Server-side rendering (SSR) refers to the process of rendering React components on the server before sending the fully populated HTML markup to the client.
With SSR, the initial request hits the server, which runs the React code, generates the static HTML, and sends the markup to the client. Once loaded in the browser, React on the client side hydrates those static HTML elements into interactive components.
- Browser request hits server ➜ React renders components into HTML string.
- The server runs React code ➜ HTML sent back to the client
- HTML rendered by React ➜
- Fully rendered HTML sent to browser
- Client hydrates static HTML ➜ React boots up and hydrates HTML
- HTML converted to interactive ➜ App is interactive on the client side
- React components
The key benefits of SSR include:
- Better SEO - Search engines can more effectively crawl and index fully rendered server-side pages. This leads to better optimization of search engines than traditional React apps with static landing pages and client-side routing.
SSR is a popular approach for solving traditional single-page applications' SEO and performance pains. However, implementing SSR does come with added complexity compared to client-side rendering only.
How Server-Side Rendering Works
When a user or search engine crawler sends a request for a page on a website using SSR:
- The request hits the server rather than just loading a static HTML file.
- The server runs the React rendering code, generating the final HTML markup.
- The server then sends the fully rendered HTML to the client.
- On the client side, React boots up and then "hydrates" the static HTML sent from the server. This converts the markup into live React components.
An important distinction is that with SSR, the same React code runs on both the server and client. The server generates static HTML markup, while the client hydrates that markup into components and takes over rendering dynamically from that point forward.
Loading data is a key aspect of SSR. To render fully populated HTML, any data needed for the initial render must be fetched on the server.
This data is then embedded into the HTML sent to the browser so the client can render without waiting for data.
React components will monitor data on the client and update dynamically after the initial render.
Implementing Server-Side Rendering
While it is possible to implement SSR manually, the complexity involved has led to powerful frameworks like Next.js simplifying the process. These tools handle tasks like code splitting, data fetching, serving static assets, and routing.
Some key steps for implementing SSR include:
- Using ReactDOMServer to render components to static markup on the server. The renderToString() method renders components to a string of HTML.
// server.js import React from 'react'; import ReactDOMServer from 'react-dom/server'; import App from './App'; const appHtml = ReactDOMServer.renderToString(<App />);
On the client side, the root component mounts by "hydrating" the server-rendered HTML rather than generating markup from scratch.
// client.js import ReactDOM from 'react-dom'; ReactDOM.hydrate(<App />, document.getElementById('root'));
Fetching and providing any necessary data for the initial render on the server. This data can be loaded into the page template or React components before rendering.
Handling client and server-side routing. The initial route is rendered server-side, then the client-side router takes over for subsequent navigation.
Frameworks like Next.js have built-in support for SSR with React. Features like automatic code splitting and data prefetching help optimize performance. Next.js apps also provide a development environment that reloads changes on save for easier SSR workflow.
Challenges with Server-Side Rendering
While the benefits clearly exist, SSR does introduce challenges, including:
- Increased Server Load - Rendering on the server places additional load and CPU usage on server infrastructure. Static assets should be served separately to keep servers focused on rendering pages.
- Repeated Rendering - Components are rendered twice - once on the server and again on the client. This repetition can impact performance if not handled properly.
- Data Fetching Complexity - Data dependencies must be handled on the server and client-side rendering. Loading data on the server adds complexity.
- Streaming Server Responses - Rendering each route on demand with SSR can produce a slow "waterfall" effect. Solutions like Next.js allow streaming responses.
- Evolving State on Client - Interactivity after the initial render can lead to a UI state that doesn't match the initial server-rendered version.
Careful performance testing and optimization help minimize these potential downsides during implementation.
Tips for Optimizing Performance
Server-side rendering improves performance for the initial page load. But optimizing the entire end-user experience requires additional considerations:
- Use Component Caching - Cache frequently rendered components on the server side to reduce computational load.
- Serve Static Assets Separately - Serve static JS/CSS bundles from a CDN rather than requiring the app server to handle.
- Delay Hydration - Wait until user interaction to hydrate static markup into interactive components.
- Load Data at Runtime - Use APIs like React Query to load and cache data on the client side at runtime.
- Add Progressive Enhancement - Detect browser capabilities and enhance experience instead of hydrating statically rendered markup.
Thorough performance testing under real-world conditions helps tune an SSR implementation to minimize tradeoffs. Monitoring tools like Lighthouse provide optimization guidance.
Server-side rendering can provide significant performance and SEO improvements for React applications. However, care must be taken to properly handle the added complexity that SSR introduces compared to client-only rendering.
Tools like Next.js reduce the configuration and boilerplate code required to get the benefits of SSR. Features like automatic code splitting, optimized data fetching, and configurable rendering options make SSR more approachable.
For newer web applications where SEO and initial load performance are priorities, SSR with React provides a compelling way to solve these common pain points of traditional single-page apps.
Careful optimization and testing are required to ensure the end-user experience is fast. However, the benefits typically outweigh the drawbacks when SSR is implemented properly.
The details may vary based on the specific application architecture and use cases. However, the core SSR approach of rendering components to HTML strings on the server and then hydrating that markup on the client side can provide a significant performance boost for initial page loads and search engine crawlability.
Frequently Asked Questions (FAQ)
- What are the main benefits of using SSR with React?
The two major benefits are faster initial page loads due to eliminating client-side rendering and better SEO since search engines can crawl fully rendered pages. SSR solves common SPA problems.
- When should SSR be avoided?
SSR provides no benefit for simple static landing pages with minimal data requirements. The increased complexity may not be worth it.
- Can React Router be used for client-side routing with SSR?
- What are the biggest challenges with implementing React SSR?
Common challenges include increased server load, repetitive client/server rendering, complex data fetching requirements, and optimizing streaming responses.
- How can an SSR React app be optimized?
Component caching, code splitting, CDNs for static assets, delayed hydration, and adding progressive enhancement all help optimize SSR apps.