There is no doubt that Linq to SQL will have an enormous impact on the way we write data access layers. I wouldn't be surprised to find out that the impact is so profound, that we might even have to reconsider the very nature of a data access layer. In fact, what is a data access layer (DAL) anyway?
Let's start by trying to create a (working) definition of a DAL. Wikipedia is usually a good place to start, but you'll find that the Wikipedia article on DAL's doesn't exactly contain all the answers. So let' give it a try ourselves.
A DAL is a layer. That means it is part of a layered architecture. Other layers use the DAL to do data access. Indeed, the DAL is the layer accessing the data (and in the context of Linq to SQL, that's relational data), and no other layers access the data directly.
That's a good start, but what is layer? Is that a special kind of component? Not in my mind it isn't. To me, a layer can contain multiple components, and that applies to a DAL as well. Let's say I have a simple banking system. It contains functionality on clients, their accounts, and the operations (such as money transfers) they do on those accounts. That might result in a vertical partitioning of the application in three modules, "Clients", "Accounts" and "Operations". Each of those would be layered, and you'd find components at the intersections of the vertical modules and the horizontal layers. So you'd have "Clients DAL", "Accounts DAL" and "Operations DAL" components. Obviously, these components are related, they have dependencies between them. The Operations DAL depends upon the Accounts DAL (and possibly the Clients DAL as well), and the accounts DAL depends on the Clients DAL.
In .NET, components like these correspond to assemblies. So our DAL would consist of several assemblies, with (non-circular) references (dependencies) to each other. Which part of the functionality do we put where?
The Clients DAL doesn't know about accounts, that's the responsibility of the Accounts DAL. That one knows about accounts, and about clients as well. After all, accounts are owned by clients. So that means the function to retrieve the list of accounts belonging to a given client sits in the Accounts DAL, not in the Clients DAL. Since this function has a client as a parameter (or at least a client id), the Accounts DAL may indeed need a reference to the Clients DAL. Each object returned by this function has a reference to the client owning the account, or at least the id of that client.
But wait, what's the impact of Linq to SQL on what I said so far? If I have a database with Clients and Accounts tables (amongst many others) with a foreign key between them, the typical Client and Account entity classes will have a relationship between them as well. The Account class will have a delay-loaded Client property, and the Client class will have an Accounts property. That's a mutual dependency, so these two classes need to sit in the same assembly. But where does that lead us to? Typically, all tables in a database are somehow related to each other, i.e. there are no disconnected islands of tables with relations between them, but no relations to other islands in the database. But that leads us to just one DAL per database! Is that what we want?
Well, SQLMetal, the Linq to SQL tool that generates an entity model based on a database schema, definitely pushes us in that direction. Typically, it generates one source code file containing one DataContext and all the entities in your data model. But that's just the entities though, that code doesn't do any data access! To actually access the data, you need to write queries! And those queries are the responsibility of our DAL components.
In our example above, we had three DAL components, and all three of them would access the same DataContext. That implies that the DataContext should exist in an assembly of its own, an assembly underlying all DAL assemblies. But that's an additional layer, isn't it?
Well maybe it is. Maybe we need to split our traditional Data Access Layer into two distinct sublayers. For lack of better terms, I'll call them the "Entity Layer" and the "Entity Access Layer".
The Entity Layer has just one assembly in it, so we might just as well refer to that assembly as the Entity Layer as well. The entire assembly is compiled from just one code file (and some housekeeping stuff perhaps, like an AssemblyInfo.cs file), generated by SQLMetal.
The Entity Access Layer (EAL) has several assemblies (three in our example), all using the Entity Layer. The EAL assemblies contain the actual queries.
Next time, we'll look at the interface between the EAL assemblies and the business layer: what parameters are used, what results are returned? Do we expose the Entity Layer types? Do we expose query expressions or query results only?
That's enough food for thought right now, and comments are more than welcome.