Am I misinterpreting the intent of the Repository pattern? I’m going to say “yeah”, but know that me and every person I’ve worked with has asked the same thing for the same reason… “You’re not thinking 4th dimensionally, Marty”. Let’s simplify it a little and stick with constructors instead of Create methods first: Editor e … Read more
Ok I’ve changed my mind. I have been trying to deal with VOs a bunch lately and after watching this http://www.infoq.com/presentations/Value-Objects-Dan-Bergh-Johnsson it clarified a couple of things for me. Commands and Event are messages (and not objects, objects are data + behavior), in some respects much like DTOs, they communicate data about an event and … Read more
The commenters are right – there are some subjective definitions at play here. But there are some principles and concepts that can help reason about the different approaches. Conway’s Law It’s not strictly the original definition, but I think the distinction can be better understood with reference to Conway’s Law: Any organization that designs a … Read more
Repositories are – like you say – an abstraction. They originate from Martin Fowler’s Object Query Pattern. Both Repositories and DTOs can simplify database persistence by mapping persisted data to equivalent collection of entity objects. However, Repositories are more coarse-grained than DAOs by providing control of an entire Aggregate Root (AG) often hiding a lot … Read more
Speaking in terms of more “classical” DDD, yes domain objects are typically not allowed anywhere outside of the domain. But it is not an absolute rule that domain objects are not used in the presentation layer. For example, Naked Objects represents a school of thought where domain objects are used directly. I myself adhere mostly … Read more
Ignore all the crazy answers about thread safe etc, that has nothing to do with DDD. (I’ve yet to see a thread safe O/R mapper or other DDD friendly dal) Imagine a value object for weights. lets say we have a KG value object. sample (edited for clarity): var kg75 = new Weight(75); joe.Weight = … Read more
I’ve read about CQRS systems that maintain a simple key value store on the command side to represent an application’s state, and others that merely correlate messages (using some sort of saga) and utilise the query store to represent an applications state instead. Either way there’ll no doubt be a persistence technology involved with these … Read more
Commands and Events can be of very different concerns. They can be technical concerns, integration concerns, domain concerns… I assume that if you ask about domain, you’re implementing a domain model (maybe even with Domain Driven Design). If this is the case I’ll try to give you a really simplified response, so you can have … Read more
I would say this practice promotes concentrating your efforts on the ‘problem space’ rather than the ‘solution space’. Driving an emergent solution (the design) by studying and really getting to know and understand the domain. One of the practices (taken from XP) would be the writing of stories that occur in the problem domain. From … Read more
You may want to look into the concept of Command Query Separation. I wouldn’t worry about typed repositories for lookup values, but I’d still probably use DTO type classes over datasets etc… You may want to spend some time reading Greg Young’s blogs starting from this one to the present. He doesn’t talk about filling … Read more