A boundary should explain a responsibility
Modularity begins with a question: which part of the product owns this behavior? A clear boundary makes it easier to change one capability without surprising another. It can be a directory, a module or a documented interface. It does not have to be a separate deployed service.
Separate decisions that change independently
Authentication, billing and the main user workflow often have different rules. Give those rules a home and make their interfaces explicit. Avoid exposing database details across every module. When one module needs information from another, prefer a small, intentional contract over access to everything.
Keep the first deployment simple
Operational boundaries bring their own work: monitoring, network failures, release coordination and access management. Introduce them when the product has a clear reason to need them. A well-structured application can provide useful separation while remaining straightforward to run.
Make change observable
A module is useful when its behavior can be checked. Test the contracts that other parts of the product depend on, document important assumptions, and watch the production paths that matter to users. Good structure should make changes easier to understand, review and recover from.
