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**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.**

---

## 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**.

---

## When ISR is NOT needed

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

There, **SSR** or **CSR** is better.

---

## Example (conceptual)

```ts
fetch('https://api.example.com/posts', {
  next: { revalidate: 60 } // revalidate once a minute
})
```

- 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)

|  | SSG | ISR | SSR |
|---|---|---|---|
| HTML ready in advance | Yes | Yes | No |
| Update without a build | No | Yes | Yes |
| Speed | Very high | Very high | High |
| Server load | None | Low | High |

---

## 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.**

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