another layers of data

So, an application is hereby defined as a set of code, objects, components, services, layers or tiers that exist within a trust boundary. They trust each other, and have no need to replicate business logic (validation, calculation, manipulation, authorization) due to lack of trust. If data is validated or manipulated in one layer, all the other layers just assume it was done correctly.

A corollary of the above statement is that the constituent entities making up an application are encapsulated -- at least in a logical sense. In other words, external entities (users or other applications) can not directly interact with code, objects, components, services, layers or tiers except through the application's formal interfaces (UI/presentation or service interfaces).

Assuming we agree on the above definition and corollary, it is very obvious that the only time a data source can be a layer within an application is if that data source is exclusive to the application.

If the data source is not exclusive, then other applications or users can directly interact with it without going through our application's presentation/UI layer. In such a case (which is the norm, not the exception), the data source cannot be a data layer within an application. It exists outside the trust boundary and thus must be considered as an external entity (like a user or another application).

What does this mean from a practical perspective?

Well it means that our application can't trust the data source. But more than that, it means that we need to fundamentally rethink the architecture of an application (or service).

What is a user? From the perspective of an application, a user is a data source and an event source. In an object-oriented world view the user could easily be characterized as just another object in our model. In a service-oriented world view the user could easily be characterized as just another service with which we interact.

In fact, we can go so far as to equate ADO.NET, XML, Web services, HTML and a GUI as all being nothing more than external interface technologies. What I' m suggesting here is that maybe there is no such thing as a presentation/UI layer -- at least not in a substantively different way from a data access layer (or a service agent layer). This idea is illustrated below.