Validation in Domain Driven Design

Question

Our team is starting a new project following Domain Driven Design (DDD). At the high level, we have an API on the top of our domain which enables a client to perform operations on the domain. One of the question I'm not very clear about is where do we perform validation on a certain property/ attribute in DDD.

Consider this example. Let us say, I have a below data contract/ DTO exposed by my API:

 public class Person
 {
    public string Email { get; set;}
    public string Name { get; set; }
 }

Now, let us say we have business validation which prevents user to enter an invalid email address and restricts user to have name more than 50 characters.

To achieve this, I can see following three approaches:

In Approach 1 , we do data validation only at the API (either through Data Annotation or Fluent Validation). I do not repeat the validation in my domain. Theoretically, this may mean my domain could go in an invalid state. But, since entry point (API) is being validated it is not possible in a real scenario.

In Approach 2, we do data validation at both API and in my domain. This approach helps us to completely remove coupling between my domain and API. The API can independently return a Bad request to the client. And since the domain performs the validation again, there is no chance of domain going to an invalid state. In this approach however, we violate DRY principle.

In Approach 3, we do validation only at Domain and do not perform validation on DTO at API level. With this approach, while we are not repeating the validation, the domain cannot throw an exception when API call tries to put it in an invalid state. Rather, we would need to wrap that exception in some Result object. This would help the API to send an appropriate response to client (eg. Bad request instead of Internal Server error). What I do not like about this approach is that I would prefer to throw a hard exception rather than putting a wrapper.

Ask

• Which approach makes the most sense and why?

• Where is the line between a business validation and a business rule? (Assuming the business rule exists in domain).

• Is there anything obvious which I'm missing here?

Note: This question may look similar to Validation in a Domain Driven Design n and where should put input validation in Domain Driven Design? but it does not really answer the specifics.

Answer 1

I have gone through the answers provided by the experts here and after much deliberation within the team we have decided to implement the validations with below key principles in mind.

• We treat the API validation and Domain validation/ Business Rules as separate concern. We put the API validation same way we would have without a DDD. This is because, API is just an interface to talk to our Domain. Tomorrow, we could have a messsage bus sit in between but our domain logic would not change. The API validations include but not limited to field length checks, regex checks etc.

• The domain validations are limited to business rules only. The field level validations like length check, regex etc which do not mean anything to business are kept out of business rule. If there is a business rule violation we simple throw a DomainException without caring about how consumer would handle it. In case of an API response, it would simple mean returning status code 500 back to the client.

• Since, we are now treating API validations and business rules as a separate concern, it does mean that there are chances of repetition of some validation rule. But we are OK with that as it makes things much simpler and easy to reason about. Since they are loosely coupled we can easily change API validation or business rule independently without impacting the other.

• There are few border line cases like email where one could have argued to have ValueObject in our Domain. But we take this on case-by-case basis. For example, we do not see any value to have a separate ValueObject for email field in our Domain at the moment. Hence, it is validated (regex check) only at the API level. If in future if business comes up with additional rules like restricting to a specific domain, then we may revisit our decision.

The above principles in mind have helped us to keep things simple and easy to reason about.

Answer 2

Theoretically, this may mean my domain could go in an invalid state.

I don't think a Name longer than 50 characters would mean an invalid domain state... The domain would still be perfectly functioning.

You have to differentiate between input validation (does the data fit in the technical slots?) and domain invariants. Some things you can validate at API level and don't matter to the domain, others have to be checked by loading domain data and thus aren't easily accessible to outer layers.

They are really two (or more) different sets of rules, more largely disconnected than you'd think.

TL;DR - there's no hard and fast answer. Try to characterize things deeper than just "business validation" and choose Approach 1, 2 or 3 wisely depending on type of rule.

Answer 3

Now, let us say we have business rule which prevents user to enter an invalid email address and restricts user to have name more than 50 characters.

Important things to notice in this example

• You don't have authority for this data. Both username and email address are assigned and managed somewhere else
• As far as the domain model is concerned, this data is opaque; you probably aren't ever going to manipulate it, or vary any computations based on the contents. As far as your business rules are concerned, they are just hash values that you are copying so that you can pass them along to something else (printing the name on an envelope, or sending an email).

Semantically, both of these values are basically just flavors of "identifier".

That being the case, the domain model doesn't care about the validation at all, except for such problems like not running out of memory. Your data model may care, if you have fixed length columns or something like that.

So this could very easily be one of the places where you care at the message boundary, but not within the domain itself.

But it's not a good proxy for the general question of where validation might live.

Contrast this case with something like a deposit amount -- it's a number, and you would reasonably expect to add/subtract it from other numbers, compare it to other numbers, and so on. There, you might look at something like Integer.MAX, and reasonably conclude that an attack/data entry error is so much more likely than a genuine use case that you will eliminate that option altogether.

Validation at the message boundary is primarily driven by the question: can you trust the source? If there is any doubt, then there is no doubt. (Deogun and Johnnson on Domain Driven Security is a good starting point).

Largely, validation at the message boundary comes down to establishing that the sequence of bytes that you have received is actually compliant with the message schema; which can of course include limits on the range of allowed values. (Example: HTTP Responses include status codes, but you aren't required to pretend that a response with status code 777 is meant to improve your afternoon).

And so it's a perfectly reasonable thing to declare that the name field in a message be no more than 50 characters, and that the email address field in the message conforms to the definition of addr_spec in RFC 5322.

And then at the boundary you make sure the bytes you get actually satisfy the message constraints, and pass it long if it does.

But within the domain model? if you don't need to make assumptions about the data, then done. "The application said these are bytes? Good enough for me!"

Somewhat more technically - the key test is whether or not the domain model has any preconditions that need to be satisfied in order to ensure the correctness of its results. If we have preconditions, then validation serves as a controlled way to detect violations.

But there's not a lot of value-add injecting the domain model with precondition checks that it doesn't need.

(Again, contrast with amount -- the domain model has a lot of interest in detecting violations before it starts moving money around indiscriminately).

Answer 4

Which approach makes the most sense and why?

Approah 2. Domain Model should always do validation. At Application Layer (API) you can do it or not, but it is better to do it if you can, as you detect the invalid data in advance without having to call the domain. I say "if you can" because there maybe business rules that cannot be validated without accessing domain.

NOTE: I follow the approach of validating the operations, not the data. A given data can be valid for performing an operation but not other.

Where is the line between a business validation and a business rule? (Assuming the business rule exists in domain).

I think you are misunderstanding/mixing concepts. A concept is "business rules", and another concept is "validation". Validation is the process we do for checking whether the business rules are satisfied or not.