What is ISR and what problem does it solve?

ISR (Incremental Static Regeneration) is a mechanism that lets you update static pages after the build, without a full application rebuild.

In short:

SSG + automatic content updates = ISR

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#What problem ISR solves

#The problem with SSG

With classic SSG:

• pages are generated once
• to update content -> you need a new build
• on a large site (1000+ pages) this is:
  • slow
  • expensive
  • inconvenient

ISR removes the dependency on a full rebuild.

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#How ISR works

Using Next.js as an example:

1. The page is generated statically
2. Users get the HTML from a CDN
3. A set amount of time passes (revalidate)
4. On the next request:

• the user gets the old page
• in the background a new version is generated

5. The new version is cached and used going forward

No downtime. No waiting.

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#What makes ISR special

• speed on par with SSG
• data updates automatically
• the server doesn't render the page on every request
• cheaper than SSR

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#When to use ISR

• blogs
• product catalogs
• marketplaces
• courses and articles
• pages with periodic data updates

The content changes, but not every second.

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#When ISR is NOT needed

• personalized data
• content depends on cookies / auth
• realtime (chats, bidding)

There, SSR or CSR is better.

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#Example (conceptual)

• 60 seconds is the acceptable "staleness" of the data
• after that, Next.js will refresh the page automatically

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#ISR vs SSG vs SSR (very brief)

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#Interview answer

ISR is a Next.js mechanism that lets you update static pages after the build without a full application rebuild, keeping SSG's speed and data freshness.