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
The following is based on my own experience and my experiments with various frameworks like Lokad.CQRS, NCQRS, etc. I’m sure there are multiple ways to handle this. I’ll post what makes most sense to me. 1. Aggregate Creation: Every time a command handler needs an aggregate, it uses a repository. The repository retrieves the respective … Read more
CQRS CQRS was introduced by Greg Young; his explanation in 2010 CQRS is simply the creation of two objects where there was previously only one. The separation occurs based upon whether the methods are a command or a query (the same definition that is used by Meyer in Command and Query Separation, a command is … Read more
In my opinion – they complement each other – but from very different perspectives. Onion Architecture is all about making the Domain/BusinessLogic independant on ‘inferior’ things like data-acccess, UI, services etc. The Onion Architecture doesn’t really care how you made the domain you have – it’s adamant about protecting it from outside dependencies. Domain Driven … Read more
From domain driven design perspective DbContext is the implementation of UnitOfWork and a DbSet<T> is the implementation of a repository. This is the point where DDD and EntityFramework contrast. DDD suggests to have a Repository per aggregate root but EntityFramework creates one per Entity. So, what is an aggregate root? Assume that we have 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
I think it is definitely popular elsewhere, especially functional languages. However, certain patterns associated with the Big Blue Book are not as applicable in dynamic languages and frameworks like Rails tend to lead people away from ideas of bounded context However, the true thrust of DDD being ubiquitous language is certainly prevalent in dynamic languages. … Read more
tl;dr No. DTOs are just one means to decouple the server side domain model from the representation exposed in HTTP resources. You can also use other means of decoupling, which is what Spring Data REST does. Details Yes, Spring Data REST inspects the domain model you have on the server side to reason about the … 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