Domain routing for locales in NextJS - Performance & Optimization

Performance: Domain routing for locales

MEDIUM IMPACT

This affects the initial page load speed by determining which locale content is served based on the domain, impacting server response and client rendering.

Serving localized content based on user domain

NextJS

export default { i18n: { locales: ['en', 'fr'], defaultLocale: 'en', domains: [ { domain: 'example.com', defaultLocale: 'en' }, { domain: 'example.fr', defaultLocale: 'fr' } ] } } // Domain routing for locales

Locale is determined server-side by domain, serving correct content immediately without redirects.

📈 Performance GainReduces redirects and layout shifts, improving LCP and CLS

Serving localized content based on user domain

NextJS

export default { i18n: { locales: ['en', 'fr'], defaultLocale: 'en', localeDetection: true } } // No domain routing, locale chosen by browser or path

Locale is detected client-side or via path, causing extra redirects or content shifts after initial load.

📉 Performance CostTriggers additional client-side redirects and layout shifts, increasing LCP and CLS

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Client-side locale detection with redirects	No extra DOM nodes	Multiple reflows due to content shifts	Higher paint cost from layout shifts	[X] Bad
Server-side domain routing for locales	No extra DOM nodes	Single reflow with stable layout	Lower paint cost with stable content	[OK] Good

Rendering Pipeline

Domain routing for locales affects the server response phase by selecting the correct locale content before sending HTML to the browser, reducing client-side locale detection work.

→Server Response

→HTML Delivery

→Layout

→Paint

⚠️ BottleneckServer Response time can increase slightly due to domain checks but reduces client-side reflows and repaints.

Core Web Vital Affected

LCP

This affects the initial page load speed by determining which locale content is served based on the domain, impacting server response and client rendering.

Optimization Tips

1Use domain routing to serve locale-specific content server-side for faster initial paint.

2Avoid client-side locale detection that causes redirects and layout shifts.

3Cache DNS and use edge caching to minimize domain routing overhead.

Performance Quiz - 3 Questions

How does domain routing for locales impact Largest Contentful Paint (LCP)?

A It has no impact on LCP.

B It worsens LCP by adding extra client-side locale detection.

C It improves LCP by serving locale-specific content immediately without redirects.

D It delays LCP by increasing JavaScript bundle size.

DevTools: Performance

How to check: Record page load and look for redirects and layout shifts in the waterfall and experience timeline.

What to look for: Fewer redirects and lower layout shift scores indicate better domain routing performance.

Practice

1. What is the main purpose of domain routing for locales in Next.js?

easy

A. To enable client-side routing only

B. To improve server performance by caching

C. To create multiple apps in one project

D. To serve different languages on different domains

Solution

1. Step 1: Understand domain routing concept

   Domain routing allows serving different languages or locales on separate domains or subdomains.

2. Step 2: Identify the main use case in Next.js

   Next.js uses domain routing to link domains to specific locales for internationalization.

3. Final Answer:

   To serve different languages on different domains -> Option D

4. Quick Check:

   Domain routing = serve languages on domains [OK]

Hint: Domain routing connects domains to languages [OK]

Common Mistakes:

• Confusing domain routing with caching
• Thinking it only affects client-side routing
• Assuming it creates multiple apps

2. Which Next.js config property is used to define domain routing for locales?

easy

A. routing.locales

B. i18n.domains

C. locales.domains

D. next.config.domains

Solution

1. Step 1: Recall Next.js i18n config structure

   The internationalization config uses the i18n key in next.config.js.

2. Step 2: Identify the property for domain routing

   The correct property to define domain-locale mapping is i18n.domains.

3. Final Answer:

   i18n.domains -> Option B

4. Quick Check:

   Domain routing config = i18n.domains [OK]

Hint: Look inside i18n config for domains key [OK]

Common Mistakes:

• Using next.config.domains instead of i18n.domains
• Confusing with routing.locales
• Trying to set domains outside i18n

3. Given this Next.js i18n config snippet, what locale will be served for requests to https://example.fr?

i18n: {
  locales: ['en', 'fr'],
  defaultLocale: 'en',
  domains: [
    { domain: 'example.com', defaultLocale: 'en' },
    { domain: 'example.fr', defaultLocale: 'fr' }
  ]
}

medium

A. fr

B. en

C. defaultLocale from browser

D. No locale, error occurs

Solution

1. Step 1: Check domain-locale mapping in config

   The domain example.fr is mapped to fr locale explicitly.

2. Step 2: Determine locale served for example.fr

   Requests to example.fr will serve the French locale fr.

3. Final Answer:

   fr -> Option A

4. Quick Check:

   example.fr domain = fr locale [OK]

Hint: Match domain to locale in i18n.domains array [OK]

Common Mistakes:

• Assuming defaultLocale 'en' applies to all domains
• Thinking browser locale overrides domain
• Believing error occurs without explicit locale

4. You wrote this middleware to detect locale by domain but it always serves the default locale. What is the likely mistake?

export function middleware(request) {
  const hostname = request.headers.get('host')
  if (hostname === 'example.fr') {
    return NextResponse.rewrite(new URL('/fr', request.url))
  }
  return NextResponse.next()
}

medium

A. Rewriting to '/fr' instead of setting locale cookie

B. Not returning a response for all domains

C. Using 'host' header instead of 'x-forwarded-host'

D. Middleware missing 'export default'

Solution

1. Step 1: Understand middleware role in locale detection

   Middleware should set locale or rewrite URL to include locale prefix properly.

2. Step 2: Identify issue with rewriting only

   Rewriting to '/fr' alone may not set locale context; locale cookie or header is needed for Next.js to recognize locale.

3. Final Answer:

   Rewriting to '/fr' instead of setting locale cookie -> Option A

4. Quick Check:

   Middleware must set locale, not just rewrite URL [OK]

Hint: Middleware must set locale, not just rewrite URL [OK]

Common Mistakes:

• Assuming host header is wrong
• Forgetting to handle all domains
• Missing export default in middleware

5. How would you configure Next.js to serve English on 'example.com' and French on 'example.fr' using domain routing, ensuring the default locale is English and URLs do not show locale prefixes?

hard

A. Set i18n.locales to ['en', 'fr'], defaultLocale to 'en', and i18n.domains with domains for example.com (fr) and example.fr (en), and use localeDetection: true

B. Set i18n.locales to ['en', 'fr'], defaultLocale to 'fr', and i18n.domains with domains for example.com (fr) and example.fr (en), and use localeDetection: true

C. Set i18n.locales to ['en', 'fr'], defaultLocale to 'en', and i18n.domains with domains for example.com (en) and example.fr (fr), and use localeDetection: false

D. Set i18n.locales to ['en', 'fr'], defaultLocale to 'en', and no domains, rely on middleware only

Solution

1. Step 1: Define locales and defaultLocale correctly

   Locales should be ['en', 'fr'] with defaultLocale 'en' to serve English by default.

2. Step 2: Configure i18n.domains for domain-locale mapping

   Map 'example.com' to 'en' and 'example.fr' to 'fr' in i18n.domains array.

3. Step 3: Disable localeDetection to avoid prefix in URLs

   Setting localeDetection: false prevents Next.js from adding locale prefixes in URLs, serving clean domain-based locales.

4. Final Answer:

   Set i18n.locales to ['en', 'fr'], defaultLocale to 'en', and i18n.domains with domains for example.com (en) and example.fr (fr), and use localeDetection: false -> Option C

5. Quick Check:

   Domain routing + defaultLocale + no prefix = Set i18n.locales to ['en', 'fr'], defaultLocale to 'en', and i18n.domains with domains for example.com (en) and example.fr (fr), and use localeDetection: false [OK]

Hint: Use i18n.domains and disable localeDetection for clean URLs [OK]

Common Mistakes:

• Swapping locales for domains
• Forgetting to disable localeDetection
• Relying only on middleware without domains