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Backend question · Software design & patterns › Layered, clean & hexagonal architecture

Orders over 500 EUR from gold customers get free shipping. Where should this rule live?

In the domain model or the application service that places the order

Business rules belong in the domain or application layer. There, you can test them without HTTP or a database, and every entry point reuses them.

Why the other places are wrong:
• "In the controller, right after validating the request body": a queue consumer or a CLI that calls the same use case would skip the rule.
• "In the repository's save method" or "In a database trigger": these hide a business rule inside infrastructure code, where it is hard to find and test.