how nextjs determines which page should be served statically by cdn

Next.js employs several rendering strategies that allow developers to optimize their applications for performance, SEO, and user experience. The framework determines how pages should be served—whether statically from a CDN, server-side rendered on each request, or through incremental static regeneration—based on the page's export methods used in its code. As of my last update in April 2023, here's how Next.js decides on the rendering strategy for a page:

Static Site Generation (SSG)

• Method: getStaticProps (optionally with getStaticPaths for dynamic routes)
• How it Works: When you export getStaticProps (and getStaticPaths for dynamic routes) in a page, Next.js pre-renders this page at build time. This generates a static HTML file along with a JSON file containing the props data. The HTML file can be served directly from a CDN, ensuring maximum performance and SEO benefits.
• CDN Serving: The statically generated pages are ideal for CDN distribution because they are immutable and can be cached indefinitely until the next build or until you invalidate the cache. For dynamic routes pre-rendered with getStaticPaths, Next.js generates the HTML for each path returned by getStaticPaths at build time.

Server-Side Rendering (SSR)

• Method: getServerSideProps
• How it Works: Pages that export getServerSideProps are rendered on the server for each request. This method fetches data on each request and generates the page's HTML on-the-fly, which means these pages cannot be served directly from a CDN as static files.
• CDN Serving: While the rendered pages themselves aren't cached by a CDN as static assets, you can still cache the API calls made during getServerSideProps using external caching mechanisms (like Redis) to improve performance. However, the HTML is generated server-side for each request.

Incremental Static Regeneration (ISR)

• Method: getStaticProps with the revalidate option
• How it Works: ISR allows you to update static content after you've deployed your application. By using getStaticProps with the revalidate option, Next.js will serve the statically generated page and regenerate it in the background if a request comes in after the revalidation period. This combines the best of both SSG and SSR, allowing for static performance with dynamic content freshness.
• CDN Serving: Initially, ISR pages are served as static files from the CDN. Once the revalidate period passes, the next request triggers a regeneration of the page, and the updated static file is again served from the CDN.

Automatic Static Optimization

• Method: Pages without getStaticProps, getServerSideProps, or getInitialProps
• How it Works: If a page doesn't use any data fetching methods or getInitialProps, Next.js automatically optimizes it as a static page. These pages are pre-rendered to static HTML at build time and can be served from a CDN.
• CDN Serving: These automatically optimized pages are served as static files from the CDN, ensuring fast load times without any configuration required from the developer for caching.

Determining the Strategy

Next.js uses the presence and configuration of these methods (getStaticProps, getServerSideProps, and revalidate) in a page's code to determine the appropriate rendering strategy. The framework's build process and the Next.js server work together to serve pages according to these configurations, leveraging CDNs for static content while dynamically rendering pages as needed based on the developer's specified data fetching requirements and rendering strategies.