Understanding React: SSR vs CSR vs ISR

As developers, we are constantly looking for the best approach to building our applications. In the realm of React, three common rendering methods frequently come up: Server-Side Rendering (SSR), Client-Side Rendering (CSR), and Incremental Static Regeneration (ISR). Each technique has its own strengths and weaknesses, making it essential to choose the right one for our specific project needs. In this article, we’ll dive deep into each of these rendering strategies, highlighting their features, advantages, and potential use cases.

What is Server-Side Rendering (SSR)?

Server-Side Rendering (SSR) refers to the technique of rendering a web page on the server and sending the fully rendered page to the client. In SSR, when a user requests a page, the server processes the request, generates the HTML content, and delivers it to the browser. This process contrasts with CSR, where rendering occurs entirely on the client side after the initial load.

How SSR Works

With SSR, the flow of data is as follows:

1. The user makes a request for a web page.
2. The server runs the React components, generating the corresponding HTML on the fly.
3. The server sends the HTML to the client, which displays the fully rendered page.
4. Subsequent interactions can be handled via React on the client side.

Example of SSR

Here’s a simple example of implementing SSR using Next.js, a React framework that supports SSR natively:

import { useEffect } from 'react';

const Page = ({ data }) => {
    useEffect(() => {
        // Client-side logic here
    }, []);

    return (
        <div>
            <h1>Server-Side Rendered Page</h1>
            <p>Data from server: {data}</p>
        </div>
    );
};

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

export default Page;

Advantages of SSR

• SEO Friendly: Since the content is rendered server-side, web crawlers can easily index it, improving search engine optimization.
• Faster Initial Page Load: Users receive a fully rendered page on their first request, leading to a more responsive initial experience.
• Performance for Static Content: Ideal for applications whose data doesn’t change frequently, benefiting from pre-rendered HTML.

Limitations of SSR

• Increased Server Load: Every page request results in a server-side render, potentially leading to higher server resource consumption.
• Longer Time for First Byte (TTFB): The time taken to generate and serve the HTML can increase, especially for dynamic data.
• Complexity: Managing server-side code and client-side code can complicate application architecture.

What is Client-Side Rendering (CSR)?

Client-Side Rendering (CSR) is the opposite of SSR. In CSR, the server sends a minimal HTML file and a JavaScript bundle to the browser. The rendering process takes place on the client side once the JavaScript is executed. This workflow is common with traditional Single Page Applications (SPAs).

How CSR Works

The flow of CSR can be summarized as follows:

1. The user requests a page, and the server responds with a basic HTML file and JavaScript.
2. The browser downloads and runs the JavaScript.
3. React builds the page dynamically in the user’s browser, fetching data as needed.

Example of CSR

Here’s a simple example of implementing CSR using React:

import React, { useEffect, useState } from 'react';

const App = () => {
    const [data, setData] = useState(null);
    
    useEffect(() => {
        fetch('https://api.example.com/data')
            .then((res) => res.json())
            .then((data) => setData(data));
    }, []);

    return (
        <div>
            <h1>Client-Side Rendered Page</h1>
            {data ? <p>Data from client: {data}</p> : <p>Loading...</p>}
        </div>
    );
};

export default App;

Advantages of CSR

• Reduced Server Load: The server only needs to serve static assets, leading to lower resource consumption.
• Dynamic Interactivity: Allows for a more fluid user experience by fetching data without full page refreshes.
• Simplified Development: Developers can focus more on building UI components without worrying about server-side logic.

Limitations of CSR

• SEO Challenges: Content rendered on the client side may not be fully indexed by search engines, which can affect visibility.
• Performance Issues: Initial load times can be slower due to the download and execution of JavaScript.
• Flash of Unstyled Content (FOUC): Users may see a blank screen or loading indicators until the JavaScript is fully loaded and executed.

What is Incremental Static Regeneration (ISR)?

Incremental Static Regeneration (ISR) is a relatively new rendering method introduced in Next.js that combines the advantages of both static generation and server-side rendering. It allows developers to create static pages that can be updated post-deployment without needing to rebuild the entire site.

How ISR Works

ISR operates in a unique way:

1. Static pages are generated at build time like traditional static sites.
2. When a request comes in for a page, it serves the pre-generated page.
3. Once a specific time threshold defined by the developer passes, it regenerates the page in the background upon the next request.

Example of ISR

A simple example of ISR can also be demonstrated with Next.js:

import { useEffect } from 'react';

const Page = ({ data }) => {
    useEffect(() => {
        //Additional client-side logic
    }, []);

    return (
        <div>
            <h1>Incremental Static Regeneration Page</h1>
            <p>Data: {data}</p>
        </div>
    );
};

export async function getStaticProps() {
    const res = await fetch('https://api.example.com/data');
    const data = await res.json();
    return {
        props: { data },
        revalidate: 10, // Re-generate every 10 seconds
    };
}

export default Page;

Advantages of ISR

• Fast Page Load: Initial loads are fast since the pages are served statically.
• SEO Benefits: Pre-rendered static pages are SEO-friendly, ensuring high visibility.
• Flexibility: Developers can easily update static content without full redeploys.

Limitations of ISR

• Complexity: While offering powerful features, it introduces additional complexity in managing revalidation and caching.
• Stale Data Risk: Depending on the revalidation time, there may be instances where the data might be stale.
• Framework Limitation: ISR is primarily tied to Next.js, limiting its use outside of this framework.

Choosing the Right Rendering Method

Selecting the appropriate rendering method in React depends on various factors such as SEO requirements, performance needs, user interactivity, and the nature of your application. Below are some scenarios to consider when making your decision:

When to Use SSR

• When SEO is critical (e.g., e-commerce sites, blogs).
• For applications where the content is dynamic and changes frequently.
• When you want a fast initial load without client-side JavaScript execution delay.

When to Use CSR

• For Single Page Applications where user engagement is high.
• When SEO is less of a concern, like internal tools and dashboards.
• For rapid applications where dynamic interactions are key.

When to Use ISR

• For applications that benefit from static site generation but need periodic updates.
• When you want the best of both SSR and CSR — fast loads and SEO-friendly content with frequent data changes.
• For blogs or marketing sites that need to display dynamic content without frequent full builds.

Conclusion

Choosing the right rendering strategy for your React application is crucial for performance, user experience, and SEO. Each approach—SSR, CSR, and ISR—has its unique use cases and benefits. By understanding the differences between them, you can select the best method to fit your application’s specific requirements. Ultimately, modern frameworks like Next.js facilitate a more flexible approach, allowing developers to combine these techniques creatively to deliver outstanding web applications.

As the web continues to evolve, staying informed about rendering methods and their implications will help you build more efficient, effective, and engaging applications. Happy coding!