This is my philosophy: Data types are object-oriented. They are responsible for ensuring that their internal state is consistent, and nothing more. They may

This is my philosophy: Data types are object-oriented. They are responsible for ensuring that their internal state is consistent, and nothing more. They may inherit if it makes sense, but it usually doesn't and most of the time aggregation is more appropriate. Business rules are functional. There's just a bag of composable, pure functions that take the various data types and perform validations, transformations, etc. External services, such as a database, are contractual. This would be an interface in Java or C# defining the required operations of the service. This allows manipulation of these external services during testing. Unit testing would mock them; integration testing would not. Workflows or services are procedural. They combine everything else into an actual use-case. They read in a linear flow of what needs to happen when to meet the use-case.