Domain Driven Design newbie, please explain 'value objects' and 'services' briefly

Question

I am reading http://en.wikipedia.org/wiki/Domain-driven_design right now, and I just need 2 quick examples so I understand what 'value objects' and 'services' are in the context of DDD.

• Value Objects: An object that describes a characteristic of a thing. Value Objects have no conceptual identity. They are typically read-only objects and may be shared using the Flyweight design pattern.

• Services: When an operation does not conceptually belong to any object. Following the natural contours of the problem, you can implement these operations in services. The Service concept is called "Pure Fabrication" in GRASP.

Value objexts: can someone give me a simple example this please?

Services: so if it isn't an object/entity, nor belong to repository/factories then its a service? I don't understand this.

Answer 1

The archetypical example of a Value Object is Money. It's very conceivable that if you build an international e-commerce application, you will want to encapsulate the concept of 'money' into a class. This will allow you to perform operations on monetary values - not only basic addition, subtraction and so forth, but possibly also currency conversions between USD and, say, Euro.

Such a Money object has no inherent identity - it contains the values you put into it, and when you dispose of it, it's gone. Additionally, two Money objects containing 10 USD are considered identical even if they are separate object instances.

Other examples of Value Objects are measurements such as length, which might contain a value and a unit, such as 9.87 km or 3 feet. Again, besides simply containing the data, such a type will likely offer conversion methods to other measurements and so forth.

Services, on the other hand, are types that performs an important Domain operation, but doesn't really fit well into the other, more 'noun'-based concepts of the Domain. You should strive to have as few Services as possible, but sometimes, a Service is the best way of encapsulating an important Domain Concept.

You can read more about Value Objects, Services and much more in the excellent book Domain-Driven Design, which I can only recommend.

Answer 2

Value Objects: a typical example is an address. Equality is based on the values of the object, hence the name, and not on identity. That means that for instance 2 Person objects have the same address if the values of their Address objects are equal, even if the Address objects are 2 completely different objects in memory or have a different primary key in the database.

Services: offer actions that do not necessarily belong to a specific domain object but do act upon domain objects. As an example, I'm thinking of a service that sends e-mail notifications in an online shop when the price of a product drops below a certain price.

InfoQ has a free book on DDD (a summary of Eric Evan's book): http://www.infoq.com/minibooks/domain-driven-design-quickly

Answer 3

This is a great example of how to identify Value Objects vs Entities. My other post also gives another example.