Modular Monoliths vs Microservices: Reclaiming Scalable Simplicity

For the better part of the last decade, microservices architecture has been the golden child of scalable software systems. Engineering leaders, CTOs, and architects raced to break apart monolithic applications in favor of distributed, independently deployable services. And with good reason — at scale, microservices can provide resilience, autonomy, and performance.

But in 2025, the narrative is evolving.

Teams are now rethinking the overhead, reassessing complexity, and rediscovering the value of the monolith — not the spaghetti-coded monolith of the past, but the modular monolith, a deliberate, well-structured architecture that blends simplicity with scalability.

In this article, we’ll unpack:

• What modular monoliths and microservices are (and aren’t)
• The tradeoffs between them
• When to use each
• How to design a modular monolith like a pro

📦 What Is a Modular Monolith?

A modular monolith is a software architecture where the application is deployed as a single unit (a monolith), but is logically divided into independent modules, each with well-defined boundaries, responsibilities, and interfaces.

Think of it as a monolith in shape, microservices in spirit.

Key Characteristics:

• Single deployment unit (one binary, one container, one process)
• Modular internal structure, often based on Domain-Driven Design (DDD)
• Strong encapsulation of business logic per module (e.g., User, Billing, Inventory)
• Clearly defined internal APIs or interfaces between modules
• Database may be shared — but access is scoped per module (sometimes enforced by code boundaries)

🧩 Microservices Recap

In contrast, microservices:

• Are independent services deployed separately
• Communicate over the network (REST, gRPC, messaging)
• Own separate databases (per service)
• Have dedicated teams per service (in large orgs)
• Require infrastructure orchestration, service discovery, and CI/CD pipelines per service

⚔️ Modular Monolith vs Microservices: The Tradeoffs

🧠 Why Modular Monoliths Are Trending (Again)

In many cases, teams jumped into microservices too early or without enough architectural maturity, leading to:

• Too many services, each too small
• Duplicated logic and tech stacks
• Hard-to-debug distributed bugs
• Complex deployments and DevOps nightmares
• Unnecessary latency and cost

Modular monoliths provide a middle ground:
✅ Maintain simplicity

✅ Avoid distributed complexity

✅ Still structure code around business domains

📐 Designing a Modular Monolith Like a Pro

Here’s how to architect a clean, scalable monolith:

1. Organize Code by Domain

Use Domain-Driven Design to separate bounded contexts:

/src
  /modules
    /users
      user.controller.js
      user.service.js
      user.model.js
    /billing
      billing.controller.js
      billing.service.js
      ...

Each module should be:

• Internally cohesive
• Only expose a public interface
• Forbidden from reaching into other module’s internals

2. Use Internal APIs

Instead of direct cross-module calls, expose clear services or interfaces:

// billing.module.ts
export class BillingService {
  chargeUser(userId, amount) { ... }
}

// user.module.ts
import { BillingService } from '../billing'

BillingService.chargeUser(user.id, 100)

This keeps contracts explicit and interchangeable later.

3. Enforce Boundaries with Linting/Build Tools

Use tools like:

• Nx or Lerna (for modular monorepos)
• ESLint custom rules to restrict cross-module imports
• Static code analysis to detect tight coupling

4. Make Migration to Microservices Possible

Design so each module could be extracted into a service if needed:

• Abstract database access via services
• Avoid global state or cross-module dependencies
• Favor pub/sub patterns internally

This makes your monolith microservice-ready, but only when it makes sense.

🚦 When Should You Choose Each?

🟢 Go Modular Monolith When:

• You’re building a new product or MVP
• Your team is < 30 engineers
• You want fast iteration with less infra complexity
• You don’t need hyper-scale or global traffic routing yet
• You want to design clean modules without premature distribution

🟡 Go Microservices When:

• You have large teams that need to work independently
• You face scaling challenges per domain
• You require multi-language support
• You need fault isolation between critical services
• You’ve outgrown your monolith and have a clear migration path

🧱 Real-World Inspiration

Several high-scale companies started as monoliths and only later split out services as pain points demanded:

• Basecamp: Modular Rails monolith for years
• Shopify: "Componentized monolith" with Rails engines
• GitHub: Still largely a monolith with strict modularity
• Amazon: Started as a monolith, moved to services gradually

The takeaway? Great systems don’t start as distributed systems — they evolve into them.

✅ Final Thoughts

The Modular Monolith isn’t a step backward — it’s a step forward in architectural clarity and development velocity.

• It’s a great way to scale without drowning in complexity
• It helps you build domain-aligned code from day one
• It allows you to migrate to microservices only when the business needs justify it

💬 Call to Action

Are you scaling a product or planning a rebuild?

💡 Let’s talk architecture. I help companies design scalable, testable full-stack systems that stay flexible as they grow.

👉 Follow me here on Dev.to, or reach out through educationgate.org to chat about your architecture, team scaling, or next big project.