Micro Frontend Architecture Patterns: A Beginner’s Guide to Designing Scalable Web Apps

In today’s dynamic digital landscape, the development of scalable web applications poses unique challenges, especially when multiple engineers collaborate on a single codebase. This is where micro frontend architecture comes into play. By breaking down the user interface into smaller, independently deliverable components, teams can enhance their agility and improve deployment efficiency. In this article, frontend engineers, tech leads, and architects will discover core concepts, various composition approaches, and practical migration advice for implementing micro frontends effectively.

What are Micro Frontends?

A micro frontend represents a self-contained UI module that can be developed, tested, and deployed independently. For example, an extensive e-commerce site can separate features like product details, the shopping cart, search functions, and user profiles into distinct micro frontends instead of bundling them into a monolithic application.

Key Principles:

• Team Autonomy: Teams own vertical slices (feature domains) from start to finish, including UI, tests, and deployment.
• Independent Deployability: Each micro frontend operates with its own build/test/release pipeline.
• Technology Agnosticism: Teams can select frameworks as needed while adhering to governance for diversity.
• Minimal Coupling: Micro frontends should limit shared, tightly-coupled code and rely on clear contracts (APIs/events).

Common Terminology:

• Host/App Shell: The container that composes micro frontends while managing global concerns like navigation and authentication.
• Micro Frontend (Remote/Module): The self-contained UI component integrated into the host.
• Composition: The process of assembling micro frontends into a complete user interface.
• Route-based vs. Region-based: Route-based micro frontends control entire routes, while region-based micro frontends inject into specific page areas (like headers or widgets).

Mirroring the principles of microservices, the micro frontend approach enables smaller, focused teams to iterate rapidly without dependency on other team efforts.

Why Use Micro Frontends? Benefits & Trade-offs

Benefits:

• Team Autonomy and Parallel Development: Teams can release features without blocking others.
• Independent Scaling: UI components can scale separately according to their unique traffic profiles.
• Incremental Migration: Facilitates replacing or modernizing a monolithic application incrementally.
• Technology Migration: Teams can experiment with new frameworks within a manageable scope.

Trade-offs and Costs:

• Increased Operational Complexity: More pipelines and runtime composition concerns arise.
• Composition Complexity: DOM conflicts, shared dependencies, and cross-app communication may complicate development.
• Potential Bundle Duplication: Multiple micro frontends may unintentionally include shared libraries.
• Governance Overhead: API versioning and UX consistency must be maintained across teams.

When They Make Sense: Micro frontends are ideal for large applications with numerous teams and those requiring frequent independent releases. However, they can be excessive for small teams or straightforward applications where the added complexity does not provide substantial benefits.

Core Architecture Patterns (Detailed)

1) Client-side Composition (Runtime Composition in the Browser)

How it Works:

• The host loads a runtime orchestrator that fetches micro frontend bundles during runtime and integrates them into DOM placeholders, typically based on routes.
• Micro frontends can be lazy-loaded and mounted as the user navigates.

Pros:

• High runtime flexibility with complete deployment independence.
• Ideal for single-page applications and gradual migration.

Cons:

• Initial renders may experience delays if numerous remotes are fetched.
• Risks of global JS/CSS conflicts without proper isolation.
• SEO and first contentful paint (FCP) may be adversely affected without server-side rendering.

Best Use Cases: Large SPAs, teams needing independent release cadences, gradual migration.

2) Server-side Composition (Stitching HTML on the Server)

How it Works:

• The server assembles HTML fragments from multiple services and stitches them together into a single HTML response before sending it to the browser.

Pros:

• Enhanced SEO and faster first render than client-only composition.
• Avoids heavy runtime orchestration in the browser.

Cons:

• Requires backend coordination and a composition layer, adding server complexity.
• Client-side hydration needs management for interactive elements.

Best Use Cases: Content-heavy sites where SEO and fast FCP are paramount.

3) Edge-side Composition (CDN/Edge Stitches Fragments)

How it Works:

• HTML fragments are composed at the CDN/edge level using edge functions, enhancing geographic responsiveness.

Pros:

• Offers low latency and improved scalability.

Cons:

• Can complicate cache invalidation strategies.
• Requires robust edge platform support.

Best Use Cases: High-traffic consumer sites requiring global performance.

4) Build-time Composition

How it Works:

• During CI, micro frontends are integrated into a single deployable artifact, allowing teams to develop independently while assembling everything at build time.

Pros:

• Simpler runtime results in fewer cross-app conflicts.
• Easier to ensure a consistent user experience (UX).

Cons:

• Loss of some deployment independence due to coordinated builds.

Best Use Cases: Teams favoring development independence despite centralized release cadences.

5) Module Federation (Webpack Runtime Sharing)

How it Works:

• Webpack Module Federation enables runtime loading of remote modules and sharing dependencies across independently built bundles.

Pros:

• Reduces duplication of shared dependencies and allows dynamic code reuse.

Cons:

• Potential version compatibility issues and caching complexities.

Best Use Cases: Multi-team SPAs desiring the performance of shared runtime modules without enforcing a single build.

Example: Minimal Webpack Module Federation Config (Host)

// host/webpack.config.js
const ModuleFederationPlugin = require('webpack').container.ModuleFederationPlugin;
module.exports = {
    // ...other config
    plugins: [
        new ModuleFederationPlugin({
            name: 'host',
            remotes: {
                product: 'product@https://cdn.example.com/product/remoteEntry.js'
            },
            shared: { react: { singleton: true, eager: false }, 'react-dom': { singleton: true } }
        })
    ]
};

Example: Remote Exposing a Module

// product/webpack.config.js
new ModuleFederationPlugin({
    name: 'product',
    filename: 'remoteEntry.js',
    exposes: { './ProductCard': './src/ProductCard' },
    shared: ['react', 'react-dom']
});

For deeper insights and considerations, refer to the official Webpack Module Federation documentation.

6) Web Components Approach

How it Works:

• Micro frontends are built as standards-based Web Components (Custom Elements + Shadow DOM), permitting consumption by any host regardless of framework.

Pros:

• Strong encapsulation (via Shadow DOM) prevents CSS leakage.
• Interoperable across frameworks.

Cons:

• Framework lifecycle and integration variances may necessitate polyfills for older browsers.

Best Use Cases: Cross-framework ecosystems, widget libraries, or maximizing reuse across disparate apps.

7) Iframe-based Isolation

How it Works:

• Each micro frontend operates within its own iframe, ensuring stringent isolation of JS/CSS and establishing security boundaries.

Pros:

• Robust isolation and sandboxing minimizes most runtime conflicts.

Cons:

• Potential user experience challenges (scrolling, focus issues) and increased resource usage.

Best Use Cases: Embedding scenarios and third-party widgets, especially when incorporating untrusted code.

Pattern Selection Guidance

• SEO/Performance Needs: Opt for server-side or edge-side composition.
• Runtime Flexibility & Independent Deployments: Consider client-side or Module Federation.
• Strict Isolation/Security Required: Choose iframes.
• Framework-Agnostic Encapsulation Preferred: Implement Web Components.

Comparison Table:

Integration Techniques & Tools

Selecting the right tools can significantly align with your composition strategy and the team’s skills.

• single-spa: A popular orchestrator for mounting multiple frameworks on the same page, supporting gradual migrations and multi-framework environments. Learn more at single-spa documentation.
• Module Federation: An official Webpack feature supporting runtime sharing of modules across bundles. Official docs can be found here.
• Web Components: Built on standards (Custom Elements, Shadow DOM). Various frameworks offer support for building web components.
• Server/Edge Stitching: Approaches like Server Side Includes (SSI), Edge Side Includes (ESI), or custom services via reverse proxies or edge functions.
• Practical Tip: Standardizing a primary composition approach avoids cognitive and operational overload. For instance, combining Module Federation and single-spa can introduce additional governance and testing requirements.

Example: Simplified single-spa Registration

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

registerApplication({
    name: '@org/product',
    app: () => import('product/ProductApp'),
    activeWhen: location => location.pathname.startsWith('/product')
});

start();

Team Organization, Deployment & CI/CD Considerations

Team Boundaries and Ownership:

• Structure teams around business domains (vertical slices) to minimize cross-team dependencies, ensuring each team manages its feature backlog, testing, and deployments.

Repository Strategies:

• Monorepo vs Multi-repo: Both choices hold merit. Monorepos simplify code sharing, while multi-repos enhance deployment independence. For insights, check our guide on Monorepo vs Multi-repo Strategies.

CI/CD and Pipelines:

• Each micro frontend needs its pipeline for building, testing, and publishing artifacts. Maintain an artifact registry or CDN for remotes, with a versioning strategy for backward compatibility. Leverage feature flags and canary releases to mitigate risks during new micro frontend deployments.

Routing and App Shell Responsibilities:

• The host should manage global routing and authentication, with micro frontends handling their internal routing and state. It’s advisable to avoid leaking global authentication states by using token-based flows or a shared authentication service.

Shared Libraries and APIs:

• When sharing utilities, prefer published packages or Module Federation rather than duplicating code across repositories. Use semantic versioning for stability in API contracts. Architectural patterns like Ports and Adapters can help decouple UI from domain logic, as discussed in our guide on Ports and Adapters Architecture.

Documentation and Communication:

• Maintain a lightweight style and UX document, conducting cross-team reviews for shared architectural transparency. For tips on engaging technical presentations, refer to Creating Engaging Technical Presentations.

Common Challenges, Testing & Security

Cross-app Communication and State

• Limit shared state. Use events (CustomEvent), a message bus, or REST/GraphQL APIs as contracts. For small persistence or client-side storage, refer to options in our discussion on Browser Storage Options.

Performance and Caching

• Minimize bundle duplication through shared dependencies (Module Federation), code-splitting, and CDN caching, implementing lazy-loading on critical content for improved FCP.

Testing Approaches

• Unit Tests: Test micro frontends in isolation.
• Integration Tests: Verify rendering within the host context.
• E2E Tests: Assess full user journeys across micro frontends with staging environments mimicking production composition.

Security Considerations

• Validate inputs at boundaries and sanitize data appropriately.
• Implement strict Content Security Policy (CSP) and relevant CORS settings.
• Avoid injecting untrusted scripts into the host; consider iframe isolation for any untrusted code, applying proper sandbox attributes.

When Not to Use Micro Frontends & Migration Advice

When to Avoid Micro Frontends

• Smaller teams or simpler applications: The complexities of multiple pipelines and composition often do not justify the overhead.
• Projects prioritizing consistent UX and tight integration over independent deployments.

Migration Strategies from a Monolith

• Strangler Pattern: Gradually replace monolithic components by routing traffic to new micro frontends, starting with low-risk features like landing or admin pages.
• Route-based Extraction: Extract one route at a time for integration into the host.
• Proof-of-Concept: Start small by extracting non-critical pages or widgets and deploying them as micro frontends with feature flags and monitoring in place.

Quick Wins to Mitigate Risks:

• Define clear API contracts before splitting code.
• Establish CI/CD templates for micro frontends and standardized build/test tooling.
• Implement centralized monitoring for observability across micro frontends.

Conclusion & Next Steps

Micro frontends offer powerful patterns for scaling frontend development across teams, yet they inherently introduce complexity. When selecting a pattern, align it with your priorities: server and edge for SEO/performance, client-side or Module Federation for team autonomy, web components for framework-agnostic reuse, and iframes for isolation.

Quick Checklist to Evaluate Readiness:

• Do you have multiple teams (>2) collaborating on a large UI?
• Is there a need for independent deployments and gradual migrations?
• Can you invest in CI/CD, monitoring, and governance?
• Are you ready to initiate with a small proof-of-concept (non-critical section)?

To kick off, try a proof-of-concept by extracting a non-critical page or widget utilizing Module Federation or single-spa. Measure metrics such as FCP, TTFB, and bundle size to validate your trade-offs.

Further Reading and References

• Micro Frontends – Foundational Concepts
• Webpack Module Federation – Official Docs
• Single-spa – Orchestration Library

Internal Resources:

• Monorepo vs Multi-repo Strategies
• Ports and Adapters (Hexagonal) Architecture
• Browser Storage Options
• Creating Engaging Technical Presentations

Good luck building your micro frontend proof-of-concept — start small, measure results, and iterate.