To continue this series that started with KISS Architecture (part 1), I will now talk more about a modern mobile multi-tier architecture. Because a picture always says more than a thousand words, let's start with a classic one.

This image should be familiar to anyone who read the Application Architecture for .NET from back in 2002. Interestingly, the Microsoft's patterns & practices team are working on a new version of this guide called Application Architecture Guide 2.0 that is already in its second beta (there is also a specific pocket guide for mobile architectures). Building on the discussions in that guide, I have taken the old diagram (because I like the colors) and created an updated version with a small modification. The modification is related to the placing of the business entities, that I think belong in the data tier. The main reason for that is that the ORM technologies (in my case, LINQ to Entities or ADO.NET Entity Framework) take care of defining the business entities. For more details on multi-tier architectures, please read the guide, it's a great resource.

A nice addition to the traditional diagram is to show how other systems ("External Systems") access my system's services that are published as service interfaces with WCF, and that is exactly how other systems ("Services") are accessed by my system.

However, the biggest changes compared to my previous architecture blueprint are probably in the data tier. Even if I was a big fan of LINQ to SQL, the ADO.NET team have announced that the long-term plan is that in ".NET 4.0, LINQ to Entities will be the recommended data access solution for LINQ to relational scenarios". Therefore, I now use LINQ to Entities (ADO.NET Entity Framework) in the data tier.

However, the changes in the data tier doesn't stop there. Another (probably even bigger) change is that the data tier is now made loosely coupled for the first time. Many attempts have been made in this direction (SQLXML to mention one) before, but I think that this time Microsoft got it right with ADO.NET Data Services. In short, it's a way of accessing data through a simple RESTful interface over HTTP using formats like ATOM and JSON.

Even if this means that it becomes very easy to access the data source directly form the presentation tier, it doesn't mean that a sound architecture should use or even suggest such nonsense. It may be interesting and fun for quick demos, but doesn't belong in any serious enterprise solution - mobile or not. Therefore, the thinking that I introduced in A New Mobile N-tier Architecture (part 2) about keeping the data and logic as logically bound together as possible (in what I call domains) still holds. I strongly believe that the data should only be accessed through the logic.

In the next part I will talk about the benefits of having tiers that are loosely coupled.