nHibernate, an n-Tier solution + request for advice

Question

I'm getting the chance to develop a mildly complex project and have been investigating the various approaches that I can use to tackle this project. Typically I would have ran with the traditional 3-Tier approach but after spending some time looking around at various options I've got an inkling that some kind of ORM might be a better fit and I'm considering nHibernate. However, I'm looking for some guidance on implementing nHibernate and more specifically how I would structure my BL and DAL in conjunction with nHibernate.

With nHibernate I would create my Objects (or DTOs?) and use nHibernate methods for my CRUD interactions all in my DAL. But what I can't get my head around is the Objects defined in the DAL would be probably be better situated within the BL, i.e. where validation and other stuff can be performed easily, and I just use the DAL from the various ObjectFactory's / ObjectRepositories. Unfortunately it seems through the many articles I've read this isn't mentioned or skirted over and I'm a tad confused.

What is the more accepted or easier method of implementation when using nHibernate in a 3 Tier system? Alternatively, what is the conventional method of exposing objects through the business layer from the data layer to the presentation?

Answer 1

My personal experience with nHibernate has led me to decide that the data access layer becomes so thin it never has made any sense to me to separate it from the business logic. Much of your data access code is already separated into xml files (or various other distinctive methods like Fluent nHibernate) and since joins are handled almost transparently your queries using criteria objects are rarely more than a few lines.

Answer 2

I suspect you're overthinking this. nHibernate is basically a pretty simple tool; what it basically does is manage the serialization of your records in your database to and from similarly structured objects in your data model. That's basically it. Nothing says you can't encapsulate your Hibernate objects in Business Layer objects for validation; that's perfectly fine. But understand that the operations of validation and serialization are fundamentally different; Hibernate manages the serialization component, and does it quite nicely. You can consider the Hibernate-serializable objects as effectively "atomic".

Basically, what you want is this: nHibernate IS your Data Access Layer. (You can, of course, have other methods of Data Access in your Data Access Layer, but if you're going to use Hibernate, you should keep to the basic Hibernate data design, i.e. simple objects that perform a relatively straightforward mapping of record to object.) If your design requires that you use a different design (deeply composited objects dependent upon multiple overlapping tables) that doesn't map well into Hibernate, you might have to abandon using Hibernate; otherwise, just go with a simple POCO approach as implied by nHibernate.

Answer 3

I'm a fan of letting the architecture emerge, but this is what my starting architecture would look like on typical ntier asp.net mvc project if I were starting it today using NHibernate.

First off, I would try to keep as much domain code out of the controller as possible. Therefore, I would create a service layer / facade over the business layer that the controller (or code behind) makes calls to. I would split my objects into two types: 1) objects with business behavior that are used on the write side, and 2) ViewModel / DTO objects that are used for displaying data and taking the initial data entry. These DTO's would have all of the view specific concerns like simple validation attributes etc... The DTOs could have their own NHibernate mappings, or they could be projected using NHibernate's AliasToBean feature. They would be mapped to business objects once they get passed the controller in operations.

As far as the Data Access layer goes, I would probably would use NHibernate directly in the service layer. I would not use the repository pattern unless I knew that I had to be able to swap out the ORM. NHibernate is already a persistence abstraction. Putting a repository over it makes you give up a lot of features.