A modular monolith is a single deployable application that is internally organised into clearly separated modules, each with its own responsibility and a well-defined boundary. It keeps the simplicity of one application to build, run and deploy, while avoiding the tangle that monoliths are infamous for. Each module knows as little as possible about the internals of the others.
Imagine a large house with well-planned rooms. It is still one building under one roof, with one front door and one set of plumbing. But the kitchen, the bedrooms and the bathroom are clearly separated, each with a clear purpose and a door you can close. You get the convenience of a single home without everything spilling into one big open space. That clean separation rests on good abstraction, where each module exposes what it does and hides how it does it.
The big advantage over microservices is cost. You get structure without the networking and operational burden of many separate services, and if one module later truly needs to scale on its own, its clear boundary makes splitting it off straightforward.
The catch is that the boundaries are only as strong as your discipline. Nothing physically stops one module from reaching into another’s database table or calling an internal function directly, and once that happens the walls quietly come down. Good teams enforce the rules with code, using separate schemas, module-level interfaces, or build checks that fail when a forbidden import sneaks in.
At TopDevs we often start clients with a modular monolith, so they get a clean, maintainable codebase now and a clear path to scale only when the business actually needs it.
