Validation in Domain Driven Design
There’s a great discussion on Jeff Palermo’s blog about entity validation patterns. Jeff takes the position that your domain objects (or entities) should not have validation logic “baked-in” to the class itself. Instead, you should separate the validation routines into separate classes that you can use to validate the object on demand.
There are two advantages to this approach. The first is that you can use different validators for the same object in different circumstances. For example, the validation you might perform prior to storing the domain object in a persistence layer could be different than the validation routine used to validate input from the GUI layer.
The second advantage is that separating the validation logic from the data itself makes it easier to work with ORM or serialization frameworks. Most of these frameworks encourage the use of Plain Old Objects, that is, objects without special attributes or interfaces that help with these mapping and serialization tasks. (See Wikipedia’s article on Plain Old Java Objects, for example.)
Those are powerful arguments, but I’m still not convinced.
Trade-offs
As a practical matter, Jeff’s advice is sound. It’s much easier to move business logic into the helper classes that surround your entity model. You get better tool support and more flexibility. But there’s two things about his approach that bother me. Judging by some of the excellent comments on his article, other programmers are bothered by them as well.
First, stripping away behavior from your domain objects is a recognized anti-pattern in object oriented code. Martin Fowler calls it the Anemic Domain Model. It harkens back to the days of procedural programming, where data and business logic were strictly separated. If you’re an OO purist, this is a red flag.
From an OO perspective, the need to validate the same object in different ways suggests that what you actually need to do is create more objects. Rather than pass a stripped-down data-transfer object (DTO) all the way from your data storage layer up to your GUI, you should have a bunch of intermediate objects to help transition the data and enforce proper behavior.
But I’m not an OO snob. Writing a whole bunch of extra classes to move information between tiers in my application is a hassle. I’ve done it before, and we’re doing it now with Infovark, but for most projects it just isn’t justified. Especially if you have to wrestle with various application frameworks to deal with correctly modeled but more complicated domain objects.
The second objection I have is that if we follow Jeff’s advice, we have to accept that bad data will creep into our domain. Jeff knows that this is a hard sell. It’s why he titled his article “The fallacy of the always-valid entity.”
Whew. That’s rough. That requires a whole different programming mindset. What about the problem of Garbage In, Garbage Out? Can we really create programs robust enough to handle business objects that might, at any moment, contain meaningless gibberish?
I don’t know. For now, as appealing as Jeff’s idea is, I’ll stick to always-valid approach. What do you think?
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- One-Way Serialization
- Domain Models in High Performance Systems
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- Getting It Out of Our System
