Application Shell

Application Shell sits at the heart of micro frontends in Module Federation. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

Application Shell Overview

Application Shell lets you structure Module Federation work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep application shell focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

const { ModuleFederationPlugin } = require('webpack').container;

module.exports = {
  plugins: [
    new ModuleFederationPlugin({
      name: 'shell',
      remotes: { catalog: 'catalog@http://localhost:3001/remoteEntry.js' },
      exposes: {},
      shared: { react: { singleton: true }, 'react-dom': { singleton: true } },
    }),
  ],
};

ModuleFederationPlugin configures which modules an app exposes, consumes, and shares.

Application Shell Example

new ModuleFederationPlugin({
  name: 'app',
  filename: 'remoteEntry.js',
  exposes: { './Widget': './src/Widget' },
  remotes: { other: 'other@http://host/remoteEntry.js' },
  shared: { react: { singleton: true } },
});

Start from a minimal Application Shell example and grow it only as needed.
Keep configuration explicit so Application Shell behaves the same in every environment.
Name things clearly so teammates understand your Application Shell at a glance.
Add tests around Application Shell early to lock in expected behaviour.

Module Federation Cheatsheet

Key Module Federation settings related to application shell.

Option	Example	Purpose
name	`name: 'shell'`	Unique container name
filename	`filename: 'remoteEntry.js'`	Remote entry manifest
exposes	`exposes: { './X': './src/X' }`	Modules a remote shares
remotes	`remotes: { app: 'app@url' }`	Remotes a host consumes
shared	`shared: { react: { singleton: true } }`	Deduplicate libraries
lazy load	`import('remote/Module')`	Load remotes on demand
Suspense	`<Suspense fallback={...}>`	Handle async loading

How Application Shell Works in Module Federation

Application Shell builds on Module Federation's ability to load code from another independently built and deployed application at runtime. Each app can be a host, a remote, or both.

ModuleFederationPlugin configures which modules an app exposes, consumes, and shares.

Remotes expose modules through a remoteEntry.js manifest.
Hosts declare remotes and import exposed modules dynamically.
Shared dependencies are deduplicated, ideally as singletons.
Each micro frontend builds and deploys on its own schedule.

Practical Guidance for Application Shell

In production, application shell needs careful version management and graceful failure handling. Align shared dependency versions and always render a fallback when a remote cannot load.

Concern	Recommendation
Shared versions	Use singletons with requiredVersion
Runtime errors	Wrap remotes in error boundaries and fallbacks
Deployment	Resolve remotes from a runtime manifest
Performance	Lazy-load remotes and cache remoteEntry.js

Common Mistakes

Skipping error handling and edge cases when wiring up application shell.
Leaving application shell untested, so regressions slip into production.
Over-engineering application shell before you actually need the extra flexibility.
Ignoring documentation, which makes application shell hard for the next developer to change.

Key Takeaways

Application Shell is a core part of working effectively with Module Federation.
Start small and keep application shell focused on a single responsibility.
Apply consistent patterns so application shell scales across your project.
Test and document application shell to keep it maintainable over time.

Pro Tip

When you get stuck on application shell, reduce it to the smallest reproducible example first — most Module Federation issues become obvious once the noise is gone.