How I Would Think of a SaaS Scalable Architecture in the First Place (Next.js Case Study)

When you start designing a SaaS platform from scratch, you want an architecture that’s easy to reason about, simple to evolve, and capable of growing with your user base. Below are two complementary approaches—first focusing on a fully modular code structure, then on domain‑driven, feature‑oriented organization. Both aim to maximize clarity, scalability, and maintainability.

Module‑Centric Architecture

A module‑based layout treats each major capability as a self‑contained unit, where API routes, UI components, business logic, data models, and types all live together. This structure minimizes cross‑module coupling, making it straightforward to add, remove, or refactor features. Each module houses its own routes, components, services, and types, while module‑specific /api folders keep related logic close to the domain. Shared utilities live in a single /shared directory to prevent duplication, and each module maintains its own test suite for isolation.

1my-saas-project/
2│
3├── .github/                  
4├── .env                      
5│
6├── src/
7│   ├── modules/              
8│   │   ├── auth/             
9│   │   │   ├── api/          
10│   │   │   ├── components/   
11│   │   │   ├── hooks/        
12│   │   │   ├── services/     
13│   │   │   └── types/        
14│   │   │
15│   │   ├── github/           
16│   │   │   ├── api/          
17│   │   │   ├── components/   
18│   │   │   ├── hooks/        
19│   │   │   ├── services/     
20│   │   │   └── types/        
21│   │   │
22│   │   ├── billing/          
23│   │   └── user/             
24│   ├── app/                  
25│   ├── shared/               
26│   └── styles/               
27├── tests/                    
28├── config/                   
29├── scripts/                  
30└── docs/                     

Example: GitHub Module

1// src/modules/github/api/githubAuth.ts
2export async function githubOAuthHandler(req, res) {
3  // OAuth logic
4}
5
6// src/modules/github/services/githubService.ts
7export class GitHubService {
8  async fetchRepositories() {
9    // Fetch logic
10  }
11}
12
13// src/modules/github/hooks/useGitHubIntegration.ts
14export function useGitHubIntegration() {
15  // Hook logic
16}

Building with a module‑centric approach delivers clear separation of concerns, independent development and testing cycles, and the ability to introduce new modules without touching existing code.

Domain‑Driven, Feature‑Based Organization

Organizing by business domain or feature aligns your code structure directly with product capabilities, simplifying onboarding and cross‑functional collaboration. In this layout, top‑level folders represent major features—authentication, billing, user management, analytics—while shared UI components and hooks reside in dedicated components/ and hooks/ directories.

1my-saas-project/
2│
3├── src/
4│   ├── app/                  
5│   ├── components/           
6│   ├── features/             
7│   │   ├── authentication/   
8│   │   ├── billing/          
9│   │   ├── user-management/  
10│   │   └── analytics/        
11│   ├── hooks/                
12│   ├── lib/                  
13│   ├── middleware/           
14│   ├── services/             
15│   ├── styles/               
16│   └── types/                
17├── tests/                    
18├── public/                   
19├── prisma/                   
20├── docker/                   
21├── config/                   
22└── docs/                     

This feature‑aligned approach provides scalability by allowing domains to scale independently and fosters consistency through shared top‑level resources.

Implementation Tips & Timeline

Start with barrel exports (index.ts) to simplify imports and implement strict typing for early error detection. Use feature flags to toggle incomplete modules and maintain independent tests for each module or feature.

Whether you choose a purely modular layout or a domain‑driven feature organization, the key is to isolate concerns, align code with business logic, and build a foundation that evolves gracefully as your SaaS scales.