ISR vs SSR vs SSG in Next.js

ISR vs SSR vs SSG in Next.js: A Practical Guide

Introduction
Next.js offers three powerful rendering strategies: SSR (Server-Side Rendering), SSG (Static Site Generation), and ISR (Incremental Static Regeneration). Each approach has its own strengths, trade-offs, and best-use scenarios. In this guide, you’ll learn what each method means, how they work in Next.js, and when to choose one (or a hybrid) for your project.

Understanding SSR, SSG, and ISR in Next.js
- SSR (Server-Side Rendering)
- Renders a page on every request using the server.
- Data is always fresh but can be slower for users because rendering happens per visit.
- Best for personalized or frequently changing content (e.g., dashboards, user-specific pages, real-time data).

- SSG (Static Site Generation)
- Renders pages at build time into static HTML.
- Very fast delivery via CDNs but uses static data unless rebuilt.
- Ideal for pages that don’t change often (marketing pages, docs, blogs, catalogs).

- ISR (Incremental Static Regeneration)
- A Next.js feature that extends SSG with on-demand/regeneration capabilities.
- Pages are static but can be updated after a specified interval or on-demand.
- Combines the speed of static generation with fresh data, without rebuilding the entire site.

How Next.js Implements Each
- SSG (Static Site Generation)
- Key functions:
- getStaticProps: fetch data at build time and pass it to the page.
- getStaticPaths: generate dynamic routes at build time.
- Example (SSG with revalidation):
- getStaticProps() fetches data.
- Return value includes props and an optional revalidate interval.
- Code snippet (conceptual):
- export async function getStaticProps() { const data = fetchData(); return { props: { data }, revalidate: 60 }; }

- SSR (Server-Side Rendering)
- Key function:
- getServerSideProps: fetch data on every request.
- Example (SSR):
- export async function getServerSideProps(context) { const data = fetchData(context); return { props: { data } }; }

ISR (Incremental Static Regeneration)
- How it works:
- Build-time static pages are generated with getStaticProps.
- The revalidate field tells Next.js how often to re-generate a page in the background.
- If a request comes in after the revalidate window, Next.js serves the cached page and re-builds it in the background for subsequent requests.
- Example:
- export async function getStaticProps() { const data = fetchData(); return { props: { data }, revalidate: 60 }; }

Pros and Cons at a Glance
- SSR
- Pros: Always fresh data, great for personalization and frequently changing content.
- Cons: Higher latency per request, requires server resources, can be more costly at scale.

- SSG
- Pros: Ultra-fast performance, simple caching via CDNs, low server load.
- Cons: Data becomes stale until rebuild, not ideal for highly dynamic content unless you use revalidation.

- ISR
- Pros: High performance with static delivery plus fresh data when needed, scalable for large sites.
- Cons: Slight complexity around revalidation timing and cache invalidation, may not suit ultra-dynamic data.

Choosing the Right Rendering Method
- When to use SSR
- User-specific content (profiles, dashboards)
- Real-time or frequently changing data
- SEO-friendly pages that rely on up-to-date data

- When to use SSG
- Content that changes rarely (marketing pages, documentation, blogs)
- High-traffic pages where speed and caching matter
- Static catalogs or landing pages

- When to use ISR
- Large sites with many pages that don’t require full rebuilds
- Pages that should stay fast but need periodic data refresh
- E-commerce product pages, FAQs, and article lists with moderate update frequency

Common Scenarios and Patterns
- Hybrid rendering (recommended)
- Use SSR for highly dynamic pages (e.g., user dashboard) and SSG/ISR for marketing pages and content-heavy routes.
- Next.js makes it easy to mix rendering strategies within the same app.

- Dynamic routes with ISR
- For pages like blog posts or product detail pages that don’t change every second, use getStaticProps with revalidate to keep pages fresh without rebuilding the entire site.

- Personalization and authentication
- Combine SSR or client-side fetching for personalized sections while serving the rest as SSG/ISR.

Performance and Best Practices
- Optimize data fetching
- Minimize data requirements in getStaticProps/getServerSideProps.
- Cache external API responses where possible.
- Use incremental regeneration wisely
- Choose reasonable revalidate intervals based on how fresh the data needs to be.
- Leverage CDN caching
- Serve static assets and generated pages from a CDN for maximum speed.
- Monitor and measure
- Use real-user monitoring and server-side metrics to decide when a strategy needs adjustment.

Migration and Hybrid Approaches
- Start simple
- Identify pages that are static by default and render them with SSG.
- Introduce ISR for large sites
- Add revalidate to statically generated pages that need refreshing.
- Move to SSR where necessary
- Convert pages with strict personalization or real-time data requirements to SSR.
- Plan a gradual hybrid
- Your Next.js app can mix SSR, SSG, and ISR across routes to optimize both performance and freshness.

Conclusion
Next.js gives you a flexible toolkit to optimize performance, data freshness, and scalability. SSR, SSG, and ISR are not mutually exclusive; a smart, hybrid approach often yields the best user experience. Start by identifying pages that need freest caching and fastest load times, then layer in ISR for updates and SSR only where personalization or real-time data is essential. With the right mix, you can deliver fast, SEO-friendly experiences at scale.

FAQs
- Is ISR the same as SSG?
- ISR extends SSG by allowing pages to be updated after a specified interval without rebuilding the entire site.
- Do I need a server to use SSR?
- Yes. SSR requires a server to render pages on each request, though hosting options vary (e.g., Vercel, Node.js servers).
- Can I mix SSR, SSG, and ISR in one Next.js project?
- Absolutely. A common pattern is to use SSG/ISR for most pages and SSR for highly dynamic or personalized routes.

If you’re building a Next.js app today, consider a hybrid rendering strategy. Start with SSG for static pages, enable ISR where appropriate, and use SSR for pages requiring fresh, per-request data. This approach delivers outstanding performance without sacrificing data accuracy.