What is ISR?

ISR (Incremental Static Regeneration) is a mechanism in Next.js that lets you update static pages on the server after the build, without rebuilding the entire project.

In other words: the page looks static, loads very fast, but its data can update over time.

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#Why ISR is needed at all

There are two extremes:

• Static generation -> very fast, but the data is "frozen"
• Server rendering on every request -> always fresh data, but slower and more expensive

ISR is the golden middle between them.

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#How ISR works step by step

1. The project is built
2. The page is generated as static
3. Users get the ready HTML (very fast)
4. Next.js tracks whether the page has gone stale
5. If the refresh time has passed:

• the server regenerates the page in the background
• the cache is updated

6. The next users get the updated version

Important: the user does not wait for a rebuild, they always get the page immediately

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#What "incremental" means

• not the whole site updates
• only specific pages
• and only when it's needed

For example:

• one article got updated
• only its page was rebuilt
• the rest of the site is untouched

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#A real-life example

Say there is:

• a product catalog
• prices change every few minutes

With ISR:

• the page loads as static
• the server refreshes it, say, once every 60 seconds
• users always see nearly up-to-date data
• without constant server requests

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#Why ISR is useful

ISR solves several problems at once:

• the speed of static pages
• refreshing data without a full build
• less load on the server
• good SEO
• scalability for large sites

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#Where ISR is used

In the Pages Router:

• through getStaticProps with a refresh setting

In the App Router (the modern approach in Next.js):

• through fetch with revalidation
• or through manually refreshing the cache on an event

The essence is the same:

the page is static, but not forever

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#Limitations of ISR

ISR does not fit when:

• the data is strictly personal
• information must be absolutely up to date every second
• the page depends on the current user

In such cases you need server rendering on every request.

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#In short

ISR is:

• static generation

• automatic updates

• without a full site rebuild

That's exactly why ISR is often used for:

• blogs
• catalogs
• marketing pages
• content platforms