How does on-demand revalidation (ISR) work?

Question

Accepted Answer

**On-demand revalidation** lets you invalidate cached/static pages **exactly when data changes** — rather than waiting for a time-based interval. It's the precise complement to time-based ISR (`revalidate: N`).

## Time-based vs on-demand

```tsx
// Time-based: page refreshes on a fixed schedule (may serve stale data up to N seconds)
export const revalidate = 3600; // every hour

// On-demand: refresh the INSTANT the underlying data changes — no waiting, no waste
```

Time-based ISR is simple but imperfect: too short = unnecessary regeneration; too long = stale content. On-demand fixes this by triggering revalidation from your own code when a change actually happens.

## The two on-demand APIs

```tsx
import { revalidatePath, revalidateTag } from "next/cache";

// 1. Tag your fetches, then invalidate by tag
await fetch("https://api/posts", { next: { tags: ["posts"] } });
revalidateTag("posts");      // refresh EVERY page/fetch tagged "posts"

// 2. Or invalidate a specific path
revalidatePath("/blog/my-post");
```

## A real workflow: CMS webhook

```ts
// app/api/revalidate/route.ts — called by your CMS when an editor publishes
import { revalidateTag } from "next/cache";
import { NextRequest, NextResponse } from "next/server";

export async function POST(req: NextRequest) {
  const secret = req.nextUrl.searchParams.get("secret");
  if (secret !== process.env.REVALIDATE_SECRET) {       // verify it's really your CMS
    return NextResponse.json({ ok: false }, { status: 401 });
  }
  revalidateTag("posts");                                // rebuild affected pages now
  return NextResponse.json({ revalidated: true });
}
```

The flow: editor publishes in the CMS → CMS calls your webhook → `revalidateTag("posts")` invalidates the cache → the next visitor gets freshly regenerated static HTML. Pages stay static-fast **and** update immediately on publish.

## After a Server Action mutation

```tsx
"use server";
export async function createPost(data) {
  await db.post.create({ data });
  revalidateTag("posts"); // user sees their new post immediately
}
```

## Why it matters

On-demand revalidation gives you the holy grail: **static performance with real-time freshness**.

Instead of guessing a revalidation interval, you invalidate precisely when content changes — via webhooks (CMS publish) or after Server Action mutations — using `revalidateTag`/`revalidatePath`.

It eliminates both stale content and wasteful over-regeneration, making ISR practical for content that updates unpredictably.