When people talk about modern software architecture, we often jump straight to microservices, serverless, or distributed systems. But many applications still live happily in monolithic structures—and with good reason. That said, there's a refined version of the monolith emerging as a serious contender: the modular monolith.

In this article, we’ll break down the key differences between monoliths and modular monoliths, why modular monoliths are gaining traction in 2025, and when to choose one over the other.

🧱 What Is a Monolith?

A monolith is a single unified codebase where all the features, business logic, UI, and data access layers live together.

Characteristics:

  • One deployment unit
  • Shared memory space
  • Often tightly coupled code
  • Easy to start, harder to scale
  • Difficult to isolate features for maintenance

Example: 

/app
  /controllers
  /services
  /models
  /routes
server.js

While this structure works fine for small teams and early-stage apps, it can become a bottleneck as the app grows.

🧩 What Is a Modular Monolith?

A modular monolith maintains a single deployment unit like a traditional monolith but introduces strict modular boundaries within the codebase.

Characteristics:

  • One deployment artifact (like a monolith)
  • Internally divided into independent modules
  • Clear ownership and isolation of features
  • Easier to maintain and test
  • A potential stepping stone to microservices

Example (using folder-level encapsulation): 

/app
  /user
    - user.controller.ts
    - user.service.ts
  /payment
    - payment.controller.ts
    - payment.service.ts
  /auth
    - auth.controller.ts
    - auth.service.ts
main.ts

Each module handles its own logic, models, and services — and ideally communicates with others via well-defined interfaces.

🔍 Key Differences

Feature Monolith Modular Monolith
Deployment Single unit Single unit
Code Organization Tightly coupled Explicit module boundaries
Scalability Hard to scale without redesign Easier to scale within monolith
Testing Hard to isolate tests Modules can be tested independently
Migration All-or-nothing changes Easier path to microservices
Team Ownership Shared responsibility Clear module ownership

🚀 Why Modular Monoliths Are Gaining Popularity in 2025

Modern teams are realizing that microservices aren't a silver bullet. They introduce distributed system complexity, operational overhead, and more moving parts.

A modular monolith, on the other hand, gives you:

✅ Simplicity in deployment

✅ Logical separation of concerns

✅ Cleaner testing and development boundaries

✅ Future-proofing for potential service extraction

Frameworks that support modular monoliths:

  • NestJS (TypeScript): uses modules as first-class citizens
  • Spring Boot (Java): encourages layered and modular architecture
  • Laravel (PHP): supports domain-driven modules

💡 When Should You Use a Modular Monolith?

Use a modular monolith when:

  • You have a small or mid-sized team
  • You're still validating your product-market fit
  • You want to avoid the early complexity of microservices
  • You want future flexibility for scaling

Use microservices when:

  • Teams are large and distributed
  • You need independent scaling per feature
  • You're dealing with highly decoupled domains or multi-tenancy at scale

🧠 Final Thoughts

You don’t need to choose between "monolith" and "microservices." Modular monoliths give you a structured, scalable, and sane middle ground. They bring the benefits of clear architecture without the operational burden of distributed systems.

Start simple. Build with boundaries. Scale with confidence.

💬 What do you prefer in 2025 — classic monoliths, modular monoliths, or microservices? Share your thoughts in the comments!