Software architecture discussions often feel polarized.

You’re told to either:

Stick with a monolith and suffer later, or

Jump to microservices and “do it right”

But there is a powerful, practical architecture that lives quietly in between — and it’s the one most successful systems evolve into first.

The modular monolith is not a compromise. It is a deliberate design choice.

What Is a Modular Monolith?

A modular monolith is:

  • One deployable application
  • One runtime

  • One database (usually) But internally:

  • Clear domain-based modules
  • Strong boundaries
  • Explicit interfaces
  • Minimal shared state

From the outside, it looks like a monolith. From the inside, it behaves like a set of well-disciplined services.

Why Most Monoliths Fail (And This One Doesn’t)

Most monoliths fail because:

  • Boundaries are implicit
  • Modules reach into each other’s internals
  • The database becomes a shared dumping ground

A modular monolith enforces:

  • Dependency direction
  • Encapsulation
  • Domain ownership

Change stays local.

The Core Principles of a Modular Monolith

  1. Domain-Driven Boundaries

Modules are organized around business capabilities, not technical layers.

Examples:

  • Identity
  • Billing
  • Orders

  • Notifications Not:

  • Controllers
  • Services
  • Repositories
  1. Explicit Module Interfaces

Modules communicate through:

  • Public APIs
  • Contracts
  • Events (even internally)
  • Direct access to internal classes is forbidden. This forces discipline early.
  1. Independent Evolution

Each module:

  • Can be developed independently
  • Has its own tests
  • Has clear ownership
  • Teams think in terms of products, not files.
  1. Controlled Data Access

The most important rule:

A module owns its data.

Other modules interact through APIs, not direct database access.

This single rule prevents most architectural decay.

Why Modular Monoliths Are So Powerful

  1. Simplicity Without Naivety

You avoid:

  • Network failures
  • Distributed tracing
  • Operational overhead

Yet still gain:

  • Clear boundaries
  • Safer refactoring
  • Predictable change
  1. Performance by Default
  • In-process calls
  • Shared memory
  • No serialization overhead You get microservice-like structure with monolith-level performance.
  1. The Best Migration Story A modular monolith is microservices-ready by design.

When the time comes:

  • Extract one module at a time
  • Replace in-process calls with network calls
  • Leave the rest untouched

No big rewrite.

Why Nobody Talks About This Architecture

Because:

  • It’s not trendy
  • It’s harder to explain in a diagram
  • It requires discipline, not tooling
  • There’s no conference talk titled:

“How We Successfully Did Nothing Fancy”

But that’s exactly why it works.

Final Thought

The modular monolith is the architecture of grown-up teams.

It respects complexity, controls change, and delays distribution until it is truly necessary.

Next, we’ll conclude this series by answering the most important question of all:

When are microservices actually worth it?

Happy Architecting!