Webpack module federation is not working with eager shared libs

Question

I was looking into Webpack 5 Module federation feature, and have some trouble understanding why my code does not work. The idea is pretty similar to what standard module federation examples do:

app1 - is the host app app2 - is a remote exposing the whole app to app1

(app1 renders the header and horizontal line, below which the app2 should be rendered)

Both app1 and app2 declares react and react-dom as their shared, singleton, eager dependencies in the weback.config.js:

// app1 webpack.config.js
module.exports = {
  entry: path.resolve(SRC_DIR, './index.js');,
  ...
  plugins: [
    new ModuleFederationPlugin({
      name: "app1",
      remotes: {
        app2: `app2@//localhost:2002/remoteEntry.js`,
      },
      shared: { react: { singleton: true, eager: true }, "react-dom": { singleton: true, eager: true } },
    }),
    ...
  ],
};

// app2 webpack.config.js
module.exports = {
  entry: path.resolve(SRC_DIR, './index.js');,
  ...
  plugins: [
    new ModuleFederationPlugin({
      name: "app2",
      library: { type: "var", name: "app2" },
      filename: "remoteEntry.js",
      exposes: {
        "./App": "./src/App",
      },
      shared: { react: { singleton: true, eager: true }, "react-dom": { singleton: true, eager: true } },
    }),
    ...
  ],
};

In the App1 index.js I have next code:

import React from "react";
import ReactDOM from "react-dom";
import App from "./App";


ReactDOM.render(<App />, document.getElementById("root"));

The App1 App.js component is next:

import React, { Suspense } from 'react';

const RemoteApp2 = React.lazy(() => import("app2/App"));

export default function App() {
  return (
    <div>
      <h1>App 1</h1>
      <p>Below will be some content</p>
      <hr/>
      <Suspense fallback={'Loading App 2'}>
        <RemoteApp2 />
      </Suspense>
    </div>
  );
}

But when I start the application I get the next error:

Uncaught Error: Shared module is not available for eager consumption: webpack/sharing/consume/default/react/react?1bb3
    at Object.__webpack_modules__.<computed> (consumes:133)
    at __webpack_require__ (bootstrap:21)
    at fn (hot module replacement:61)
    at Module../src/index.js (main.bundle.a8d89941f5dd9a37d429.js:239)
    at __webpack_require__ (bootstrap:21)
    at startup:4
    at startup:6

If I extract everything from index.js to bootstrap.js and in index.js will do

import('./bootstrap');

Everything works just fine.

This confuses me as official docs and blog posts from the creator states that you can do either bootstrap.js way OR declare dependency as an eager one.

Would appreciate any help/insights on why it does not work without bootstrap.js pattern.

Here is a link to full GitHub sandbox I was building: https://github.com/vovkvlad/webpack-module-fedaration-sandbox/tree/master/simple

Solution 1

In order to make it work you need to change the way you are loading remote entry.

1. Update your ModuleFederationPlugin config in webpack.config.js for app1 to this:

...

new ModuleFederationPlugin({
    name: "app1",
    remoteType: 'var',
    remotes: {
      app2: 'app2',
    },
    shared: {
      ...packageJsonDeps,
      react: { singleton: true, eager: true, requiredVersion: packageJsonDeps.react },
      "react-dom": { singleton: true, eager: true, requiredVersion: packageJsonDeps["react-dom"] }
    },
}),

...

2. Add script tag to the head of your index.html in app1 :

<script src="http://localhost:2002/remoteEntry.js"></script>

Good look with further hacking!

UPDATE:

Just for the sake of it: I've create a PR to your sandbox repo with fixes described above: https://github.com/vovkvlad/webpack-module-fedaration-sandbox/pull/2

Solution 2

Just to make it clear for those who might miss the comment to the initial answer:

It seems like the main reason of why it failed initially was that remoteEntry.js file was loaded after the code that actually runs the host app.

Both bootstrap.js approach and adding direct script <script src="http://localhost:2002/remoteEntry.js"></script> to the <head></head> tag has exactly the same outcome - they make remoteEntry.js be loaded and parsed BEFORE the main app's code.

In case of bootstrap the order is next:

1. main_bundle is loaded
2. as the main code is extracted into bootstrap.js file - remoteEntry.js is loaded
3. bootstrap.js is loaded which actually runs the main app

with proposed variant by Oleg Vodolazsky events order is next:

1. remoteEntry.js is loaded first as it is directly added to html file and webpack's main_bundle is being appended to <head></head> after remoteEntry link
2. main_bundle is loaded and runs the application

and in case of just trying to run app without bootstrap and without hardcoded script in the <head></head> main_bundle is loaded before remoteEntry.js and as main_bundle tries to actually run the app, it fails with an error:

Solution 3

You can set the dependency as eager inside the advanced API of Module Federation, which doesn’t put the modules in an async chunk, but provides them synchronously. This allows us to use these shared modules in the initial chunk. But be careful as all provided and fallback modules will always be downloaded. It’s recommended to provide it only at one point of your application, e.g. the shell.

Webpack's website strongly recommend using an asynchronous boundary. It will split out the initialization code of a larger chunk to avoid any additional round trips and improve performance in general.

For example, your entry looked like this:

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';
ReactDOM.render(<App />, document.getElementById('root'));

Let's create bootstrap.js file and move contents of the entry into it, and import that bootstrap into the entry:

index.js

+ import('./bootstrap');
- import React from 'react';
- import ReactDOM from 'react-dom';
- import App from './App';
- ReactDOM.render(<App />, document.getElementById('root'));

bootstrap.js

+ import React from 'react';
+ import ReactDOM from 'react-dom';
+ import App from './App';
+ ReactDOM.render(<App />, document.getElementById('root'));

This method works but can have limitations or drawbacks.

Setting eager: true for dependency via the ModuleFederationPlugin

webpack.config.js

// ...
new ModuleFederationPlugin({
  shared: {
    ...deps,
    react: {
      eager: true,
    },
  },
});

Source

Comments

[volk]

So just to make things clear clear: your proposed changes do work, but as far as I understand from playing around with it, is that the bottom line is that the remoteEntry.js should be loaded BEFORE the bundle that actually runs the app. And that's basically, what bootstrap.js does - it makes the main app to load after main_bundle and remoteEntry.js When using your proposed change, it does the same - it downloads remoteEntry.js before the main_bundle as main_bundle is being added by webpack after hardcoded script tag with remoteEntry.js

[volk]

Here is a link with screenshots of all three situations: imgur.com/a/63WTCMg

[pauldcomanici]

I'm having the same problem. And the suggested solution does not scale, if we are talking about development or a POC is ok, but when you have production-ready applications with 3+ environments this solution will not work. If anyone has a solution that works with dynamic imports, I'm really interested how you solved this issue.

[krutoo]

Is there any news regarding a solution without bootstrap file or add scripts to all remote entry?

[Chunky Chunk]

Besides import(".bootstrap") are the remaining imports and ReactDom.Render() necessary in the index.js file?

[hassan ketabi]

@ChunkyChunk No it isn't necessary. Because we have it in bootstrap file.

[Chunky Chunk]

I didn't notice the "+" and "-". Now I understand what you wrote.