Validation in a Domain Driven Design
How do you deal with validation on complex aggregates in a domain driven design? Do you consolidate your business rules/validation logic?
I understand argument validation. And I understand property validation which can be attached to the models themselves and do things like check that an email address or zipcode is valid or that a first name has a minimum and maximum length.
But what about complex validation that involves multiple models? Where do you typically place these rules & methods within your architecture? And what patterns if any do you use to implement them?
Answer
Instead of relying on IsValid(xx) calls all over your application, consider taking some advice from Greg Young:
Don't ever let your entities get into an invalid state.
What this basically means is that you transition from thinking of entities as pure data containers and more about objects with behaviors.
Consider the example of a person's address:
person.Address = "123 my street";
person.City = "Houston";
person.State = "TX";
person.Zip = 12345;
Between any of those calls your entity is invalid (because you would have properties that don't agree with each other. Now consider this:
person.ChangeAddress(.......);
all of the calls relating to the behavior of changing an address are now an atomic unit. Your entity is never invalid here.
If you take this idea of modeling behaviors rather than state, then you can reach a model that doesn't allow invalid entities.
For a good discussion on this, check out this infoq interview: http://www.infoq.com/interviews/greg-young-ddd