March 10, 2011, 11:08 p.m.
posted by wdwx
Requirements on the Persistence Infrastructure
Let's repeat again that I want the infrastructure to stay out of the way of the Domain Model as much as possible so that I can focus on creating a powerful Domain Model for solving the business problem without having more distractions than are necessary. For example, I believe that I should be able to work with as high a level of Persistence Ignorance (PI) as possible, so that's a requirement of the infrastructure.
At the same time, it's also important not to put too much responsibility on the consumer of the Domain Model. I want to provide the consumer with a very simple API so that the consumer programmer can focus on what is important to him: creating a good experience for consumer users.
I have already defined certain desirable features for the life cycle of the persistent entities in Chapter 5, "Moving Further with Domain-Driven Design," but let's repeat it again here in Figure.
Of course, this varies widely, not only concerning the requirements, but also because a certain persistence framework can sometimes fail and sometimes succeed for the same mix of non-functional requirements. The reason for this is that the success or failure depends on the type of application, the usage patterns, and so on. I have to leave it up to the reader to define her own mix of non-functional requirements and to check whether a certain persistence framework can fulfill the requirements by carrying out careful tests. Just remember that if your appetite for non-functional requirements becomes too big, it will cost you a lot somewhere else.
Most often it's very hard to get the customer to express any non-functional requirements at all. If you succeed, the risk is that he will want everything to 100%, and that's when it's good to remember the cost side of it.
For further discussion of non-functional requirements, see [POSA 1], [Fowler PoEAA] or [Nilsson NED].
As you may have noticed, I didn't mention it as a requirement on the persistence framework to be able to have a network between the consumer and the Domain Model. Building it is very possible, as we did in Valhalla [Valhalla]. However, it's probably better to think along the lines of explicit boundary where networks are involved and design the crossing borders in a service-oriented way. This is why I seldom see the need for the shortcut that remoting represents.
So, we have defined requirements to fulfill for the Domain Model's sake, and we have defined how to use the API for the persistence framework.
With the requirements now in place, let's get going and make some choices about the infrastructure. First is the location for the persistent data.