What is a legitimate module dependency?

Goal & mental model verified

Spring Modulith derives application modules from a Boot application structure and supports boundary verification. Modules expose APIs/named interfaces while keeping implementation details internal.

Worked example · design exercise synthesis

Separate quotes, policies and billing into modules. Quotes calls the published policy API or emits a domain event; it should not reach directly into billing’s internal persistence package.

Engineering decision synthesis

Use a modular monolith when one deployment can satisfy operational needs. This preserves local interactions while clarifying boundaries, but independent scaling/deployment may later justify extraction.

Pitfall & diagnosis synthesis

Package names alone cannot prevent coupling if public internals are widely imported. Replacing calls with events changes consistency and failure semantics; it is not automatically an improvement.

Improve & validate synthesis

Run module dependency verification and focused module tests. Track coupling and change ownership before extracting a service; choose extraction based on an actual deployment constraint.