Ok, so you mean detaching entities when returning them to the view layer
(wicket)?
How do you propose updating the underlying entities? Send the detached
entities back to the service layer and copying their changes to attached
entities? Or ...?
Op 12-2-2012 14:22, schreef Martin Makundi:
Why don't you just detach the entity/data objects with deep or shallow
clone or similar? Minimal duplication...
**
Martin
2012/2/12 Bas Gooren<b...@iswd.nl>:
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!
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