How do you use CDN with Next.js?

Using a Content Delivery Network (CDN) with Next.js is a common approach to improve the performance, scalability, and global reach of your web application. CDNs help by caching your static assets closer to your users, reducing latency and offloading traffic from your origin server. However, integrating a CDN with Next.js requires understanding both how Next.js serves content and how CDNs work in practice.

In this answer, I’ll walk through the core concepts of using CDNs with Next.js, share real-world examples, discuss best practices, common pitfalls, and performance considerations. I'll also touch on security aspects and provide practical interview tips to help you explain this topic confidently.

Understanding Next.js Asset Serving and CDN Basics

Next.js is a React framework that supports server-side rendering (SSR), static site generation (SSG), and client-side rendering. It generates several types of assets:

• Static assets: Images, fonts, CSS, JavaScript bundles, and other files under the public/ folder or generated during the build.
• Server-rendered pages: HTML generated on the server at request time (SSR) or at build time (SSG).
• API routes: Backend endpoints running on the server.

A CDN caches and serves static content from edge locations worldwide, reducing the distance and time it takes for users to download your assets. When you use a CDN with Next.js, you typically want to serve your static assets (JS, CSS, images) through the CDN, and optionally cache SSR pages or API responses depending on your setup.

Why Use a CDN with Next.js?

• Faster load times: Assets are served from edge servers closer to users.
• Reduced origin server load: Offloads traffic, improving scalability.
• Improved reliability: CDNs have built-in failover and redundancy.
• Better SEO: Faster page loads improve search engine rankings.

How to Use CDN with Next.js: Core Approaches

There are several ways to integrate a CDN with Next.js, depending on your hosting environment and deployment strategy.

1. Using the next.config.js assetPrefix Setting

The simplest way to tell Next.js to serve static assets from a CDN is by setting the assetPrefix option in next.config.js. This prefix is prepended to all static asset URLs like JS bundles, CSS, and images in the public/ folder.

module.exports = {
  assetPrefix: 'https://cdn.example.com',
};

With this, Next.js will generate URLs like https://cdn.example.com/_next/static/chunks/main.js instead of serving them from your origin server.

How this works in practice: You upload your static assets (usually the /.next/static folder contents) to your CDN’s storage or origin, and the CDN serves these files globally.

2. Using a CDN in Front of Your Hosting Provider

If you deploy Next.js on platforms like Vercel, AWS Amplify, or your own server, you can put a CDN (like Cloudflare, AWS CloudFront, or Fastly) in front of your entire domain. This setup caches static assets and optionally SSR pages or API responses.

For example, with Cloudflare, you configure your DNS to point to your origin server and enable caching rules for static assets. Cloudflare will cache and serve those assets from its edge locations.

3. Using Image CDN with Next.js Image Component

Next.js has a built-in next/image component that optimizes images on-demand. It supports external image CDNs by configuring the domains array in next.config.js or by using a custom loader.

For example, if you use an image CDN like Imgix, Cloudinary, or Akamai, you can configure Next.js to load images from those domains, benefiting from image resizing, format conversion, and caching.

module.exports = {
  images: {
    domains: ['cdn.example.com'],
  },
};

Real-World Example: Deploying Next.js with AWS CloudFront

Imagine you have a Next.js app deployed on AWS Amplify or an EC2 instance. You want to serve static assets via CloudFront to improve performance globally.

1. Build your Next.js app: next build
2. Upload the /.next/static folder and public/ assets to an S3 bucket configured as the CloudFront origin.
3. Set assetPrefix in next.config.js to your CloudFront distribution URL.
4. Deploy your server or serverless functions for SSR pages and API routes.
5. Configure CloudFront behaviors to cache static assets aggressively and optionally cache SSR pages with proper cache-control headers.

This setup ensures your static files are served from CloudFront edge locations, while SSR pages are generated on your origin server.

Best Practices When Using CDN with Next.js

• Set proper cache headers: Static assets should have long cache lifetimes with cache busting via content hashes (Next.js does this by default).
• Use assetPrefix carefully: Make sure your CDN origin has the correct files and paths to avoid 404s.
• Invalidate cache selectively: When deploying updates, invalidate CDN cache for changed assets to prevent stale content.
• Optimize images: Use Next.js next/image with an image CDN or built-in optimization to reduce payload size.
• Test CDN behavior: Check cache hit/miss rates and latency to ensure your CDN is configured correctly.
• Consider SSR caching: If your app uses SSR heavily, consider caching HTML responses at the CDN layer with short TTLs or stale-while-revalidate strategies.

Common Mistakes Developers Make

• Not uploading static assets to the CDN origin: Setting assetPrefix without actually uploading assets to the CDN results in broken links.
• Ignoring cache invalidation: Deploying new versions without clearing CDN cache causes users to get stale JS/CSS files.
• Misconfiguring image domains: Forgetting to whitelist CDN domains in next.config.js leads to image loading errors.
• Over-caching SSR pages: Caching dynamic pages too aggressively at the CDN can cause stale or incorrect content.
• Not testing in production: CDN setups can behave differently in staging vs production; always test with real traffic.

Performance Considerations

CDNs dramatically reduce latency for static assets, but the overall performance depends on how you configure caching and asset delivery.

Aspect	Impact	Best Practice
Cache TTL	Long TTL reduces origin hits but risks stale content	Use content hashes for cache busting and set long TTLs
Cache Invalidation	Ensures users get latest assets after deploy	Automate invalidation on deploy or use versioned URLs
SSR Page Caching	Improves response times but can serve stale data	Use short TTLs or stale-while-revalidate headers
Image Optimization	Reduces payload size and speeds up load	Use Next.js Image component with CDN or external loader

Security Considerations

When using a CDN with Next.js, security is often overlooked but critical:

• HTTPS Everywhere: Ensure your CDN supports HTTPS and your assetPrefix URLs use https:// to avoid mixed content issues.
• Access Controls: If your CDN origin is an S3 bucket or similar, configure bucket policies to allow only the CDN to fetch assets, preventing direct access.
• Cache Poisoning: Use proper cache keys and headers to avoid malicious users poisoning your CDN cache.
• CORS: Configure Cross-Origin Resource Sharing headers correctly if your assets are served from a different domain.

Interview Tips: How to Talk About CDN with Next.js

• Explain the problem CDN solves: Start by describing why CDNs matter—latency, scalability, and reliability.
• Describe Next.js asset types: Mention static assets, SSR pages, and API routes, and how CDNs fit into each.
• Walk through assetPrefix: This is a key Next.js feature for CDN integration.
• Discuss caching strategies: Show you understand cache headers, invalidation, and trade-offs.
• Bring up image optimization: Next.js Image component is a common production feature.
• Share real-world experience: Talk about deployments you’ve done with CDNs and challenges faced.
• Mention common pitfalls: This shows practical knowledge beyond theory.

Comparison with Alternatives

Approach	Pros	Cons	Use Case
assetPrefix + CDN Storage	Simple, explicit control over static assets	Requires manual upload or automation of assets	Static sites or SSR with separate static asset hosting
CDN in front of entire domain	Transparent, caches everything including SSR	Complex cache invalidation, risk of stale dynamic content	Apps with mostly static content or cacheable SSR
Next.js Image + External Image CDN	Automatic image optimization and CDN benefits	Limited to images, requires configuration	Image-heavy apps needing resizing and format conversion

Summary

Using a CDN with Next.js is a practical way to boost your app’s performance and scalability. The key is understanding how Next.js serves static assets and pages, and configuring your CDN accordingly. The assetPrefix option is your friend for static assets, while putting a CDN in front of your entire domain can cache SSR pages but requires careful cache management.

Always remember to handle cache invalidation properly, optimize images, and secure your CDN setup. Sharing your hands-on experience with these concepts during interviews will demonstrate both your technical depth and your ability to deliver performant production apps.