It sounds like you have a lot of layers/scaffolding (=high maintenance) in your design.
The "exposed domain model" pattern was created to avoid all those extra layers (especially DAOs and DTOs) in recognition of the fact that for most applications they *feel* redundant and *seem* like code duplication. I understand the puritan goals that bring about the adoption of DTOs but IMHO introducing them forms a slippery slope to "productivity impedance". We adopted the Exposed Domain Model (EDM) pattern with "Open [Session|PersistenceManager] In View" many years ago and haven't created a DAO or DTO since - and development has been easier, faster and more maintainable since. EDM still does provide for Respositories (object search, looks ups, etc., - where your queries reside) and Services (where you orchestrate non trivial model changes, updates) but for the most part if the UI is simply CRUDing a model object we expose that model object and let it call the setters and getters directly. We even allow some trivial persistent objects to be instantiated directly by the UI where that is appropriate. With 'persistence by reachability' (we use JDO - JPA might also have this) it's not even necessary to call persist() on the newly constructed instances if they are reachable by at least one already persisted object. In certain cases where instantiation of an object graph must be orchestrated according to some business rules or some constraints implicit in the model we do either of the following: 1. make the domain class' constructors private and force instantiation to take place via a static method on the particular class. 2. make the domain class' constructors protected and force instantiation to take place via a Service provided for the package in which the new class exists. It's been working flawlessly for us and our domain model now has over 300 persistent classes. I doubt we would have been able to grow it this fast if we had the extra development burden of developing and creating DAO and DTO classes on the way. Also the code base would probably be close to double what it is today. Having let our hair down this much we must state that we still are 100% puritan when it comes to separation of Model and Presentation layer. The model itself has ZERO dependencies on any UI code and can be compiled completely independently of the UI. >-----Original Message----- >From: James Carman [mailto:jcar...@carmanconsulting.com] >Sent: Monday, 13 February 2012 4:40 AM >To: users@wicket.apache.org >Subject: Re: Architectural question > >Well setters/getters somewhat go against ddd. We just have to figure out >what works for us. It's all about finding what gets the job done most >effectively. >On Feb 12, 2012 10:45 AM, "Bas Gooren" <b...@iswd.nl> wrote: > >> We already use OSIV, thanks to guice-persist. >> This means the read-side of things is rather trivial, and that the >service >> and dao layers do need to be aware of the exact data the view needs >(since >> lazy loading is possible). >> >> With regard to the write-side of things: we do what you do (call update >> explicitly) right now. >> >> Suppose you are using DDD in a project, how would you go about >> constructing and populating a new object? Without DDD my entities were >mere >> containers for data with some validation and JPA annotations. So it was >> simply a matter of creating a Model which wraps a "new Customer", and >> wicket pushing its fields to that customer object. >> >> However, when applying DDD, that Customer is no longer a simple >container, >> but a business object. Having wicket push changes directly to fields >(those >> fields may even be private-access only, without setters) seems to go >> against DDD fashion. >> >> So I feel it would be better to create a DTO for a certain view (e.g. >> NewCustomerDTO for a CreateCustomerPanel), and putting all validation >> annotations on the DTO, too. When it's time to persist the new customer, >> the service layer simply creates a new customer and copies all fields >from >> the DTO. >> >> But then again, maybe this is overcomplicating things. DDD seems like a >> good match for most of my projects though, and I'd like to be able to >> properly integrate my domain objects with wicket. >> >> Op 12-2-2012 15:17, schreef James Carman: >> >>> I just use open session in view. You can still retrieve stuff outside a >>> transaction. I explicitly call update to persist. >>> On Feb 12, 2012 7:54 AM, "Bas Gooren"<b...@iswd.nl> wrote: >>> >>> Hi All, >>>> >>>> I have an architectural question about wicket, DDD and the service >layer. >>>> >>>> Let's say we have a simple JPA entity (Customer), and a few simple >CRUDL >>>> screens. >>>> For database access, we have a DAO layer (CustomerDao) which delegates >to >>>> an EntityManager, and provides some convenience methods for searching. >>>> We also like to have clear boundaries, so we have a thin service layer >>>> which wraps persist() and delete() calls in a transaction before >>>> forwarding >>>> them to the DAO layer (@Transactional, as provided by guice-persist). >>>> >>>> A wicket model fetches one or more customers (by id or by running a >>>> search), and attaches to a form. In the form we use PropertyModels >which >>>> push their changes to the entity, and in onSubmit() we call >>>> service.persist(entity). >>>> This means that the actual changes to the model happen outside of the >>>> transaction (in wicket code), and within the transaction (/service >layer) >>>> we merely call persist() and flush(). >>>> >>>> Then parts of the app need something a bit more advanced, so we decide >to >>>> apply parts of DDD and put logic where it belongs (on the domain >models). >>>> However, some logic coordinates multiple models, so we add a domain- or >>>> application-service for that. >>>> The good thing about DDD is that it's a lot more clear what happens >>>> (intent). We now realize that having a persist() method on a entity- >based >>>> service now looks like a bit of a code smell, since it does not capture >>>> intent at all. Also, since the changes to the model happen in wicket, >>>> before the service layer is called, I feel that the service layer is >not >>>> doing anything to act as a boundary. We might as well mark the >persist() >>>> method on our daos @transactional and remove the service layer. >>>> >>>> The only clean way to fix this seems to be either: >>>> (a) using DTO's so the UI/wicket is not actually modifying domain >>>> entities >>>> upside: the state of the domain is not modified by wicket itself >>>> downside: duplication of models (actual model + DTO); >>>> downside: validation is currently set-up in wicket by scanning >fields >>>> for validation annotations, so we would need to duplicate those on the >>>> DTO? >>>> >>>> (b) using a concept from CQRS: sending commands to the domain through a >>>> bus. This clearly and cleanly defines the intent and captures the exact >>>> change. >>>> upside: the state of the domain is not modified by wicket itself >>>> downside: likely overkill for what we are trying to achieve; lot of >>>> extra complexity >>>> >>>> (c) wrapping the entire request in a transaction >>>> upside: easy to implement >>>> downside: since anything in the request can fetch a dao, read some >>>> entities and modify them, this means we can lose track of what happens >>>> in a >>>> request; >>>> downside: feels like moving backwards >>>> >>>> (d) simplify by removing thin services and, where necessary, putting >more >>>> logic in the dao's >>>> upside: simple api contract: want to save/update an entity? use the >>>> dao >>>> directly >>>> downside: dao's contain logic which does not really belong there >>>> downside: if at some point we really do need a service, the api >>>> contract becomes less clear: for X and Y you can use the dao, for Z you >>>> have to use a service >>>> >>>> (a) and (b) provide a way to capture a change and execute all of the >>>> change inside a transaction. >>>> >>>> So my question to the list is: what are your experiences with this? How >>>> do >>>> you deal with this in simple to moderately complex webapps? >>>> >>>> Thanks for reading! >>>> >>>> --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscr...@wicket.apache.org For additional commands, e-mail: users-h...@wicket.apache.org
