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**ISR (Incremental Static Regeneration)** in **Next.js** is a hybrid of **SSG + background update**, where a page stays **static and fast**, but is **periodically rebuilt** without a full rebuild of the site.

## How ISR works

1. On `next build` the page is **generated once** (like SSG)
2. Users get **static HTML**
3. `revalidate` seconds pass
4. **Next request**:
  - the old HTML is served right away
  - the new one is rebuilt **in the background**
5. Subsequent users will see the updated version

```
build → static HTML → users
                 ↓ (revalidate)
              background rebuild
```

A user **never waits** for the rebuild.

---

## How to enable ISR (App Router)

### Via `revalidate`

```ts
// app/blog/page.tsx
export const revalidate = 60; // seconds

export default async function Page() {
  const posts = await getPosts();
  return <Posts posts={posts} />;
}
```

### Via `fetch`

```ts
await fetch("https://api.example.com/posts", {
  next: { revalidate: 60 },
});
```

---

## When to use ISR

ISR is a great fit when:

- the content is **public**
- the data **gets updated**
- **SEO** is needed
- **speed** matters

Typical cases:

- a blog
- a product catalog
- roadmaps
- articles
- marketing pages with updates

---

## Pros of ISR

- almost as fast as SSG
- data updates automatically
- no load like with SSR
- no full rebuild needed
- great for a CDN

---

## Cons of ISR

- data is not always "right now"
- a stale version is possible until revalidate
- not suited for personalization
- cannot be used for auth data

---

## ISR vs SSG vs SSR

| | SSG | ISR | SSR |
| --- | --- | --- | --- |
| Speed | high | high | medium |
| Freshness | no | partial | yes |
| Server | no | partial | yes |
| SEO | yes | yes | yes |

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