Modular Monoliths Are Not Just Better Folder Structure

A modular monolith is more than just organizing code into folders — it enforces strict boundaries between modules through project-level isolation, controlled public APIs, and database schema separation. Using a running tracker app as a concrete example, the key differences between a regular monolith and a modular monolith are demonstrated: in a regular monolith, bounded contexts slowly bleed into each other through direct entity references and database joins, making future extraction painful. In a modular monolith, each module owns its domain, infrastructure, and a clearly defined public API that is the only allowed coupling point between modules. This approach enables easier extraction into microservices later, better control over side effects, and optional database-level access policies per schema.