Motivation

Multiple separate builds should form a single application. These separate builds act like containers and can expose and consume code among themselves, creating a single, unified application.

This is often known as Micro-Frontends, but is not limited to that.

Low-level concepts

We distinguish between local and remote modules. Local modules are regular modules that are part of the current build. Remote modules are modules that are not part of the current build but are loaded at runtime from a remote container.

Loading remote modules is considered an asynchronous operation. When using a remote module, these asynchronous operations will be placed in the next chunk loading operation(s) that are between the remote module and the entrypoint. It's not possible to use a remote module without a chunk loading operation.

A chunk loading operation is usually animport() call, but older constructs likerequire.ensure orrequire([...]) are supported as well.

A container is created through a container entry, which exposes asynchronous access to the specific modules. The exposed access is separated into two steps:

1. loading the module (asynchronous)
2. evaluating the module (synchronous).

Step 1 will be done during the chunk loading. Step 2 will be done during the module evaluation interleaved with other (local and remote) modules. This way, evaluation order is unaffected by converting a module from local to remote or the other way around.

It is possible to nest containers. Containers can use modules from other containers. Circular dependencies between containers are also possible.

High-level concepts

Each build acts as a container and also consumes other builds as containers. This way, each build is able to access any other exposed module by loading it from its container.

Shared modules are modules that are both overridable and provided as overrides to nested containers. They usually point to the same module in each build, e.g., the same library.

ThepackageName option allows setting a package name to look for arequiredVersion. It is automatically inferred for the module requests by default, setrequiredVersion tofalse when automatic infer should be disabled.

Building blocks

ContainerPlugin (low level)

This plugin creates an additional container entry with the specified exposed modules.

ContainerReferencePlugin (low level)

This plugin adds specific references to containers as externals and allows to import remote modules from these containers. It also calls theoverride API of these containers to provide overrides to them. Local overrides (via__webpack_override__ oroverride API when build is also a container) and specified overrides are provided to all referenced containers.

ModuleFederationPlugin (high level)

ModuleFederationPlugin combinesContainerPlugin andContainerReferencePlugin.

Concept goals

• It should be possible to expose and consume any module type that webpack supports.
• Chunk loading should load everything needed in parallel (web: single round-trip to server).
• Control from consumer to container
  • Overriding modules is a one-directional operation.
  • Sibling containers cannot override each other's modules.
• Concept should be environment-independent.
  • Usable in web, Node.js, etc.
• Relative and absolute request in shared:
  • Will always be provided, even if not used.
  • Will resolve relative toconfig.context.
  • Does not use arequiredVersion by default.
• Module requests in shared:
  • Are only provided when they are used.
  • Will match all used equal module requests in your build.
  • Will provide all matching modules.
  • Will extractrequiredVersion from package.json at this position in the graph.
  • Could provide and consume multiple different versions when you have nested node_modules.
• Module requests with trailing/ in shared will match all module requests with this prefix.

Use cases

Separate builds per page

Each page of a Single Page Application is exposed from container build in a separate build. The application shell is also a separate build referencing all pages as remote modules. This way each page can be separately deployed. The application shell is deployed when routes are updated or new routes are added. The application shell defines commonly used libraries as shared modules to avoid duplication of them in the page builds.

Components library as container

Many applications share a common components library which could be built as a container with each component exposed. Each application consumes components from the components library container. Changes to the components library can be separately deployed without the need to re-deploy all applications. The application automatically uses the up-to-date version of the components library.

Dynamic Remote Containers

The container interface supportsget andinit methods.init is anasync compatible method that is called with one argument: the shared scope object. This object is used as a shared scope in the remote container and is filled with the provided modules from a host.It can be leveraged to connect remote containers to a host container dynamically at runtime.

init.js

(async()=>{// Initializes the shared scope. Fills it with known provided modules from this build and all remotesawait__webpack_init_sharing__('default');const container= window.someContainer;// or get the container somewhere else// Initialize the container, it may provide shared modulesawait container.init(__webpack_share_scopes__.default);const module=await container.get('./module');})();

The container tries to provide shared modules, but if the shared module has already been used, a warning and the provided shared module will be ignored. The container might still use it as a fallback.

This way you could dynamically load an A/B test which provides a different version of a shared module.

functionloadComponent(scope, module){returnasync()=>{// Initializes the shared scope. Fills it with known provided modules from this build and all remotesawait__webpack_init_sharing__('default');const container= window[scope];// or get the container somewhere else// Initialize the container, it may provide shared modulesawait container.init(__webpack_share_scopes__.default);const factory=await window[scope].get(module);const Module=factory();return Module;};}loadComponent('abtests','test123');

See full implementation

Promise Based Dynamic Remotes

Generally, remotes are configured using URL's like in this example:

module.exports={  plugins:[newModuleFederationPlugin({      name:'host',      remotes:{        app1:'app1@http://localhost:3001/remoteEntry.js',},}),],};

But you can also pass in a promise to this remote, which will be resolved at runtime. You should resolve this promise with any module that fits theget/init interface described above. For example, if you wanted to pass in which version of a federated module you should use, via a query parameter you could do something like the following:

module.exports={  plugins:[newModuleFederationPlugin({      name:'host',      remotes:{        app1:`promise new Promise(resolve => {      const urlParams = new URLSearchParams(window.location.search)      const version = urlParams.get('app1VersionParam')      // This part depends on how you plan on hosting and versioning your federated modules      const remoteUrlWithVersion = 'http://localhost:3001/' + version + '/remoteEntry.js'      const script = document.createElement('script')      script.src = remoteUrlWithVersion      script.onload = () => {        // the injected script has loaded and is available on window        // we can now resolve this Promise        const proxy = {          get: (request) => window.app1.get(request),          init: (...arg) => {            try {              return window.app1.init(...arg)            } catch(e) {              console.log('remote container already initialized')            }          }        }        resolve(proxy)      }      // inject this script with the src set to the versioned remoteEntry.js      document.head.appendChild(script);    })`,},// ...}),],};

Note that when using this API youhave to resolve an object which contains the get/init API.

Dynamic Public Path

Offer a host API to set the publicPath

One could allow the host to set the publicPath of a remote module at runtime by exposing a method from that remote module.

This approach is particularly helpful when you mount independently deployed child applications on the sub path of the host domain.

Scenario:

You have a host app hosted onhttps://my-host.com/app/* and a child app hosted onhttps://foo-app.com. The child app is also mounted on the host domain, hence,https://foo-app.com is expected to be accessible viahttps://my-host.com/app/foo-app andhttps://my-host.com/app/foo-app/* requests are redirected tohttps://foo-app.com/* via a proxy.

Example:

webpack.config.js (remote)

module.exports={  entry:{    remote:'./public-path',},  plugins:[newModuleFederationPlugin({      name:'remote',// this name needs to match with the entry name      exposes:['./public-path'],// ...}),],};

public-path.js (remote)

exportfunctionset(value){  __webpack_public_path__= value;}

src/index.js (host)

const publicPath=awaitimport('remote/public-path');publicPath.set('/your-public-path');//bootstrap app  e.g. import('./bootstrap.js')

Infer publicPath from script

One could infer the publicPath from the script tag fromdocument.currentScript.src and set it with the__webpack_public_path__ module variable at runtime.

Example:

webpack.config.js (remote)

module.exports={  entry:{    remote:'./setup-public-path',},  plugins:[newModuleFederationPlugin({      name:'remote',// this name needs to match with the entry name// ...}),],};

setup-public-path.js (remote)

// derive the publicPath with your own logic and set it with the __webpack_public_path__ API__webpack_public_path__= document.currentScript.src+'/../';

There is also an'auto' value available tooutput.publicPath which automatically determines the publicPath for you.

Troubleshooting

Uncaught Error: Shared module is not available for eager consumption

The application is eagerly executing an application that is operating as an omnidirectional host. There are options to choose from:

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.

We 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 Reactfrom'react';import ReactDOMfrom'react-dom';import Appfrom'./App';ReactDOM.render(<App/>, document.getElementById('root'));

Let's createbootstrap.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.

Settingeager: true for dependency via theModuleFederationPlugin

webpack.config.js

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

Uncaught Error: Module "./Button" does not exist in container.

It likely does not say"./Button", but the error message will look similar. This issue is typically seen if you are upgrading from webpack beta.16 to webpack beta.17.

Within ModuleFederationPlugin. Change the exposes from:

new ModuleFederationPlugin({ exposes: {-   'Button': './src/Button'+   './Button':'./src/Button' }});

Uncaught TypeError: fn is not a function

You are likely missing the remote container, make sure it's added.If you have the container loaded for the remote you are trying to consume, but still see this error, add the host container's remote container file to the HTML as well.

Collision between modules from different remotes

If you're going to load multiple modules from different remotes, it's advised to set theoutput.uniqueName option for your remote builds to avoid collisions between multiple webpack runtimes.

Further Reading

• Webpack 5 Module Federation: A game-changer in JavaScript architecture
• Explanations and Examples
• Module Federation YouTube Playlist

11 Contributors