Monolith to Micro Frontends

Understanding monolith to micro frontends helps you work with single-spa confidently. Here you will learn the core ideas behind monolith to micro frontends, see working code, and pick up best practices used on real teams.

Monolith to Micro Frontends Overview

Monolith to Micro Frontends is a building block you will reach for often in single-spa. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn monolith to micro frontends properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real single-spa projects.

import { registerApplication, start } from 'single-spa';

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

start();

single-spa orchestrates multiple framework apps on one page through a root config.

Monolith to Micro Frontends Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to monolith to micro frontends.

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 Monolith to Micro Frontends Works in Single-SPA

Monolith to Micro Frontends 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.

single-spa orchestrates multiple framework apps on one page through a root config.

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 Monolith to Micro Frontends

For reliable micro frontends, monolith to micro frontends 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

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

Key Takeaways

Monolith to Micro Frontends is a core part of working effectively with single-spa.
Start small and keep monolith to micro frontends focused on a single responsibility.
Apply consistent patterns so monolith to micro frontends scales across your project.
Test and document monolith to micro frontends to keep it maintainable over time.

Pro Tip

Pair monolith to micro frontends with automated tests from day one. It is far cheaper to catch single-spa regressions in CI than in production.