What caching layers does Next.js have?

Question

Accepted Answer

The App Router has **multiple caching layers** that work together. Understanding them explains why data sometimes looks stale and how to control freshness.

## The four caches

```text
1. Request Memoization — dedupes identical fetch() calls within ONE render pass
2. Data Cache          — persists fetch() results across requests & deploys (server)
3. Full Route Cache    — caches the rendered HTML/RSC of static routes (build time)
4. Router Cache        — client-side cache of visited routes (in the browser, per session)
```

## 1. Request Memoization

```tsx
// the SAME fetch in a layout AND a page → only ONE network request per render
const user = await fetch("/api/user"); // automatically deduped
```

## 2. Data Cache (the one you tune most)

```tsx
fetch(url, { cache: "force-cache" });       // cached (default) — like SSG
fetch(url, { cache: "no-store" });          // never cached — fresh every request
fetch(url, { next: { revalidate: 60 } });   // cached, refreshed every 60s — ISR
fetch(url, { next: { tags: ["posts"] } });  // taggable for on-demand invalidation
```

## 3 & 4. Route caches

The **Full Route Cache** stores statically rendered routes; the **Router Cache** keeps previously visited routes in the browser so back/forward and re-navigation are instant.

## Invalidating caches on mutation

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

revalidatePath("/blog");        // invalidate a specific route's cache
revalidateTag("posts");         // invalidate every fetch tagged "posts"
router.refresh();               // (client) clear the Router Cache for current route
```

After creating/updating data (often in a Server Action), you call `revalidatePath`/`revalidateTag` so the affected pages re-fetch and users see the change.

## The common gotcha

```text
"Why is my data stale after an update?" → a cache (usually Data or Router cache)
is serving the old value. Fix: revalidateTag/revalidatePath after the mutation,
or use cache: "no-store" for always-fresh data.
```

(Note: cache defaults have shifted across Next versions — check your version, as some recent versions cache less aggressively by default.)

## Why it matters

Next.js's aggressive multi-layer caching is what makes it fast, but it's also the #1 source of "why is my data stale?" confusion.

Knowing the four layers, controlling the Data Cache per-fetch (`no-store`/`revalidate`/tags), and invalidating with `revalidatePath`/`revalidateTag` after mutations is essential to getting both performance and correct freshness.