SSR renders pages on every request using getServerSideProps, ideal for dynamic, personalized content with fresh data; SSG generates pages at build time using getStaticProps, perfect for static content like blogs and docs; ISR combines both by regenerating static pages in the background with revalidate, suitable for large sites needing occasional updates. Choose SSG for static content, ISR for scalable sites with frequent small changes, and SSR for real-time, user-specific data, and mix strategies as needed within the same app.

When building modern web applications with Next.js, you’ve probably come across the terms SSR (Server-Side Rendering), SSG (Static Site Generation), and ISR (Incremental Static Regeneration). These aren't just buzzwords—they represent different rendering strategies that affect performance, SEO, and user experience. Let’s break them down in simple terms with practical insights.

What Is SSR? (Server-Side Rendering)

SSR means the page is rendered on the server for every incoming request. The HTML is generated fresh each time someone visits the page.

In Next.js, you use getServerSideProps to enable SSR.

export async function getServerSideProps() {
  const res = await fetch('https://api.example.com/data');
  const data = await res.json();

  return { props: { data } };
}

When to use SSR:

• Your data changes frequently (e.g., user dashboards, real-time analytics).
• You need up-to-date data on every request.
• SEO is important, but content can't be prebuilt at build time.

Pros:

• Always fresh data.
• Good for personalized content.
• SEO-friendly (search engines see fully rendered HTML).

Cons:

• Slower than SSG because rendering happens at request time.
• Higher server load.

Think of SSR like a chef preparing a meal from scratch every time a customer orders. Fresh, but takes time.

What Is SSG? (Static Site Generation)

SSG means the page is generated at build time. Once built, the HTML is stored and served as static files—no server processing needed per request.

In Next.js, use getStaticProps (and optionally getStaticPaths for dynamic routes).

export async function getStaticProps() {
  const res = await fetch('https://api.example.com/posts');
  const posts = await res.json();

  return { props: { posts }, revalidate: false };
}

When to use SSG:

• Content doesn’t change often (e.g., blogs, documentation, marketing sites).
• You want maximum performance and low hosting costs.

Pros:

• Super fast loading (served from CDN).
• Low server cost.
• Great for SEO.

Cons:

• Content is only as fresh as your last build.
• Not suitable for user-specific or real-time data.

SSG is like printing a cookbook. Once it's printed, every reader gets the same copy—fast and efficient, but updates require a new print run.

What Is ISR? (Incremental Static Regeneration)

ISR is the best of both worlds: you get the speed of static generation with the flexibility to update pages after deployment—without rebuilding the entire site.

With revalidate in getStaticProps, Next.js regenerates the page in the background after a set time.

export async function getStaticProps() {
  const res = await fetch('https://api.example.com/articles');
  const articles = await res.json();

  return { props: { articles }, revalidate: 60 }; // Rebuild every 60 seconds
}

How ISR works:

1. First visitor requests a page → served from static cache (if exists) or generated on-demand.
2. After 60 seconds (in this example), the next request triggers a background regeneration.
3. The stale page is served until the new one is ready.
4. Future visitors get the updated version.

When to use ISR:

• Large sites where full rebuilds are expensive (e.g., e-commerce product pages, news sites).
• You want mostly static performance but occasional updates.

Pros:

• Near-static performance.
• No need to rebuild the entire site for small content changes.
• Reduces build time and CI/CD strain.

Cons:

• Slight complexity in cache invalidation logic.
• Users might briefly see stale data during regeneration.

ISR is like updating a few pages in a printed book without reprinting the whole thing—efficient and scalable.

Key Differences at a Glance

Which One Should You Choose?

It’s not about which is “best”—it’s about fitting the strategy to your content:

• Use SSG for blogs, docs, landing pages—anything that doesn’t change often.
• Use ISR for large-scale sites (like e-commerce) where rebuilding everything is costly.
• Use SSR when you need real-time data or user-specific content on every visit.

And remember: you can mix them in the same app. Your homepage might use SSG, your blog uses ISR, and your dashboard uses SSR.

Basically, Next.js gives you the tools to pick the right rendering strategy for each page. Understanding SSR, SSG, and ISR lets you balance speed, freshness, and scalability—without over-engineering.

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