## <a id="private-cache"></a>8. Private Cache Mechanics
### Pattern from Tests: Private Cache Structure
**Test Source**: `test/e2e/app-dir/use-cache-private/app/cookies/page.tsx`
```typescript
// THE EXACT PATTERN FROM TESTS:
export default function Page() {
return (
<Suspense fallback={<p>Loading...</p>}>
<Private />
</Suspense>
)
}
async function Private() {
'use cache: private'
cacheLife({ stale: 420 })
const cookie = (await cookies()).get('test-cookie')
const { headers } = await fetch('https://...', {
headers: { 'x-test-cookie': cookie?.value ?? '' }
}).then(res => res.json())
return (
<pre>
test-cookie: <span id="test-cookie">{headers['x-test-cookie'] || '<empty>'}</span>
</pre>
)
}
// TEST BEHAVIOR:
// 1. Set cookie to 'foo'
// 2. Page displays: 'foo'
// 3. Change cookie to 'bar'
// 4. Refresh page
// 5. Page displays: 'bar' (per-user cache updated!)
```
### Private Cache Rules from Tests:
1. **MUST be wrapped in Suspense** (build error if not)
2. **Can access cookies()** ✅
3. **Can access headers()** ✅
4. **Can access searchParams** ✅
5. **Can access params** ✅
6. **CANNOT use connection()** ❌ (throws error)
7. **Excluded from static prerender** (always dynamic)
8. **Included in runtime prefetch** (if stale >= 30s)
---
## <a id="private-inclusion"></a>9. When Private Cache is Included/Excluded
### Test Pattern: Stale Time Threshold
**Test Source**: `test/e2e/app-dir/segment-cache/prefetch-runtime/prefetch-runtime.test.ts` (lines 752-1030)
```typescript
// CRITICAL THRESHOLD: 30 seconds (RUNTIME_PREFETCH_DYNAMIC_STALE)
// Pattern 1: Private cache with cacheLife('seconds')
async function ShortLivedCache() {
'use cache: private'
cacheLife('seconds') // stale: 0, revalidate: 1, expire: 1
// ... BUT cacheLife('seconds') is special: stale is set to 30s!
return <div id="cached-value">{Date.now()}</div>
}
// TEST BEHAVIOR:
// Static prefetch: ❌ NOT included (expire < 5min)
// Runtime prefetch: ✅ INCLUDED (stale = 30s, meets 30s threshold)
// Pattern 2: Private cache with short stale time
async function TooShort() {
'use cache: private'
cacheLife({ stale: 20, revalidate: 100, expire: 200 })
return <div>{Date.now()}</div>
}
// TEST BEHAVIOR:
// Static prefetch: ❌ NOT included (expire < 5min)
// Runtime prefetch: ❌ NOT included (stale < 30s)
// Navigation: Fetched at request time
```
### Inclusion Matrix from Tests:
| Cache Type | Stale Time | Expire Time | Static Prefetch | Runtime Prefetch |
| ---------- | ---------- | ----------- | --------------- | ---------------- |
| Public | Any | >= 5min | ✅ Included | ✅ Included |
| Public | >= 30s | < 5min | ❌ Excluded | ✅ Included |
| Public | < 30s | < 5min | ❌ Excluded | ❌ Excluded |
| Private | >= 30s | Any | ❌ Excluded | ✅ Included |
| Private | < 30s | Any | ❌ Excluded | ❌ Excluded |
---
