page that hasn’t been generated yet.

fallback: false: If the page doesn’t exist, a 404 error is shown.
fallback: true: If the page isn’t generated yet, Next.js will serve a static fallback page (a loading screen) until the regeneration is complete.
fallback: 'blocking': This option blocks the user from seeing the page until the regeneration is complete. Once the regeneration finishes, the new page is served.

8. Handling Edge Cases with ISR

Fallback Pages: If you’re using fallback: true, you may want to add a loading state or spinner while waiting for the page to be generated.
Data Staleness: If your data changes frequently (e.g., in real-time apps), consider using shorter revalidation intervals or other strategies like Server-Side Rendering (SSR).

9. Performance Considerations

While ISR helps with content freshness and performance, you should still be mindful of:

API Latency: If the data-fetching API is slow, it could impact regeneration times.
Build Time: Regenerating static pages in the background can introduce some latency, but it won’t block the user from seeing the previous version of the page.
Caching: ISR works well with CDNs, and caching strategies should be used to ensure fast page delivery.