Modular Monolith Architecture

Overview

A modular monolith keeps one deployable application while enforcing strong module boundaries internally.

Why It Works

• Simpler operations than microservices
• Clear domain ownership by module
• Easier transactional consistency

flowchart TD
    A[Single Deployable App] --> B[Orders Module]
    A --> C[Billing Module]
    A --> D[Catalog Module]
    B --> E[(Shared DB with module boundaries)]
    C --> E
    D --> E

Practical Example

A module exposes only application services, while internal entities remain private to prevent cross-module coupling.

Production Example

Growing teams often start with a modular monolith and extract selected modules into services only when scaling or release constraints justify the cost.

Common Pitfalls

• Treating modules as folders without dependency rules
• Direct table access across module boundaries
• Allowing shared utility modules to become dumping grounds

Key Takeaways

• Modular monolith is a strong default for many products.
• Boundary enforcement is critical for long-term maintainability.
• Extraction to microservices should be evidence-driven.