Correct use of Repository / Service Classes

Im working on a fairly light weight client application (desktop app in .Net as connect to other applications maybe needed in the future) for tracking of RMAs within a business for the engineers, engineers will view and update the status in the application, customers will create/view RMAs on the website.

I want to know if my intended use of service and repository classes ar开发者_开发问答e correct. A web service provides the data in JSON format from which I can construct an RMAobject. This web service is accessed by http://myRMA.com/repairs/2234/RMA.JSON

Would the code to get the RMA sit within RMARepository class with a method like GetRMA(int RMAId), and should the RMARepository follow the singleton design pattern?

Whereas if the RMA is updated, i.e. Item is inspecting and pending a quote acceptance, is this completed in a service class, which calls a RMARepository to submit an update. Would the IsExists(int RMAId) be in this service class or be in the repository?

I am familiar with Presentation - Application (Logic) - Data Access - Data (Database) artitecture, and also with MVP. But i do wonder where the Service classes fit in.

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The repository classes are only used to access and store information in the database. They should have no other logic.

Services are used to fetch information from a data source (most likely a repository), process the information and return the result to the caller.

A service class can use multiple repositories to achieve the wanted result. In this layer you can also have a event system which can allow one service to take actions when another service have generated an event.

An example would be that the the UserService calls the FeedService to create a new RSS feed item for each newly created user.

The Repository layer can be represented by a ORM such as nhibernate.