Next.js Data Fetching

Overview

Provides patterns for data fetching in Next.js App Router: server-side fetching, SWR/React Query integration, ISR, revalidation, error boundaries, and loading states.

When to Use

• Implementing data fetching in Next.js App Router
• Choosing between Server Components and Client Components
• Setting up SWR or React Query for client-side caching
• Configuring ISR, time-based, or on-demand revalidation
• Handling loading and error states
• Building forms with Server Actions

Instructions

Server Component Fetching

Fetch directly in async Server Components:

async function getPosts() {
  const res = await fetch('https://api.example.com/posts');
  if (!res.ok) throw new Error('Failed to fetch posts');
  return res.json();
}

export default async function PostsPage() {
  const posts = await getPosts();
  return (
    <ul>
      {posts.map((post) => (
        <li key={post.id}>{post.title}</li>
      ))}
    </ul>
  );
}

Parallel Data Fetching

Use Promise.all() for independent requests:

async function getDashboardData() {
  const [user, posts, analytics] = await Promise.all([
    fetch('/api/user').then(r => r.json()),
    fetch('/api/posts').then(r => r.json()),
    fetch('/api/analytics').then(r => r.json()),
  ]);
  return { user, posts, analytics };
}

export default async function DashboardPage() {
  const { user, posts, analytics } = await getDashboardData();
  // Render dashboard
}

Sequential Data Fetching (When Dependencies Exist)

async function getUserPosts(userId: string) {
  const user = await fetch(`/api/users/${userId}`).then(r => r.json());
  const posts = await fetch(`/api/users/${userId}/posts`).then(r => r.json());
  return { user, posts };
}

Time-based Revalidation (ISR)

async function getPosts() {
  const res = await fetch('https://api.example.com/posts', {
    next: { revalidate: 60 } // Revalidate every 60 seconds
  });
  return res.json();
}

On-Demand Revalidation

// app/api/revalidate/route.ts
import { revalidateTag } from 'next/cache';
import { NextRequest } from 'next/server';

export async function POST(request: NextRequest) {
  const tag = request.nextUrl.searchParams.get('tag');
  if (tag) {
    revalidateTag(tag);
    return Response.json({ revalidated: true });
  }
  return Response.json({ revalidated: false }, { status: 400 });
}

Tag data for selective revalidation:

async function getPosts() {
  const res = await fetch('https://api.example.com/posts', {
    next: { tags: ['posts'], revalidate: 3600 }
  });
  return res.json();
}

Opt-out of Caching

async function getRealTimeData() {
  const res = await fetch('https://api.example.com/data', {
    cache: 'no-store'
  });
  return res.json();
}

// Or:
export const dynamic = 'force-dynamic';

Client-Side Data Fetching

SWR Integration

Install: npm install swr

'use client';

import useSWR from 'swr';

const fetcher = (url: string) => fetch(url).then(r => r.json());

export function Posts() {
  const { data, error, isLoading } = useSWR('/api/posts', fetcher, {
    refreshInterval: 5000,
    revalidateOnFocus: true,
  });

  if (isLoading) return <div>Loading...</div>;
  if (error) return <div>Failed to load posts</div>;

  return (
    <ul>
      {data.map((post: any) => (
        <li key={post.id}>{post.title}</li>
      ))}
    </ul>
  );
}

React Query Integration

Install: npm install @tanstack/react-query

// app/providers.tsx
'use client';

import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
import { useState } from 'react';

export function Providers({ children }: { children: React.ReactNode }) {
  const [queryClient] = useState(() => new QueryClient({
    defaultOptions: {
      queries: {
        staleTime: 60 * 1000,
        refetchOnWindowFocus: false,
      },
    },
  }));

  return (
    <QueryClientProvider client={queryClient}>
      {children}
    </QueryClientProvider>
  );
}

See react-query.md for mutations, optimistic updates, infinite queries, and advanced patterns.

Error Boundaries

Wrap client-side data fetching in Error Boundaries to handle failures gracefully:

See error-boundaries.md for full ErrorBoundary implementations (basic, with reset callback) and usage examples with data fetching.

Server Actions

Use Server Actions for mutations with cache revalidation:

See server-actions.md for complete examples including form validation with useActionState, error handling, and cache invalidation.

Loading States

loading.tsx Pattern

// app/posts/loading.tsx
export default function PostsLoading() {
  return (
    <div className="space-y-4">
      {[...Array(5)].map((_, i) => (
        <div key={i} className="h-16 bg-gray-200 animate-pulse rounded" />
      ))}
    </div>
  );
}

Suspense Boundaries

// app/posts/page.tsx
import { Suspense } from 'react';
import { PostsList } from './PostsList';
import { PostsSkeleton } from './PostsSkeleton';

export default function PostsPage() {
  return (
    <div>
      <h1>Posts</h1>
      <Suspense fallback={<PostsSkeleton />}>
        <PostsList />
      </Suspense>
    </div>
  );
}

Best Practices

1. Default to Server Components — Fetch in Server Components for better performance
2. Use parallel fetching — Promise.all() for independent requests to reduce latency
3. Choose appropriate caching:
   • Static data: long revalidation intervals
   • Dynamic data: short revalidation or cache: 'no-store'
   • User-specific data: use dynamic rendering
4. Handle errors gracefully — Wrap client data fetching in error boundaries
5. Implement loading states — Use loading.tsx or Suspense boundaries
6. Prefer SWR/React Query for: real-time data, user interactions, background updates
7. Use Server Actions for: form submissions, mutations requiring cache revalidation

Constraints and Warnings

Critical Constraints

• Server Components cannot use hooks (useState, useEffect) or client data fetching libraries
• Client Components must include the 'use client' directive
• The fetch API in Next.js extends standard Web fetch with Next.js-specific caching options
• Server Actions require 'use server' and can only be called from Client Components or form actions

Common Pitfalls

1. Fetching in loops — Avoid sequential fetches in Server Components; use parallel fetching
2. Cache poisoning — Do not use force-cache for user-specific or personalized data
3. Memory leaks — Clean up subscriptions in Client Components when using real-time data
4. Hydration mismatches — Ensure server and client render the same initial state with React Query hydration

Examples

Example 1: Blog with ISR

Input: Create a blog page that fetches posts and updates every hour.

// app/blog/page.tsx
async function getPosts() {
  const res = await fetch('https://api.example.com/posts', {
    next: { revalidate: 3600 }
  });
  return res.json();
}

export default async function BlogPage() {
  const posts = await getPosts();
  return (
    <main>
      <h1>Blog Posts</h1>
      {posts.map(post => (
        <article key={post.id}>
          <h2>{post.title}</h2>
          <p>{post.excerpt}</p>
        </article>
      ))}
    </main>
  );
}

Output: Page statically generated at build time, revalidated every hour.

Example 2: Dashboard with Parallel Fetching

Input: Build a dashboard showing user profile, stats, and recent activity in parallel.

// app/dashboard/page.tsx
async function getDashboardData() {
  const [user, stats, activity] = await Promise.all([
    fetch('/api/user').then(r => r.json()),
    fetch('/api/stats').then(r => r.json()),
    fetch('/api/activity').then(r => r.json()),
  ]);
  return { user, stats, activity };
}

export default async function DashboardPage() {
  const { user, stats, activity } = await getDashboardData();
  return (
    <div className="dashboard">
      <UserProfile user={user} />
      <StatsCards stats={stats} />
      <ActivityFeed activity={activity} />
    </div>
  );
}

Output: All three requests execute concurrently, minimizing total load time.