Tanstack Integration Best Practices

# cache-single-source: Let TanStack Query Manage Caching

## Priority: MEDIUM

## Explanation

When using TanStack Router with TanStack Query, let Query be the single source of truth for caching. Disable Router's built-in cache with `defaultPreloadStaleTime: 0` to avoid confusion about which cache is authoritative.

## Bad Example

```tsx
// Both Router and Query caching enabled - confusing
const router = createRouter({
  routeTree,
  context: { queryClient },
  // Default router caching enabled
  // defaultPreloadStaleTime: 30000 (default)
})

export const Route = createFileRoute('/posts')({
  loader: async () => {
    // Fetches directly - cached by Router
    const posts = await fetchPosts()
    return { posts }
  },
  component: PostsPage,
})

function PostsPage() {
  // Also uses Query cache - which is authoritative?
  const { data } = useQuery({
    queryKey: ['posts'],
    queryFn: fetchPosts,
  })

  // Now there are TWO caches with potentially different data
}
```

## Good Example

```tsx
// router.tsx - Disable router cache when using Query
import { QueryClient } from '@tanstack/react-query'
import { createRouter } from '@tanstack/react-router'
import { setupRouterSsrQueryIntegration } from '@tanstack/react-router-ssr-query'
import { routeTree } from './routeTree.gen'

export function getRouter() {
  const queryClient = new QueryClient({
    defaultOptions: {
      queries: {
        staleTime: 1000 * 60 * 2, // 2 minutes
        refetchOnWindowFocus: false,
      },
    },
  })

  const router = createRouter({
    routeTree,
    context: { queryClient },
    defaultPreload: 'intent',
    defaultPreloadStaleTime: 0, // Let Query manage caching
    scrollRestoration: true,
  })

  setupRouterSsrQueryIntegration({
    router,
    queryClient,
  })

  return router
}

// routes/posts.tsx
export const Route = createFileRoute('/posts')({
  loader: async ({ context: { queryClient } }) => {
    // Query is the single cache source
    await queryClient.ensureQueryData(postQueries.all())
    // No return needed - data lives in Query cache
  },
  component: PostsPage,
})

function PostsPage() {
  // Single source of truth
  const { data: posts } = useSuspenseQuery(postQueries.all())

  return <PostList posts={posts} />
}
```

## Cache Comparison

| Feature | Router Cache | Query Cache |
|---------|-------------|-------------|
| Invalidation | Manual/time-based | Query keys, patterns |
| Background refetch | No | Yes |
| Optimistic updates | No | Yes |
| Mutations | No built-in | Full support |
| DevTools | Limited | Rich debugging |
| Cross-route sharing | Full | Full |

## Good Example: Coordinated Caching Config

```tsx
// router.tsx
export function getRouter() {
  const queryClient = new QueryClient({
    defaultOptions: {
      queries: {
        staleTime: 60 * 1000,       // Fresh for 1 minute
        gcTime: 10 * 60 * 1000,     // Cache for 10 minutes
        refetchOnWindowFocus: true, // Refetch when tab focused
        retry: 1,
      },
    },
  })

  const router = createRouter({
    routeTree,
    context: { queryClient },
    defaultPreload: 'intent',
    defaultPreloadStaleTime: 0, // Router defers to Query
    scrollRestoration: true,
    defaultStructuralSharing: true,
  })

  setupRouterSsrQueryIntegration({
    router,
    queryClient,
  })

  return router
}
```

## Good Example: Preload Still Works

```tsx
// Preloading still works - it just uses Query's cache
export function getRouter() {
  const queryClient = new QueryClient()

  const router = createRouter({
    routeTree,
    context: { queryClient },
    defaultPreload: 'intent',     // Preload on hover
    defaultPreloadStaleTime: 0,   // Query decides if data is stale
  })

  setupRouterSsrQueryIntegration({ router, queryClient })

  return router
}

// When user hovers a Link:
// 1. Router triggers preload
// 2. Loader runs ensureQueryData
// 3. Query checks its cache - fresh? skip fetch. stale? refetch.
// 4. User clicks - data already in Query ca