The Purpose of a Service Layer and ASP.NET MVC 2

In a MVC pattern you have responsibilities separated between the 3 players: Model, View and Controller.

The Model is responsible for doing the business stuff, the View presents the results of the business (providing also input to the business from the user) while the Controller acts like the glue between the Model and the View, separating the inner workings of each from the other.

The Model is usually backed up by a database so you have some DAOs accessing that. Your business does some...well... business and stores or retrieves data in/from the database.

But who coordinates the DAOs? The Controller? No! The Model should.

Enter the Service layer. The Service layer will provide high service to the controller and will manage other (lower level) players (DAOs, other services etc) behind the scenes. It contains the business logic of your app.

What happens if you don't use it?

You will have to put the business logic somewhere and the victim is usually the controller.

If the controller is web centric it will have to receive its input and provide response as HTTP requests, responses. But what if I want to call my app (and get access to the business it provides) from a Windows application which communicates with RPC or some other thing? What then?

Well, you will have to rewrite the controller and make the logic client agnostic. But with the Service layer you already have that. Yyou don't need to rewrite things.

The service layer provides communication with DTOs which are not tied to a specific controller implementation. If the controller (no matter what type of controller) provides the appropriate data (no mater the source) your service layer will do its thing providing a service to the caller and hiding the caller from all responsibilities of the business logic involved.

I have to say I agree with dpb with the above, the wrapper i.e. Service Layer is reusable, mockable, I am currently in the process of including this layer inside my app... here are some of the issues/ requirements I am pondering over (very quickly :p ) that could be off help to youeself...
1. Multiple portals (e.g. Bloggers portal, client portal, internal portal) which will be needed to be accessed by many different users. They all must be separate ASP.NET MVC Applications (an important requirement)
2. Within the apps themselves some calls to the database will be similar, the methods and the way the data is handled from the Repository layer. Without doubt some controllers from each module/ portal will make exactly or an overloaded version of the same call, hence a possible need for a service layer (code to interfaces) which I will then compile in a separate class project.
3.If I create a separate class project for my service layer I may need to do the same for the Data Layer or combine it with the Service Layer and keep the model away from the Web project itself. At least this way as my project grows I can throw out the data access layer (i.e. LinqToSql -> NHibernate), or a team member can without working on any code in any other project. The downside could be they could blow everything up lol...