ISR (Incremental Static Regeneration).

There are broadly three ways to generate a page. With static site generation (SSG), the HTML is created once at build time and then delivered unchanged: very fast, but only as current as the last build. With server-side rendering, the HTML is created afresh on every request: always current, but with computing effort per request. ISR sits in between: the page is generated statically and cached, but can be regenerated individually while the site is live.

The classic flow is time-based. A page is given a revalidation interval, for example one hour. If someone requests the page after that interval has expired, they first still receive the existing version while Next.js generates a new one in the background. The next visitor then sees the updated page. This pattern is called 'stale-while-revalidate': better to show the old version briefly than to make the visitor wait.

On-demand revalidation is more widely used today. Here a page is regenerated not on a timer but deliberately as soon as its data changes, for instance when an article is saved in the headless CMS or a price is adjusted in the shop. The CMS calls a protected endpoint that revalidates exactly that page, or all pages with a particular tag. Content stays current without rebuilding a thousand pages for every small change.

ISR suits search engines well, because every page comes as finished HTML from the cache. That ensures a short time to first byte and fully readable content without JavaScript. Pages that did not yet exist at build time can be generated on first request and then cached as well, which is practical for large catalogues where not every page should be built in advance.