Where to put global rules validation in DDD

Question

I\'m new to DDD, and I\'m trying to apply it in real life. There is no questions about such validation logic, as null check, empty strings check, etc - that goes directly to

Answer 1

I’m not an expert on DDD but I have asked myself the same questions and this is what I came up with: Validation logic should normally go into the constructor/factory and setters. This way you guarantee that you always have valid domain objects. But if the validation involves database queries that impact your performance, an efficient implementation requires a different design.

(1) Injecting Entities: Injecting entities can be technical difficult and also makes managing application performance very hard due to the fragmentation of you database logic. Seemingly simple operations can now have an unexpectedly performance impact. It also makes it impossible to optimize your domain object for operations on groups of the same kind of entities, you no longer can write a single group query, and instead you always have individual queries for each entity.

(2) Injecting repository: You should not put any business logic in repositories. Keep repositories simple and focused. They should act as if they were collections and only contain logic for adding, removing and finding objects (some even spinoff the find methods to other objects).

(3) Domain service This seems the most logical place to handle the validation that requires database querying. A good implementation would make the constructor/factory and setters involved package private, so that the entities can only be created / modified with the domain service.