Modular Monolith

The modular monolith addresses the problem of maintaining strong internal boundaries and cohesive change units without paying the operational cost of distributed deployment.

Cohesive Formulation

A modular monolith is primarily a system-structure practice:

Modules define boundaries inside one deployment.
Relations and flows across modules are explicit.
Persistence and transactions may be shared, but semantic ownership must still be scoped.
Observers may be local calls, handlers, processes, or in-process message dispatch.

In the Model

The monolith is a realization choice. The modules are the important semantic and structural boundaries. A modular monolith can preserve bounded contexts, aggregate boundaries, interaction contracts, and ownership without forcing network boundaries.

Failure Modes

The pattern fails when shared process and shared database are mistaken for shared semantics. Without explicit module boundaries, the result is only a monolith, not a modular one.