#Server-Side Rendering

Server-Side Rendering (SSR) generates complete HTML pages on the server and sends them to the browser for direct display, without requiring additional client-side rendering.

#Core Advantages

• Faster First Screen: Server-side pre-rendering allows the browser to display content directly without waiting for JavaScript execution
• Better SEO: Search engines can directly index complete HTML content
• Out of the Box: No need to write complex server-side logic or maintain separate services

import { defineConfig } from '@modern-js/app-tools';

export default defineConfig({
  server: {
    ssr: {
      mode: 'string', // Traditional SSR mode
    },
  },
});

#Enabling SSR

Enabling SSR in Modern.js is straightforward. Simply set server.ssr to true:

import { defineConfig } from '@modern-js/app-tools';

export default defineConfig({
  server: {
    ssr: true, // Streaming rendering enabled by default
  },
});

#Data Fetching

Modern.js provides Data Loader, enabling developers to fetch data isomorphically under both SSR and CSR. Each route module (such as layout.tsx and page.tsx) can define its own Data Loader:

export const loader = () => {
  return {
    message: 'Hello World',
  };
};

Access data in components through Hooks API:

import { useLoaderData } from '@modern-js/runtime/router';
export default () => {
  const data = useLoaderData();
  return <div>{data.message}</div>;
};

#Using Client Loader

By default, in SSR applications, the loader function only executes on the server. However, in some scenarios, developers might want requests made on the client-side to bypass the SSR service and directly fetch data from the source. For example:

1. Reducing network consumption on the client-side by directly fetching from the data source.
2. The application has data cached on the client side and doesn't want to fetch data from the SSR service.

Modern.js supports adding a .data.client file, also named exported as loader, in SSR applications. If the Data Loader fails to execute on the server side, or when navigating on the client side, it will execute the loader function on the client side instead of sending a data request to the SSR service.

import cache from 'my-cache';

export async function loader({ params }) {
  if (cache.has(params.id)) {
    return cache.get(params.id);
  }
  const res = await fetch(`URL_ADDRESS?id=${params.id}`);
  const data = await res.json();
  return {
    message: data.message,
  }
}

#SSR Fallback

In Modern.js, if an application encounters an error during SSR, it automatically falls back to CSR mode and re-fetches data, ensuring the page can display correctly. SSR fallback can occur for two main reasons:

1. Data Loader execution error.
2. React component rendering error on the server side.

#Data Loader Execution Error

By default, if the loader function for a route throws an error, the framework renders the <ErrorBoundary> component directly on the server, displaying the error message. This is the default behavior of most frameworks.

Modern.js also supports customizing the fallback strategy through the loaderFailureMode field in the server.ssr configuration. Setting this field to clientRender immediately falls back to CSR mode and re-fetches the data.

If a Client Loader is defined for the route, it will be used to re-fetch the data. If re-rendering fails again, the <ErrorBoundary> component will be displayed.

#Component Rendering Error

If the Data Loader executes correctly but the component rendering fails, SSR rendering will partially or completely fail, as shown in the following code:

import { Await, useLoaderData } from '@modern-js/runtime/router';
import { Suspense } from 'react';

const Page = () => {
  const data = useLoaderData();
  const isNode = typeof window === 'undefined';
  const undefinedVars = data.unDefined;
  const definedVars = data.defined;

  return (
    <div>
      {isNode ? undefinedVars.msg : definedVars.msg}
    </div>
  );
};

export default Page;

In this case, Modern.js will fallback the page to CSR and use the existing data from the Data Loader to render. If the rendering still fails, the <ErrorBoundary> component will be rendered.

#Logging and Monitoring

#Page Caching

Modern.js has built-in caching capabilities. Refer to Rendering Cache for details.

#Differences in Runtime Environment

SSR applications run on both the server and the client, with differing Web and Node APIs.

When enabling SSR, Modern.js uses the same entry to build both SSR and CSR bundles. Therefore, having Web APIs in the SSR bundle or Node APIs in the CSR bundle can lead to runtime errors. This usually happens in two scenarios:

• There are issues in the application's own code.
• The dependency package contains side effects.

#Issues with Own Code

This scenario often arises when migrating from CSR to SSR. CSR applications typically import Web APIs in the code. For example, an application might set up global event listeners:

document.addEventListener('load', () => {
  console.log('document load');
});
const App = () => {
  return <div>Hello World</div>;
};
export default App;

In such cases, you can use Modern.js built-in environment variables MODERN_TARGET to remove unused code during the build:

if (process.env.MODERN_TARGET === 'browser') {
  document.addEventListener('load', () => {
    console.log('document load');
  });
}

After packing in the development environment, the SSR and CSR bundles will compile as follows. Therefore, Web API errors will not occur in the SSR environment:

// SSR Bundle
if (false) {
}

// CSR Bundle
if (true) {
  document.addEventListener('load', () => {
    console.log('document load');
  });
}

#Side Effects in Dependencies

This scenario can occur at any time in SSR applications because not all community packages support running in both environments. Some packages only need to run in one. For instance, importing package A that has a side effect using Web APIs:

document.addEventListener('load', () => {
  console.log('document load');
});

export const doSomething = () => {}

Directly referencing this in a component will cause SSR to throw errors, even if you use environment variables to conditionally load the code. The side effects in the dependency will still execute.

import { doSomething } from 'packageA';

export const Page = () => {
  if (process.env.MODERN_TARGET === 'browser') {
    doSomething();
  }
  return <div>Hello World</div>
}

Modern.js supports distinguishing between SSR and CSR bundles by using .server. suffix files. You can create .ts and .server.ts files with the same name to create a proxy:

export { doSomething } from 'packageA';

export const doSomething: any = () => {};

Import ./a in the file, and the SSR bundle will prioritize the .server.ts files, while the CSR bundle will prioritize the .ts files.

import { doSomething } from './a'

export const Page = () => {
  doSomething();
  return <div>Hello World</div>
}

#Common Issues

#Ensuring Consistent Rendering

In SSR applications, it is crucial to ensure that the rendering results on the server are consistent with the hydration results in the browser. Inconsistent rendering may lead to unexpected outcomes. Here’s an example demonstrating issues when SSR and CSR render differently. Add the following code to your component:

{
  typeof window !== 'undefined' ? <div>browser content</div> : null;
}

After starting the application and visiting the page, you will notice a warning in the browser console:

Warning: Expected server HTML to contain a matching <div> in <div>.

This warning is caused by a mismatch between the React hydrate results and the SSR rendering results. Although the current page appears normal, complex applications may experience DOM hierarchy disruptions or style issues.

The application needs to maintain consistency between SSR and CSR rendering results. If inconsistencies occur, it indicates that some content should not be rendered by SSR. Modern.js provides the <NoSSR> utility component for such scenarios:

import { NoSSR } from '@modern-js/runtime/ssr';

Wrap elements that should not be server-side rendered with the NoSSR component:

<NoSSR>
  <div>browser content</div>
</NoSSR>

After modifying the code, refresh the page and notice that the previous warning has disappeared. Open the browser's developer tools and check the Network tab. The returned HTML document will not contain the content wrapped by the NoSSR component.

In practical scenarios, some UI displays might be affected by the user's device, such as UA information. Modern.js also provides APIs like use(RuntimeContext), allowing components to access complete request information and maintaining SSR and CSR rendering consistency. For detailed usage, please refer to Runtime Context.

#Attention to Memory Leaks

With SSR, each browser request triggers server-side component rendering. Therefore, you should avoid defining any data structures that continually grow globally, subscribing to global events, or creating non-disposable streams.

For example, when using redux-observable, developers accustomed to CSR might code as follows:

/* Code is just an example and not executable */
import { createEpicMiddleware, combineEpics } from 'redux-observable';

const epicMiddleware = createEpicMiddleware();
const rootEpic = combineEpics();

export default function Test() {
  epicMiddleware.run(rootEpic);
  return <div>Hello Modern.js</div>;
}

In this case, the epicMiddleware instance is created outside the component, and epicMiddleware.run is called within the component.

This code does not cause issues on the client-side. However, in SSR, the Middleware instance remains non-disposable. Each time the component renders, calling epicMiddleware.run(rootEpic) adds new event bindings internally, causing the entire object to grow continuously, ultimately affecting application performance.

Such issues are not easily noticed in CSR. When transitioning from CSR to SSR, if you're unsure whether your application has such hidden pitfalls, consider stress testing the application.