Modular Monolith

A traditional monolith often starts cleanly but drifts toward broad coupling as modules reach into one another's data and internals. Jumping directly to Microservices can solve one problem by creating many new ones in networking, deployment, and consistency. Modular Monolith exists for the middle ground where boundaries matter, but distributed operations are still too expensive.

Many teams learned that splitting too early into Microservices did not create autonomy if the domain boundaries were still wrong. At the same time, staying with a loose monolith made those boundaries impossible to practice. Modular Monolith became attractive again because it lets teams test and preserve domain boundaries before deciding whether distribution is truly worth it.

Each module exposes a narrow public contract and hides its internals. Runtime and deployment stay shared, but code dependencies are constrained at the module edge. Even when a database is shared, ownership still needs to be treated as module-local rather than globally open.

Compared with a plain Monolith, the difference is not the deployment unit but the strength of internal boundaries. Compared with Microservices, the difference is that collaboration still happens in-process, without network and operational overhead. It is often a better fit when domain boundaries matter more than independent scaling right now.

It works well when domains are getting more complex, future service splits are possible, but the team still wants one deployment path and simple operations. It can be a stepping stone toward Microservices or a durable end state in its own right. The real success criterion is whether module boundaries stay visible in the dependency graph.