Next.js App Router Patterns

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**Server vs Client Components:**
- Default to Server Components; add `"use client"` only when you need hooks, event handlers, or browser APIs
- Push `"use client"` boundaries as far down the tree as possible to maximize server rendering
- Never import server-only modules (db, fs, secrets) inside Client Components — use server actions or API routes
- Use `server-only` package to enforce the boundary at build time

**Data Fetching Patterns:**
- Fetch data in Server Components directly — no useEffect, no client-side fetch for initial load
- Use `fetch()` with Next.js cache options: `{ cache: "force-cache" }` (static), `{ next: { revalidate: 60 } }` (ISR), `{ cache: "no-store" }` (dynamic)
- Parallel fetch with `Promise.all()` to avoid sequential waterfalls in layouts and pages
- Use `React.cache()` to deduplicate identical requests within a render pass

**Streaming & Suspense:**
- Wrap slow data-fetching components in `<Suspense fallback={<Skeleton />}>` to stream progressively
- Use `loading.tsx` for route-level skeletons that activate instantly on navigation
- Prefer fine-grained Suspense boundaries over a single page-level boundary
- Use `<Suspense>` with `use(promise)` in Client Components for deferred server data

**Parallel & Intercepting Routes:**
- Use `@slot` parallel routes for independent, simultaneously rendered UI sections (modals, sidebars)
- Intercepting routes (`(.)`, `(..)`, `(...)`) enable modal patterns without losing URL shareability
- Combine parallel + intercepting routes for photo gallery or settings modal UX patterns

**Caching Strategy:**
- Understand the four caches: Request Memoization, Data Cache, Full Route Cache, Router Cache
- Use `revalidatePath()` or `revalidateTag()` in Server Actions to invalidate cache after mutations
- Tag fetch calls with `{ next: { tags: ["posts"] } }` for targeted invalidation
- Set `export const dynamic = "force-dynamic"` on routes that must never be cached

**Middleware:**
- Use middleware (`middleware.ts`) for auth redirects, locale detection, A/B testing, and header injection
- Keep middleware fast — avoid database calls; use lightweight JWT or cookie checks only
- Use `matcher` config to restrict middleware to relevant paths and avoid unnecessary invocations

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