Shared Event Bus

In this lesson you will learn shared event bus in single-spa, why it matters within utility modules, and how to use it correctly with clear, copy-ready examples.

Shared Event Bus Overview

Shared Event Bus lets you structure single-spa 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 shared event bus focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

// shared utility module exposed via the import map
export const authService = {
  getToken: () => localStorage.getItem('token'),
};

// consumed by any micro frontend
import { authService } from '@org/utils';

Shared utility modules provide cross-app services like auth without tight coupling.

Shared Event Bus Example

registerApplication({
  name: '@org/app',
  app: () => System.import('@org/app'),
  activeWhen: ['/app'],
});
start();

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

Single-SPA Cheatsheet

Core single-spa APIs related to shared event bus.

API	Example	Purpose
registerApplication	`registerApplication({ name, app, activeWhen })`	Register a micro frontend
activeWhen	`activeWhen: ['/checkout']`	Route ownership
start	`start()`	Begin routing
bootstrap	`export async function bootstrap()`	One-time setup
mount	`export async function mount(props)`	Render the app
unmount	`export async function unmount(props)`	Clean up the app
import map	`systemjs-importmap`	Locate app bundles

How Shared Event Bus Works in Single-SPA

Shared Event Bus is part of how single-spa lets multiple applications — even in different frameworks — coexist on one page. A root config registers each app and controls when it is active.

Shared utility modules provide cross-app services like auth without tight coupling.

A root config registers apps and calls start().
Each app exports bootstrap, mount, and unmount lifecycles.
activeWhen decides which routes each app owns.
Import maps resolve each app's bundle at runtime.

Practical Guidance for Shared Event Bus

For reliable micro frontends, shared event bus should isolate failures and keep shared state minimal. Let each team own its app end to end while agreeing on a few shared contracts.

Concern	Recommendation
Isolation	One app's crash should not break others
Shared state	Prefer shared utility modules over globals
Routing	Keep activeWhen rules explicit and non-overlapping
Deployment	Release via import-map updates per app

Common Mistakes

Copying shared event bus snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up shared event bus.
Leaving shared event bus untested, so regressions slip into production.
Over-engineering shared event bus before you actually need the extra flexibility.

Key Takeaways

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

Pro Tip

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