Incremental Static Regeneration (ISR): A flexible way to cache dynamic content

What caching buys you

Before evaluating ISR, it's worth restating what any caching layer should accomplish. Storing and reusing previously computed responses reduces the number of requests that hit your origin servers, cuts the bandwidth consumed between server and client, and lowers latency because a cached response is served faster than a freshly generated one. Caching also improves availability: even if the upstream provider fails, end users can still retrieve the last cached version of a page. These gains translate directly into higher traffic capacity, lower infrastructure costs, and a more responsive application.

How ISR combines static and dynamic rendering

Incremental Static Regeneration (ISR) is a server-side caching strategy that blends the strengths of static site generation (SSG) and server-side rendering (SSR). Rather than choosing between prerendered pages that go stale and fully dynamic pages that are slow to generate, ISR lets you have both. The key capabilities it introduces:

• Stale-while-revalidate delivery: Users receive edge-cached static content immediately while fresh data is fetched and stored in the background. This is the same model HTTP caching calls "stale-while-revalidate."
• Page-level regeneration: Individual pages can be updated independently without triggering a full site rebuild. This is a major time saver for content-heavy sites where a complete rebuild would be costly.
• On-demand invalidation: Specific pages can be refreshed at the moment new content is published or an update is needed, without waiting for a scheduled revalidation window.
• Deterministic cache lifetime: Cached content remains valid for whatever duration you configure. The cache is only invalidated by your own actions, with one exception: content that has not been accessed for 31 days may be evicted.

ISR is most effective when used with hybrid rendering frameworks, which support mixing prerendered (SSG) content and per-request (SSR) content within the same application. This allows each route to use the rendering strategy that fits it best, while ISR fills the gap for pages that are neither fully static nor fully dynamic.