Micro Frontends: Why, When, and How?

Micro Frontends is an architectural approach in which a frontend application is broken down into smaller, independent, and decoupled parts, where each part represents a distinct feature or function. These individual pieces, called micro frontends, can be developed, deployed, and maintained independently.

Let’s explore why, when, and how to implement Micro Frontends.

Why Use Micro Frontends?

Scalability: As frontend applications grow, they can become monolithic, difficult to maintain, and slow to develop. Micro frontends break down large applications into smaller parts, enabling teams to scale development efforts across different modules.
Team Autonomy: Different teams can own, develop, and maintain different parts of the frontend independently, allowing faster development cycles. Teams can choose different technologies, frameworks, or libraries based on the needs of their module.
Independent Deployment: Each micro frontend can be deployed independently without affecting the rest of the application. This leads to faster releases and fewer deployment bottlenecks.
Reuse: Components from different micro frontends can be reused across other applications, enabling consistent functionality and user experience.
Technology Agnostic: Teams are free to choose different frontend technologies (e.g., React, Angular, Vue) for their part of the frontend. This flexibility allows teams to adopt newer or more suitable technologies without rewriting the entire application.
Resilience: If a single micro frontend fails or has a bug, it doesn’t bring down the entire application. Other parts of the application can continue to work independently.

When to Use Micro Frontends?

Micro frontends are a good solution when you encounter the following situations:

Large Teams and Complex Applications: When you have a large development team or multiple teams working on a massive frontend application, micro frontends help split responsibilities across teams.
Independent Domain Areas: If your application consists of distinct domain areas (e.g., a shopping platform with separate modules for product listings, checkout, user profile), micro frontends allow each domain to evolve independently.
Multiple Frameworks or Legacy Systems: If your organization is using a mix of frameworks or wants to migrate from a legacy system incrementally, micro frontends allow parts of the frontend to be developed in newer frameworks without a complete rewrite.
Frequent Releases: If certain parts of your application require frequent updates or releases, micro frontends allow you to deploy just the changed parts without affecting the entire frontend.
Maintenance and Code Ownership Challenges: When code ownership becomes fragmented, and a single monolithic codebase is becoming hard to maintain, micro frontends offer a way to create well-defined boundaries of ownership.

How to Implement Micro Frontends?

There are multiple ways to implement micro frontends, depending on your use case and the technologies you are comfortable with. Here’s how you can implement micro frontends:

1. Architecture Approaches

a. Client-Side Integration (JavaScript Frameworks)

In this approach, each micro frontend is loaded directly in the browser, using iframes or other techniques like module federation.

Module Federation (Webpack 5): This is a popular way to implement micro frontends by loading modules dynamically at runtime. It allows applications to share dependencies and load other applications’ modules as part of the build process.

Example:
```
const MicroFrontend = React.lazy(() => import('app1/ButtonComponent'));
```
Single-SPA: A framework for bringing together multiple micro frontends into a single application. It allows you to mix and match frameworks like React, Angular, and Vue.

Example:
```
registerApplication(
  'app1',
  () => import('app1'),
  (location) => location.pathname.startsWith('/app1')
);
start();
```

b. Server-Side Integration (Edge-Side Includes, SSR)

This approach involves assembling micro frontends on the server side. Each fragment of the page is fetched and composed on the server or at the edge.

Edge-Side Includes (ESI): This is a web standard that allows pieces of content to be injected dynamically into a page at the edge (usually using a CDN).
SSR (Server-Side Rendering): The server renders parts of the frontend independently, fetching the relevant micro frontends before sending the full page to the client.

c. Iframe Integration

Though less commonly used in modern applications, iframes can isolate each micro frontend in its own window. This allows micro frontends to be completely decoupled but comes with performance and usability concerns.

2. Communication and State Management

A key challenge of micro frontends is managing shared state or communication between the micro frontends.

Custom Events and Pub/Sub: Micro frontends can communicate using custom events in the browser or by using a publish-subscribe pattern where one micro frontend publishes events and others subscribe to them.

Example:
```
window.dispatchEvent(new CustomEvent('userLoggedIn', { detail: { userId: 1234 } }));
```
Shared State Stores: Use a shared state management solution like Redux, RxJS, or Context API to manage state across micro frontends.

Example with Redux Store:
```
const store = createStore(rootReducer);
<Provider store={store}>
  <App />
</Provider>
```
URL-based Communication: Micro frontends can also communicate by encoding state in the URL, such as query parameters or URL fragments, allowing different micro frontends to respond based on the current state.

3. Deployment and Versioning

Independent Deployment: Micro frontends should be deployed independently. You can use CI/CD pipelines to automate this process, where each micro frontend has its own release process.
Versioning: Use versioning strategies to ensure compatibility between different micro frontends, especially when sharing dependencies or common libraries.

4. Shared Dependencies

To avoid duplication, shared libraries or dependencies (e.g., React, CSS frameworks) should be managed. Module federation in Webpack 5 allows you to share dependencies between different micro frontends.

Example:

module.exports = {
  name: 'app1',
  exposes: {
    './ButtonComponent': './src/ButtonComponent',
  },
  shared: { react: { singleton: true }, 'react-dom': { singleton: true } },
};

Example Scenario of Micro Frontends in Action

Imagine you’re building a large e-commerce platform with several distinct areas such as product listings, checkout, and user profile. You can break down the frontend like this:

Product Listing Micro Frontend (owned by Team A) built with React.
Checkout Micro Frontend (owned by Team B) built with Angular.
User Profile Micro Frontend (owned by Team C) built with Vue.

Each team can develop, test, and deploy their micro frontend independently, reducing bottlenecks. When the user navigates through the e-commerce site, these micro frontends load dynamically, either using client-side integration (like module federation) or server-side composition.

Challenges of Micro Frontends

Increased Complexity: Managing multiple repositories, build pipelines, and deployments can introduce additional complexity.
Shared State Management: Handling shared state or cross-component communication can become difficult in large applications.
Performance Overhead: Loading multiple independent frontend bundles can increase load times if not optimized carefully.
Testing: Integration testing becomes more complex when micro frontends need to work together seamlessly.

Conclusion

Micro frontends offer an excellent solution for scaling large frontend applications, especially when you have multiple teams working on different parts of the frontend. They enable independent development, deployment, and technology flexibility, providing faster time to market and maintainable codebases. However, they also introduce challenges such as complexity in state management, deployment, and communication, which need careful planning and implementation.

When building large, complex applications or dealing with frequent deployments, micro frontends offer the architectural flexibility to scale your frontend development while keeping teams autonomous.