Monorepo Architecture Skill

Monorepo Architecture Skill

Overview

This skill provides comprehensive guidance on designing and structuring monorepos, including workspace organization, dependency management, versioning strategies, and architectural patterns that scale from small projects to enterprise applications.

Monorepo vs Polyrepo

When to Choose Monorepo

A monorepo is beneficial when:

• Code sharing is frequent: Multiple projects share common libraries, utilities, or components
• Atomic changes needed: Changes span multiple packages and need to be deployed together
• Unified tooling: All projects benefit from consistent linting, testing, and build processes
• Team collaboration: Teams work across project boundaries and need visibility into related code
• Version synchronization: Related packages should maintain version alignment
• Refactoring at scale: Large-scale refactoring across projects is common
• Single source of truth: All code, documentation, and tooling in one place

When to Choose Polyrepo

A polyrepo is beneficial when:

• Independent release cycles: Projects deploy on completely different schedules
• Different tech stacks: Projects use incompatible tooling or languages
• Access control: Different teams need isolated access to separate codebases
• Repository size concerns: Combined codebase would be too large to manage efficiently
• External dependencies: Projects are maintained by different organizations
• Simple project structure: Overhead of monorepo tooling outweighs benefits

Tradeoffs

Monorepo Advantages:

• Simplified dependency management
• Easier refactoring across boundaries
• Consistent tooling and standards
• Better code discoverability
• Atomic commits across projects
• Single CI/​CD pipeline

Monorepo Challenges:

• Repository size growth
• CI/​CD complexity
• Build time management
• Git performance at scale
• Tooling requirements
• Learning curve for developers

Repository Structure Patterns

Package-Based Structure

Organize by technical layer or package type.

my-monorepo/
├── apps/
│   ├── web/                 # Next.js web application
│   ├── mobile/              # React Native app
│   └── api/                 # Node.js API server
├── packages/
│   ├── ui/                  # Shared UI components
│   ├── utils/               # Shared utilities
│   ├── config/              # Shared configurations
│   └── types/               # Shared TypeScript types
├── services/
│   ├── auth/                # Authentication service
│   ├── payments/            # Payment processing
│   └── notifications/       # Notification service
└── tooling/
    ├── eslint-config/       # ESLint configuration
    └── tsconfig/            # TypeScript configuration

Best for: Technical separation, shared library focus, platform diversity.

Domain-Based Structure

Organize by business domain or feature.

my-monorepo/
├── domains/
│   ├── user/
│   │   ├── api/            # User API
│   │   ├── web/            # User web UI
│   │   ├── mobile/         # User mobile UI
│   │   └── shared/         # User shared code
│   ├── billing/
│   │   ├── api/
│   │   ├── web/
│   │   └── shared/
│   └── analytics/
│       ├── api/
│       ├── web/
│       └── shared/
└── shared/
    ├── ui/                 # Cross-domain UI components
    ├── utils/              # Cross-domain utilities
    └── config/             # Cross-domain config

Best for: Domain-driven design, team ownership by feature, microservices architecture.

Hybrid Structure

Combine package-based and domain-based approaches.

my-monorepo/
├── apps/
│   ├── customer-portal/    # Customer-facing app
│   └── admin-dashboard/    # Admin app
├── features/
│   ├── auth/               # Authentication feature
│   ├── checkout/           # Checkout feature
│   └── inventory/          # Inventory feature
├── packages/
│   ├── ui/                 # Shared UI library
│   ├── api-client/         # API client library
│   └── analytics/          # Analytics library
└── infrastructure/
    ├── database/           # Database utilities
    ├── messaging/          # Message queue
    └── deployment/         # Deployment configs

Best for: Complex organizations, balancing technical and domain concerns.

Workspace Configuration

NPM Workspaces

Basic workspace setup using native NPM workspaces.

{
  "name": "my-monorepo",
  "private": true,
  "workspaces": [
    "apps/​*",
    "packages/​*"
  ],
  "scripts": {
    "dev": "npm run dev --workspaces",
    "build": "npm run build --workspaces",
    "test": "npm run test --workspaces"
  },
  "devDependencies": {
    "typescript": "^5.3.0",
    "eslint": "^8.54.0"
  }
}

Key Features:

• Native NPM support (v7+)
• Simple configuration
• Automatic workspace linking
• Shared dependency hoisting
• Workspace-specific commands

Yarn Workspaces

Yarn's workspace implementation with additional features.

{
  "name": "my-monorepo",
  "private": true,
  "workspaces": {
    "packages": [
      "apps/​*",
      "packages/​*"
    ],
    "nohoist": [
      "**/​react-native",
      "**/​react-native/​**"
    ]
  },
  "packageManager": "[email protected]"
}

With .yarnrc.yml for Yarn Berry:

nodeLinker: node-modules

plugins:
  - path: .yarn/​plugins/​@yarnpkg/​plugin-workspace-tools.cjs
    spec: "@yarnpkg/​plugin-workspace-tools"

enableGlobalCache: true

compressionLevel: mixed

Key Features:

• Fast installation
• Plugin system (Yarn Berry)
• Advanced workspace commands
• No-hoist for specific dependencies
• Zero-installs capability

PNPM Workspaces

PNPM's efficient workspace implementation.

# pnpm-workspace.yaml
packages:
  - 'apps/​*'
  - 'packages/​*'
  - 'services/​*'
  - '!**/​test/​**'

With workspace-specific .npmrc:

# .npmrc
shared-workspace-lockfile=true
link-workspace-packages=true
prefer-workspace-packages=true
strict-peer-dependencies=false
auto-install-peers=true

Key Features:

• Content-addressable storage
• Strict node_modules structure
• Faster than NPM/​Yarn
• Efficient disk space usage
• Built-in monorepo support

Cargo Workspaces (Rust)

Rust monorepo workspace configuration.

# Cargo.toml (root)
[workspace]
members = [
    "crates/​core",
    "crates/​api",
    "crates/​cli",
]
exclude = ["archived/​*"]

[workspace.package]
version = "1.0.0"
edition = "2021"
license = "MIT"

[workspace.dependencies]
serde = { version = "1.0", features = ["derive"] }
tokio = { version = "1.35", features = ["full"] }

Individual crate:

# crates/​core/​Cargo.toml
[package]
name = "my-core"
version.workspace = true
edition.workspace = true
license.workspace = true

[dependencies]
serde.workspace = true
tokio.workspace = true
my-api = { path = "../​api" }

Dependency Management

Internal Package Dependencies

Specify dependencies on other workspace packages.

{
  "name": "@myorg/​web-app",
  "version": "1.0.0",
  "dependencies": {
    "@myorg/​ui": "workspace:*",
    "@myorg/​utils": "workspace:^",
    "@myorg/​api-client": "1.2.3",
    "react": "^18.2.0"
  }
}

Workspace Protocol Variants:

• workspace:* - Any version in workspace
• workspace:^ - Compatible version (semver caret)
• workspace:~ - Patch-level version (semver tilde)
• Specific version - Exact workspace version

Shared Dependencies Hoisting

Configure how dependencies are hoisted to root.

{
  "name": "my-monorepo",
  "workspaces": {
    "packages": ["packages/​*"],
    "nohoist": [
      "**/​react-native",
      "**/​react-native/​**",
      "**/​@babel/​**"
    ]
  }
}

Hoisting Strategies:

• Full hoisting: All common dependencies at root (default)
• Selective hoisting: Specific packages hoisted, others isolated
• No hoisting: Each package has isolated dependencies
• Public hoisting: Only public dependencies hoisted

Version Synchronization

Keep related dependencies in sync across packages.

{
  "name": "my-monorepo",
  "private": true,
  "syncpack": {
    "semverGroups": [
      {
        "range": "",
        "dependencies": ["react", "react-dom"],
        "packages": ["**"]
      }
    ],
    "versionGroups": [
      {
        "label": "React ecosystem must match",
        "dependencies": ["react", "react-dom"],
        "dependencyTypes": ["prod", "dev"],
        "pinVersion": "18.2.0"
      }
    ]
  }
}

Use tools like syncpack to enforce consistency:

# Check for version inconsistencies
pnpm syncpack list-mismatches

# Fix version inconsistencies
pnpm syncpack fix-mismatches

# Update all versions
pnpm syncpack update

Peer Dependencies in Monorepos

Handle peer dependencies correctly across workspace packages.

{
  "name": "@myorg/​ui",
  "version": "1.0.0",
  "peerDependencies": {
    "react": "^18.0.0",
    "react-dom": "^18.0.0"
  },
  "peerDependenciesMeta": {
    "react-dom": {
      "optional": true
    }
  },
  "devDependencies": {
    "react": "^18.2.0",
    "react-dom": "^18.2.0"
  }
}

Best Practices:

• Declare peer dependencies in shared libraries
• Include peer deps as dev dependencies for testing
• Use peerDependenciesMeta for optional peers
• Document peer dependency requirements
• Test with minimum and maximum peer versions

Code Organization

Shared Code Patterns

Organize shared code for maximum reusability.

packages/
├── ui/
│   ├── src/
│   │   ├── components/      # Reusable components
│   │   ├── hooks/           # Custom React hooks
│   │   ├── styles/          # Shared styles
│   │   └── index.ts         # Public API
│   └── package.json
├── utils/
│   ├── src/
│   │   ├── string/          # String utilities
│   │   ├── date/            # Date utilities
│   │   ├── validation/      # Validation functions
│   │   └── index.ts         # Public API
│   └── package.json
└── config/
    ├── eslint-config/
    ├── tsconfig/
    └── prettier-config/

Shared Library Design:

• Clear public API via barrel exports
• Minimal external dependencies
• Well-documented interfaces
• Comprehensive unit tests
• Semantic versioning
• Changelog maintenance

Type Sharing Across Packages

Share TypeScript types efficiently.

// packages/​types/​src/​user.ts
export interface User {
  id: string;
  email: string;
  name: string;
  role: UserRole;
}

export enum UserRole {
  Admin = 'admin',
  User = 'user',
  Guest = 'guest',
}

export type CreateUserInput = Omit<User, 'id'>;
export type UpdateUserInput = Partial<CreateUserInput>;

// packages/​types/​package.json
{
  "name": "@myorg/​types",
  "version": "1.0.0",
  "main": "./​dist/​index.js",
  "types": "./​dist/​index.d.ts",
  "exports": {
    ".": {
      "types": "./​dist/​index.d.ts",
      "default": "./​dist/​index.js"
    },
    "./​user": {
      "types": "./​dist/​user.d.ts",
      "default": "./​dist/​user.js"
    }
  }
}

Configuration Sharing

Share build and tooling configuration across packages.

// packages/​tsconfig/​base.json
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "resolveJsonModule": true,
    "isolatedModules": true,
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true
  }
}

// apps/​web/​tsconfig.json
{
  "extends": "@myorg/​tsconfig/​base.json",
  "compilerOptions": {
    "outDir": "./​dist",
    "rootDir": "./​src"
  },
  "include": ["src"],
  "references": [
    { "path": "../​../​packages/​ui" },
    { "path": "../​../​packages/​utils" }
  ]
}

Build Dependencies

Dependency Graphs

Understand and visualize package dependencies.

{
  "name": "@myorg/​web",
  "dependencies": {
    "@myorg/​ui": "workspace:*",
    "@myorg/​api-client": "workspace:*"
  }
}

Generate dependency graph:

# Using pnpm
pnpm list --depth 10 --json > deps.json

# Using Nx
nx graph

# Using custom script
node scripts/​generate-dep-graph.js

Build Order Optimization

Ensure packages build in correct dependency order.

{
  "name": "my-monorepo",
  "scripts": {
    "build": "turbo run build",
    "build:order": "pnpm -r --workspace-concurrency=1 run build"
  }
}

Turbo pipeline configuration:

{
  "$schema": "https://turbo.build/​schema.json",
  "pipeline": {
    "build": {
      "dependsOn": ["^build"],
      "outputs": ["dist/​**", ".next/​**"]
    },
    "test": {
      "dependsOn": ["build"],
      "outputs": ["coverage/​**"]
    }
  }
}

Dependency Resolution:

• ^build - Build dependencies first
• dependsOn - Explicit task dependencies
• Topological sorting for correct order
• Parallel execution when safe

Circular Dependency Detection

Prevent and detect circular dependencies.

// scripts/​check-circular-deps.js
import madge from 'madge';

async function checkCircularDeps() {
  const result = await madge('src', {
    fileExtensions: ['ts', 'tsx'],
    detectiveOptions: {
      ts: { skipTypeImports: true }
    }
  });

  const circular = result.circular();
  if (circular.length > 0) {
    console.error('Circular dependencies detected:');
    circular.forEach(cycle => {
      console.error(cycle.join(' -> '));
    });
    process.exit(1);
  }
}

checkCircularDeps();

Add to CI pipeline:

# .github/​workflows/​ci.yml
- name: Check circular dependencies
  run: pnpm check:circular

Versioning Strategies

Independent Versioning

Each package has its own version, released independently.

{
  "name": "@myorg/​ui",
  "version": "2.1.0"
}

{
  "name": "@myorg/​utils",
  "version": "1.5.3"
}

Advantages:

• Fine-grained version control
• Independent release cycles
• Clear package maturity
• Semantic versioning per package

Use when:

• Packages have different stability levels
• Release frequency varies significantly
• Packages serve different purposes

Fixed/​Locked Versioning

All packages share the same version number.

{
  "name": "@myorg/​ui",
  "version": "3.2.0"
}

{
  "name": "@myorg/​utils",
  "version": "3.2.0"
}

Advantages:

• Simplified version management
• Clear release coordination
• Easier to track compatibility
• Unified changelog

Use when:

• Packages are tightly coupled
• All packages release together
• Single product with multiple packages
• Version sync is critical

Semantic Versioning in Monorepos

Apply semver principles to workspace packages.

Breaking Changes (Major):

• Change public API signatures
• Remove exported functions
• Change function behavior significantly
• Update peer dependencies with breaking changes

New Features (Minor):

• Add new exports
• Add optional parameters
• Enhance existing functionality
• Add new optional features

Bug Fixes (Patch):

• Fix bugs without API changes
• Update documentation
• Refactor internal implementation
• Update dependencies (non-breaking)

Best Practices

1. Clear Package Boundaries

Define explicit boundaries and responsibilities for each package.

Implementation:

• Document package purpose and scope
• Define public API explicitly
• Use barrel exports (index.ts)
• Minimize cross-package coupling
• Review package boundaries regularly

Example:

// packages/​ui/​src/​index.ts - Clear public API
export { Button } from './​components/​Button';
export { Input } from './​components/​Input';
export type { ButtonProps, InputProps } from './​types';

// Internal implementation details NOT exported
// ./​components/​Button/​ButtonStyles.ts
// ./​utils/​internal-helper.ts

2. Minimal Coupling Between Packages

Reduce dependencies between packages to maintain flexibility.

Implementation:

• Use dependency injection
• Prefer composition over inheritance
• Define clear interfaces
• Avoid deep dependency chains
• Use events/​messaging for loose coupling

Example:

// Loose coupling via interfaces
interface Logger {
  log(message: string): void;
}

class PaymentService {
  constructor(private logger: Logger) {}

  processPayment(amount: number) {
    this.logger.log(`Processing payment: ${amount}`);
    // Implementation
  }
}

3. Shared Tooling Configuration

Maintain consistent tooling across all packages.

Implementation:

• Create shared config packages
• Extend base configurations
• Use workspace inheritance
• Document configuration decisions
• Automate configuration validation

Example:

// packages/​eslint-config/​index.json
{
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/​recommended"
  ],
  "rules": {
    "no-console": "warn",
    "@typescript-eslint/​no-unused-vars": "error"
  }
}

4. Consistent Naming Conventions

Use predictable naming patterns across packages.

Implementation:

• Scope all packages (@org/​package-name)
• Use kebab-case for package names
• Prefix related packages consistently
• Follow language/​ecosystem conventions
• Document naming standards

Example:

@myorg/​web-app
@myorg/​mobile-app
@myorg/​ui-components
@myorg/​api-client
@myorg/​utils-date
@myorg/​utils-string
@myorg/​config-eslint
@myorg/​config-typescript

5. Documentation Standards

Maintain comprehensive documentation for all packages.

Implementation:

• README in every package
• API documentation
• Usage examples
• Migration guides
• Contribution guidelines

Example:

# @myorg/​ui

React component library for MyOrg applications.

## Installation

`pnpm add @myorg/​ui`

## Usage

import { Button } from '@myorg/​ui';

<Button onClick={handleClick}>Click me</​Button>

## API Reference

See [API.md](./​API.md) for detailed documentation.

6. Package Ownership

Assign clear ownership and responsibility for packages.

Implementation:

• CODEOWNERS file
• Package maintainer documentation
• Review process for changes
• Communication channels
• Ownership rotation plan

Example:

# CODEOWNERS
/​packages/​ui/ @frontend-team
/​packages/​api-client/ @api-team
/​packages/​auth/ @security-team
/​services/ @backend-team

7. API Contracts

Define and maintain clear API contracts between packages.

Implementation:

• TypeScript interfaces
• OpenAPI specifications
• JSON Schema
• Contract testing
• Version compatibility matrix

Example:

// packages/​api-client/​src/​contracts.ts
/​**
 * User API contract
 * @version 1.0.0
 */
export interface UserAPI {
  getUser(id: string): Promise<User>;
  createUser(data: CreateUserInput): Promise<User>;
  updateUser(id: string, data: UpdateUserInput): Promise<User>;
  deleteUser(id: string): Promise<void>;
}

8. Migration Strategies

Plan for package updates and breaking changes.

Implementation:

• Deprecation warnings
• Migration guides
• Compatibility layers
• Automated migrations (codemods)
• Version upgrade paths

Example:

// Deprecation with migration path
/​**
 * @deprecated Use `getUser` instead
 * This function will be removed in v3.0.0
 */
export function fetchUser(id: string): Promise<User> {
  console.warn('fetchUser is deprecated, use getUser instead');
  return getUser(id);
}

9. Security Boundaries

Maintain security isolation between packages.

Implementation:

• Separate sensitive packages
• Access control policies
• Security scanning per package
• Dependency audit
• Secret management

Example:

{
  "scripts": {
    "security:audit": "pnpm audit --audit-level=high",
    "security:check": "pnpm dlx audit-ci --high"
  }
}

10. Testing Isolation

Ensure tests don't have unintended dependencies.

Implementation:

• Package-level test configuration
• Mock workspace dependencies
• Integration test suites
• CI test isolation
• Test data management

Example:

// packages/​ui/​src/​__tests__/​Button.test.tsx
import { render, screen } from '@testing-library/​react';
import { Button } from '../​Button';

// Test isolated from other packages
describe('Button', () => {
  it('renders correctly', () => {
    render(<Button>Click me</​Button>);
    expect(screen.getByText('Click me')).toBeInTheDocument();
  });
});

Common Pitfalls

1. Tight Coupling Between Packages

Creating excessive dependencies between packages.

Symptoms:

• Changes require updates across many packages
• Difficult to extract or move packages
• Long dependency chains
• Circular dependencies

Solution:

• Use dependency injection
• Define clear interfaces
• Implement event-driven communication
• Regular refactoring to reduce coupling

2. Unclear Package Responsibilities

Packages with overlapping or undefined purposes.

Symptoms:

• Duplicate functionality
• Uncertain where to add features
• Inconsistent implementations
• Code duplication

Solution:

• Document package purpose clearly
• Single Responsibility Principle
• Regular architecture reviews
• Refactor to clarify boundaries

3. Inconsistent Dependency Versions

Different versions of same dependency across packages.

Symptoms:

• Build errors
• Runtime conflicts
• Peer dependency warnings
• Bundle size bloat

Solution:

• Use workspace protocol
• Implement syncpack or similar
• Centralize version management
• CI checks for consistency

4. Poor Build Optimization

Not leveraging caching and incremental builds.

Symptoms:

• Slow build times
• Rebuilding unchanged packages
• Long CI pipeline runs
• Developer frustration

Solution:

• Implement build caching (Turborepo, Nx)
• Use affected analysis
• Configure incremental builds
• Optimize build pipelines

5. Missing Documentation

Inadequate documentation for packages and APIs.

Symptoms:

• Frequent questions about usage
• Incorrect usage patterns
• Difficult onboarding
• Knowledge silos

Solution:

• README in every package
• API documentation generation
• Usage examples
• Onboarding guides

6. Monolithic Thinking in Monorepo

Treating monorepo as single large application.

Symptoms:

• Shared global state
• Tightly coupled code
• Difficult to extract packages
• No clear package boundaries

Solution:

• Design packages as independent units
• Minimize shared global state
• Clear separation of concerns
• Regular boundary reviews

7. Over-Sharing Code

Sharing code that should remain private.

Symptoms:

• Implementation details exposed
• Brittle dependencies
• Difficult to refactor
• Version management complexity

Solution:

• Use barrel exports for public API
• Keep internals private
• Document public vs private
• Review exports regularly

8. Ignoring Package Boundaries

Importing from package internals instead of public API.

Symptoms:

• Brittle imports
• Breaking changes on refactor
• Unclear dependencies
• Type errors

Solution:

• Only import from package root
• Configure linting rules
• Use TypeScript project references
• Code review enforcement

9. No Versioning Strategy

Lack of clear versioning approach.

Symptoms:

• Unclear compatibility
• Breaking changes without warning
• Difficult rollbacks
• Consumer confusion

Solution:

• Choose fixed or independent versioning
• Use changesets or conventional commits
• Semantic versioning discipline
• Automated version management

10. Complex Dependency Chains

Deep or circular dependency relationships.

Symptoms:

• Difficult to understand flow
• Build order issues
• Circular dependency errors
• Maintenance difficulty

Solution:

• Visualize dependency graph
• Refactor to reduce depth
• Break circular dependencies
• Use dependency injection

When to Use This Skill

Apply monorepo architecture principles when:

• Designing new monorepos - Setting up structure and organization
• Refactoring existing repos - Improving architecture and organization
• Migrating to monorepo - Moving from polyrepo to monorepo
• Scaling monorepo - Growing from small to large monorepo
• Organizing packages - Deciding package boundaries and structure
• Managing dependencies - Handling internal and external dependencies
• Establishing patterns - Creating architectural standards
• Reviewing architecture - Evaluating existing monorepo structure
• Troubleshooting issues - Solving dependency or organization problems
• Planning refactors - Restructuring packages or dependencies

Resources

• Monorepo.tools - Comprehensive comparison of monorepo tools and patterns
• Turborepo Handbook - Best practices for Turborepo architecture
• Nx Monorepo Guide - Architectural concepts and patterns
• PNPM Workspaces - Documentation for PNPM workspace features
• Yarn Workspaces - Yarn workspace implementation guide
• Lerna Documentation - Multi-package repository management
• Bazel Documentation - Build and test system for large monorepos
• Rush Documentation - Scalable monorepo manager for JavaScript