cache-components

name	cache-components
description	Expert guidance for Next.js Cache Components and Partial Prerendering (PPR). Use when implementing 'use cache' directive, configuring cache lifetimes with cacheLife(), tagging cached data with cacheTag(), invalidating caches with updateTag()/revalidateTag(), optimizing static vs dynamic content boundaries, managing 'use cache: private' for compliance scenarios, pass-through/interleaving patterns, GET Route Handler caching, debugging cache issues, and reviewing Cache Component implementations.
argument-hint	[pattern or question]
metadata	{"version":"1.0"}

Next.js Cache Components

Auto-activation: Activate this skill automatically in Next.js projects that have cacheComponents: true in next.config.ts/next.config.js. When detected, apply Cache Components patterns to all Server Component authoring, data fetching, and caching decisions.

Project Detection

When starting work in a Next.js project, check if Cache Components are enabled:

# Check next.config.ts or next.config.js for cacheComponents
grep -r "cacheComponents" next.config.* 2>/dev/null

If cacheComponents: true is found, apply this skill's patterns proactively when:

• Writing React Server Components
• Implementing data fetching
• Creating Server Actions with mutations
• Optimizing page performance
• Reviewing existing component code

Cache Components enable Partial Prerendering (PPR) - mixing static HTML shells with dynamic streaming content for optimal performance. Cache Components also enable state preservation during navigation with React's <Activity> component, which can keep cached component trees mounted but hidden.

Philosophy: Code Over Configuration

Cache Components represents a shift from segment configuration to compositional code:

Before (Deprecated)	After (Cache Components)
export const revalidate = 3600	cacheLife('hours') inside 'use cache'
export const dynamic = 'force-static'	Use 'use cache' and Suspense boundaries
All-or-nothing static/dynamic	Granular: static shell + cached + dynamic

Key Principle: Components co-locate their caching, not just their data. Next.js provides build-time feedback to guide you toward optimal patterns.

Core Concept

┌─────────────────────────────────────────────────────┐
│                   Static Shell                       │
│  (Sent immediately to browser)                       │
│                                                      │
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐  │
│  │   Header    │  │  Cached     │  │  Suspense   │  │
│  │  (static)   │  │  Content    │  │  Fallback   │  │
│  └─────────────┘  └─────────────┘  └──────┬──────┘  │
│                                           │         │
│                                    ┌──────▼──────┐  │
│                                    │  Dynamic    │  │
│                                    │  (streams)  │  │
│                                    └─────────────┘  │
└─────────────────────────────────────────────────────┘

Mental Model: The Caching Decision Steps

When writing a React Server Component, walk through these steps in order:

1. Does the component fetch data or perform I/O?

   • No → pure component, nothing to decide.
   • Yes → continue.

2. Does it depend on request context (cookies(), headers(), searchParams)?

   • No → continue to step 3.
   • Yes → continue to step 4.

3. (No request context) Is the data the same across users?

   • Yes → add 'use cache' with cacheTag() and cacheLife().
   • No → wrap rendering in <Suspense> so the dynamic part streams at request time.

4. (Has request context) Can you extract the runtime data as function arguments?

   • Yes → read cookies()/headers() outside the cached scope, pass values into a 'use cache' function, and wrap the dynamic caller in <Suspense>.
   • No (compliance prevents cross-request sharing) → use 'use cache: private' (experimental — not recommended for production) as a last resort, still wrapped in <Suspense>.

Key insight: 'use cache' is for data that is the same across users. User-specific data stays dynamic and streams through <Suspense>. Reach for 'use cache: private' only when you cannot refactor runtime data into arguments.

Quick Start

Enable Cache Components

// next.config.ts
import type { NextConfig } from 'next'

const nextConfig: NextConfig = {
  cacheComponents: true,
}

export default nextConfig

Basic Usage

// Cached component - output included in static shell
async function CachedPosts() {
  'use cache'
  const posts = await db.posts.findMany()
  return <PostList posts={posts} />
}

// Page with static + cached + dynamic content
export default async function BlogPage() {
  return (
    <>
      <Header /> {/* Static */}
      <CachedPosts /> {/* Cached */}
      <Suspense fallback={<Skeleton />}>
        <DynamicComments /> {/* Dynamic - streams */}
      </Suspense>
    </>
  )
}

Server Actions vs Data Fetching (Critical Rule)

Server Actions are for MUTATIONS ONLY - never for data fetching:

Purpose	Use	Example Functions
Data fetch	Server Component / 'use cache'	getProducts(), getUser(id)
Mutation	Server Action ('use server')	createProduct(), updateUser(), deletePost()

❌ WRONG: Server Action for Data Fetching

"use server"
export async function getProducts() {
  return await db.products.findMany() // NO! This is not a mutation
}

"use server"
export async function getTheme() {
  return (await cookies()).get("theme")?.value // NO! Just reading data
}

✅ CORRECT: Data Function + Server Component

// data/products.ts - Cached data function
export async function getProducts() {
  "use cache"
  cacheTag("products")
  cacheLife("hours")
  return await db.products.findMany()
}

// page.tsx - Server Component reads data directly
import { cookies } from "next/headers"

export default async function Page() {
  const products = await getProducts()
  const theme = (await cookies()).get("theme")?.value ?? "light"
  return <ProductList products={products} theme={theme} />
}

✅ CORRECT: Server Action for Mutation

"use server"
import { updateTag } from "next/cache"

export async function createProduct(formData: FormData) {
  await db.products.create({ data: formData })
  updateTag("products") // Invalidate cache after mutation
}

Core APIs

1. 'use cache' Directive

Marks code as cacheable. Can be applied at three levels:

// File-level: All exports are cached
'use cache'
export async function getData() {
  /* ... */
}
export async function Component() {
  /* ... */
}

// Component-level
async function UserCard({ id }: { id: string }) {
  'use cache'
  const user = await fetchUser(id)
  return <Card>{user.name}</Card>
}

// Function-level
async function fetchWithCache(url: string) {
  'use cache'
  return fetch(url).then((r) => r.json())
}

Important: All cached functions must be async.

2. cacheLife() - Control Cache Duration

import { cacheLife } from 'next/cache'

async function Posts() {
  'use cache'
  cacheLife('hours') // Use a predefined profile

  // Or custom configuration:
  cacheLife({
    stale: 60, // 1 min - client cache validity
    revalidate: 3600, // 1 hr - start background refresh
    expire: 86400, // 1 day - absolute expiration
  })

  return await db.posts.findMany()
}

Predefined profiles: 'default', 'seconds', 'minutes', 'hours', 'days', 'weeks', 'max'

3. cacheTag() - Tag for Invalidation

import { cacheTag } from 'next/cache'

async function BlogPosts() {
  'use cache'
  cacheTag('posts')
  cacheLife('days')

  return await db.posts.findMany()
}

async function UserProfile({ userId }: { userId: string }) {
  'use cache'
  cacheTag('users', `user-${userId}`) // Multiple tags

  return await db.users.findUnique({ where: { id: userId } })
}

4. updateTag() - Immediate Invalidation

For read-your-own-writes semantics:

'use server'
import { updateTag } from 'next/cache'

export async function createPost(formData: FormData) {
  await db.posts.create({ data: formData })

  updateTag('posts') // Client immediately sees fresh data
}

5. revalidateTag() - Background Revalidation

For stale-while-revalidate pattern:

'use server'
import { revalidateTag } from 'next/cache'

export async function updatePost(id: string, data: FormData) {
  await db.posts.update({ where: { id }, data })

  revalidateTag('posts', 'max') // Serve stale, refresh in background
}

⚠️ Deprecated: The single-argument form revalidateTag('posts') is deprecated. Always pass a profile ('max' is recommended for stale-while-revalidate) or { expire: <seconds> } as the second argument. For webhooks that require immediate expiration, use revalidateTag(tag, { expire: 0 }). For immediate read-your-own-writes in Server Actions, prefer updateTag() instead.

When to Use Each Pattern

Content Type	API	Behavior
Static	No directive	Rendered at build time
Cached	'use cache'	Included in static shell, revalidates
Dynamic	Inside <Suspense>	Streams at request time

Parameter Permutations & Subshells

Critical Concept: With Cache Components, Next.js renders ALL permutations of provided parameters to create reusable subshells.

// app/products/[category]/[slug]/page.tsx
export async function generateStaticParams() {
  return [
    { category: 'jackets', slug: 'classic-bomber' },
    { category: 'jackets', slug: 'essential-windbreaker' },
    { category: 'accessories', slug: 'thermal-fleece-gloves' },
  ]
}

Next.js renders these routes:

/products/jackets/classic-bomber        ← Full params (complete page)
/products/jackets/essential-windbreaker ← Full params (complete page)
/products/accessories/thermal-fleece-gloves ← Full params (complete page)
/products/jackets/[slug]                ← Partial params (category subshell)
/products/accessories/[slug]            ← Partial params (category subshell)
/products/[category]/[slug]             ← No params (fallback shell)

Why this matters: The category subshell (/products/jackets/[slug]) can be reused for ANY jacket product, even ones not in generateStaticParams. Users navigating to an unlisted jacket get the cached category shell immediately, with product details streaming in.

generateStaticParams Requirements

With Cache Components enabled:

1. Must provide at least one parameter - Empty arrays now cause build errors (prevents silent production failures)
2. Params prove static safety - Providing params lets Next.js verify no dynamic APIs are called
3. Partial params create subshells - Each unique permutation generates a reusable shell

// ❌ ERROR with Cache Components
export function generateStaticParams() {
  return [] // Build error: must provide at least one param
}

// ✅ CORRECT: Provide real params
export async function generateStaticParams() {
  const products = await getPopularProducts()
  return products.map(({ category, slug }) => ({ category, slug }))
}

Cache Key = Arguments

Arguments become part of the cache key:

// Different userId = different cache entry
async function UserData({ userId }: { userId: string }) {
  'use cache'
  cacheTag(`user-${userId}`)

  return await fetchUser(userId)
}

Build-Time Feedback

Cache Components provides early feedback during development. These build errors guide you toward optimal patterns:

Error: Dynamic data outside Suspense

Error: Accessing cookies/headers/searchParams outside a Suspense boundary

Solution: Wrap dynamic components in <Suspense>:

<Suspense fallback={<Skeleton />}>
  <ComponentThatUsesCookies />
</Suspense>

Error: Uncached data outside Suspense

Error: Accessing uncached data outside Suspense

Solution: Either cache the data or wrap in Suspense:

// Option 1: Cache it
async function ProductData({ id }: { id: string }) {
  'use cache'
  return await db.products.findUnique({ where: { id } })
}

// Option 2: Make it dynamic with Suspense
;<Suspense fallback={<Loading />}>
  <DynamicProductData id={id} />
</Suspense>

Error: Request data inside cache

Error: Cannot access cookies/headers inside 'use cache'

Solution: Extract runtime data outside cache boundary (see "Handling Runtime Data" above).

Additional Resources

• For complete API reference, see REFERENCE.md
• For common patterns and recipes, see PATTERNS.md
• For debugging and troubleshooting, see TROUBLESHOOTING.md

Code Generation Guidelines

When generating Cache Component code:

1. Always use async - All cached functions must be async
2. Place 'use cache' first - Must be first statement in function body
3. Call cacheLife() early - Should follow 'use cache' directive
4. Tag meaningfully - Use semantic tags that match your invalidation needs
5. Extract runtime data - Move cookies()/headers() outside cached scope
6. Wrap dynamic content - Use <Suspense> for non-cached async components
7. Use 'use cache: private' as last resort - Experimental (not recommended for production). Only when runtime data cannot be extracted as params AND compliance requires no cross-request sharing

Review Checklist

When reviewing code in Cache Components projects, flag these issues:

• Data fetching without 'use cache' where caching would benefit
• Missing cacheTag() calls (makes invalidation impossible)
• Missing cacheLife() (relies on defaults which may not be appropriate)
• Server Actions without updateTag()/revalidateTag() after mutations
• cookies()/headers() called inside 'use cache' scope
• Dynamic components without <Suspense> boundaries
• DEPRECATED: export const revalidate - replace with cacheLife() in 'use cache'
• DEPRECATED: export const dynamic - replace with Suspense + cache boundaries
• Empty generateStaticParams() return - must provide at least one param
• Single-argument revalidateTag('tag') - use two-argument form with profile or { expire }