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🧩 Modular Monolith

✅ Overview​

A structural style that is a Monolith but configured internally with "clearly separated modules".

Conceptual Diagram​

✅ Problems Addressed​

  • Tight coupling, change collision, and ambiguous boundaries of Monolith.
  • Want to avoid "operational cost" and "complexity" of Microservices.
  • Want to separate only the internal structure firmly.

✅ Basic Philosophy & Rules​

  • Single deployment, but internally split by clear boundaries.
  • Modules are configured as independent domains/packages.
  • Inter-module communication is limited to public interfaces.

✅ Suitable Applications​

  • Initial stage of medium to large-scale products.
  • Areas with clear domain boundaries.
  • Pre-structure when considering future microservices migration.

❌ Unsuitable Cases​

  • Teams that don't observe boundaries and end up tightly coupled.
  • Strong availability requirements (cannot tolerate partial outage).
  • Scale characteristics differ extremely per service.

✅ History​

  • Attracted attention with the emergence of DDD.
  • Re-evaluated as a reaction to Microservices.

✅ Related Styles​

✅ Representative Frameworks​

  • Spring Boot + Module Boundary (Package by Feature)
    Representative configuration of Modular Monolith protecting model boundaries.

  • NestJS (Module Structure)
    Supports clear module boundaries, suitable for Modular Monolith.

  • .NET (Modular Monolith Template)
    Can realize DDD + Module boundaries with official templates.

  • Laravel / Rails (Engine / Module Pattern)
    Configuration to cut out modules partially is possible.

✅ Design Patterns Supporting This Style​

  • Facade
    Functions as an entrance of API exposed to outside of the module.

  • Mediator
    Commonizes coordination logic between modules.

  • Adapter
    Used when performing inter-module communication via interfaces.

  • Strategy
    Makes behavior inside module switchable.

  • Proxy
    Useful for module boundary protection and access control.

✅ Summary​

Modular Monolith is a structural style that balances
"Simplicity of Monolith" and "Boundary awareness of Microservices".