Help w/ DDD, SOA and PI

Question

Without getting into all of the gory details, I am trying to design a service-based solution that will be consumed by several client applications. The solution allows admins to create and modify document templates which are used by regular users to perform data entry. It is my intent to make the application a learning tool for best practices, techniques, etc.

And, at the same time, I have to accomodate a schizophrenic environment because the 'powers that be' cannot ever stick to their decisions regarding technologies and tools. For example, I am using Linq-to-SQL today because they aren't ready to go to EF4 but there is also discussion about switching over to NHibernate. So, I have to make the code as persistent ignorant as possible to minimize the work required should we change OR/M tools.

At this point, I am also limited to using the partial class approach to extend the Linq-to-SQL classes so they implement interfaces defined in my business layer. I cannot go with POCOs because management insists that we leverage all built-in tooling, etc. so I must support the Linq-to-SQL designer.

That said, my service interface has a StartSession method that accepts a template identifier in its signature. The operation flows like this:

1. If a session already exists in the database for the current user and specified template, update the record to show the current activity. If not, create a new session object.
    
2. The session is associated with an instance of the template, call it the "form". So if the session is new, I need to retrieve the template information to create the new "form", associate it with the session then save the session to the database. On the other hand, if the session already existed, then I need to also load the "form" with the data entered by the user and stored in the session previously.
    
3. Finally, the session (with form definition and data) is returned to the caller.
    

My first objective is to create clean separation between the logical layers of my application. The second is to maintain persistence ignorance (as mentioned above). Third, I have to be able to test everything so all dependencies must be externalized for easy mocking. I am using Unity as an IoC tool to help in this area.

To accomplish this, I have defined my service class and data contracts as needed to support the service interface. The service class will have a dependency injected from the business layer that actually performs the work. And here's where it has gotten messy for me.

I've been try to go the Unit of Work and Repository route to help with persistance ignorance. I have an ITemplateRepository and an ISessionRepository which I can access from my IUnitOfWork implementation. The service class gets an instance of my SessionManager class (in my BLL) injected. The SessionManager receives the IUnitOfWork implementation through constructor injection and will delegate all persistence to the UoW but I find myself playing a shell game with the various logic.

Should all of the logic described above be in the SessionManager class or perhaps the UoW implementation? I want as little logic as possible in the repository implementations because changing the data access platform could result in unwanted changes to the application logic. Since my repository is working against an interface, how do I best go about creating the new session (keeping in mind that a valid session has a reference to the template, er, form being used)? Would it be better to still use POCOs even though I have to support the designer and use a tool like AutoMapper inside the repository implementation to handle translating the objects?

Ugh!

I know I am just stuck in analysis paralysis so a little nudge is probably all I need. What would be ideal would be if someone could provide an example how you would you would solve the problem given the business rules and architectural constraints I've defined.

Answer 1

If you don't use POCOs then your not really going to be data store agnostic. And using POCOs will allow you to get your system up and running with memory based repositories which is what you'll likely want to use for your unit tests anyhow.

The AutoMapper sounds nice but I wouldn't consider it a deal breaker. Mapping POCOs to EF4, LinqToSql, nHibernate isn't that time consuming unless you have hundreds of tables. When/If your POCOs begin to diverge from your persistence layer then you might find that an AutoMapper wont really fit the bill.