Event Bus

Event Bus sits at the heart of communication in Module Federation. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

Event Bus Overview

Event Bus 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 event bus 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.

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

Module Federation Cheatsheet

Key Module Federation settings related to event bus.

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 Event Bus Works in Module Federation

Event Bus 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 Event Bus

In production, event bus 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 event bus.
Leaving event bus untested, so regressions slip into production.
Over-engineering event bus before you actually need the extra flexibility.
Ignoring documentation, which makes event bus hard for the next developer to change.

Key Takeaways

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

Pro Tip

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