Silverlight RIA Services allow serializing LINQ queries between the client and server. So the client can create the LINQ query and make it run on the server side and get back the results. So the client has greater flexibility on this side.

Microsoft WCF RIA Services simplifies the traditional n-tier application pattern by bringing together the ASP.NET and Silverlight platforms. RIA Services provides a pattern to write application logic that runs on the mid-tier and controls access to data for queries, changes and custom operations. It also provides end-to-end support for common tasks such as data validation, authentication and roles by integrating with Silverlight components on the client and ASP.NET on the mid-tier.

Silverlight can take advantage of WCF RIA Services and its features to help build business applications. WCF RIA Services makes n-tier development as simple as traditional two-tier development by including enterprise-class networking and data access for building n-tier applications with transactional support and data paging. WCF RIA Services allows you to expose an object model on the server through an optimized .NET to .NET binary format as well as a set of open extensions to the ATOM formatted known as OData and an open JavaScript Object Notation (JSON) format to Silverlight application.

The RIA Services can either Sit on top of WCF (essentially wrapping the WCF services for the Client App to consume) or Replace the WCF layer with RIA Services using alternatice data source (eg. ADO.NET Entity Data Model/Entity Framework.

RIA Services pattern, it can make it very easy to build out a n-tier silverlight application. an aim to make it nearly as simply as building traditional 2 tier applications. In this space, you should think of RIA services as a more perceptive layer on top of ADO.NET Data Services and WCF. For the data querying and updating aspects of the model we will be using ADO.NET DataServices.

Terminology - RIA Services (A.K.A WCF RIA Services) - RIA stands for Rich Internet Applications - RIA is a global term that applies to different technologies, one of them being Microsoft technologies. - WCF stands for Windows Communication Foundation. - WCF RIA Services is a framework and code generation tool built to ease the data communication development between the Silverlight and ASP.NET application clients and the server. WCF RIA Services is geared to serve Silverlight client model andASP.NET client model (other clients models have other choices such as WCF Data Services). - LINQ integrates query semantics into the languages (C#, VB, etc) providing a common abstraction for working with data; whether that data is in memory CLR objects, XML, relational data, or anything else. With RIA Services (we are) extending this pattern by offering a prescriptive model for exposing your domain logic over LINQ. (Ref #1).

Problem (Why we need it?) - Silverlight versions 2.0 and up caught the eyes of developers working on web application development using the .NET platform who wanted to escape the browser compatibility and script coding. - Silverlight can't talk to databases behind firewalls for security reasons, so it had to use WCF. - Using WCF and Silverlight may requires considerable coding for large application - Coding across tiers requires sharing business classes between tiers. - Manually coding the shared classes between tiers would obviously lead to problems in large projects.

Solution - The framework part of the RIA Services technology provides advanced data management, authentication/authorization management, and querying features; the code generation part is able to generate code in a new project based on the code in the existing project, so that hand coding the classes across tiers may be eliminated. - WCF RIA Services technology is concerned with the middle tier of the application. You are not forced to use specific data access technology.

How is RIA Different from WCF - RIA Web Services uses WCF whereas WCF does not use RIA Web Services. WCF is concerned with technical aspects of communication between servers across tiers and message data management whereas RIA Web Services delegates this functionality to WCF. WCF does not generate source code to address class business logic such as validation that may need to be shared across tiers like RIA Web Services does. WCF is a generic service that could be used in Windows Forms for example whereas RIA Web Services is focused on Silverlight and ASP.NET clients

References The above information reflects my current understanding from serveal readings and mostly from the following sources - If the above is inaccurate, I blame myself not the references!

1- What is .NET RIA Services?

2-Pro Business Applications with Silverlight 4 - By Chris Anderson

WCF RIA, introduced in .NET Framework 4.0, is a superset of the traditional WCF functionality included in .NET Framework 3.0 and all subsequent versions.

WCF RIA helps building n-tier-applications in that you can share code ansd behavior of the data contracts between different tiers. As in traditional WCF, an automatic proxy is generated on client side when you include the WCF reference into your project. But while WCF is limited (among others) in some ways:

• you cannot modify data elements provided by the data classes
• you cannot add sophisticated data validation that was not included in the data classes you provide

WCF RIA overcomes these shortcomings in that you can extend the data classes with your own annotations. These annotations are not limited to the Attributes contained in the System.Data.Annotations namespace. You can even include self-written classes or partial classes that extend the functionality of the data classes. This code is then replicated to the client. You can use this functionality for replication of validation checkers to the client without writing validation code twice. You can even include additional functionality not present in the data class (e.g. automatic retrieval of dependent objects on member access) through code originally written in the server layer, but then replicated to the client layer and used before data is sent to the server. With this, you can reduce network traffic, increase the client's responsiveness to the user and improve security of your n-tier application by validating data on both sides of the communication channel in dientical ways.

.NET RIA Services was created for Silverlight that runs in the browser. Silverlight is running a special version of the the .NET framework and in an N-tier application Silverlight is unable to share assemblies with the server side. By employing some clever code generation

.NET RIA Services makes this gap almost invisible to the developer. Classes similar to the domain classes are code generated on the client side, and ways to move objects back and forth between client and server are also made available.

We will probably be able to call into a .NET RIA Service from Windows Mobile, but I don't think it will particular easy and currently you may in fact have to reverse engineer what's sent on the wire (JSON is used). WCF on the other has a much more broad scope, but doesn't support Silverlight development in the same way that .NET RIA Services does.

If you are writing a Silverlight only N-tier application .NET RIA Services are very powerful. If however Silverlight is only one of several clients WCF is probably a better choice.

Our thinking on the RIA Services work really grow out of the LINQ project a few years ago. LINQ integrates query semantics into the languages (C#, VB, etc) providing a common abstraction for working with data; whether that data is in memory CLR objects, XML, relational data, or anything else. With RIA Services we are extending this pattern by offering a prescriptive model for exposing your domain logic over LINQ.

To answer this question let's first look at MSDN.

WCF RIA Services

Microsoft WCF RIA Services simplifies the traditional n-tier application pattern by bringing together the ASP.NET and Silverlight platforms. RIA Services provides a pattern to write application logic that runs on the mid-tier and controls access to data for queries, changes and custom operations. It also provides end-to-end support for common tasks such as data validation, authentication and roles by integrating with Silverlight components on the client and ASP.NET on the mid-tier.

What Is Windows Communication Foundation

Windows Communication Foundation (WCF) is a framework for building service-oriented applications. Using WCF, you can send data as asynchronous messages from one service endpoint to another. A service endpoint can be part of a continuously available service hosted by IIS, or it can be a service hosted in an application. An endpoint can be a client of a service that requests data from a service endpoint.

As an example Microsoft BizTalk Server R2, Windows Workflow Foundation and definitely Microsoft Silverlight utilize WCF as a communication technology.

So, in fact RIA services is a new technology that is built on top of WCF services and in Silverlight applications it helps you to make client-server integration "transparent" for a developer. At compile time RIA services generate client side classes based on the actual server side classes. Then the client can use generated client side functionality in almost the same way as if it was executing actual server side class. For example RIA servises support serializing LINQ queries between the client and the server, so the client can create the LINQ query and have it run server side without writing any code to handle client server communication. So, this allows RIA Services to provide a much richer client side object model by including any business rules and relationships between objects.

I have to mention that there are few limitations for RIA services right now. For example RIA services serializer doesn't support some data types that WCF does (reference) or RIA Services do not recognize many-to-many relationships in an Entity Framework model (reference). However I believe they will soon be addressed in the future releases.

So, to answer the question WCF is generic and provides you great flexibility and abilities to handle asynchronous messages from one service endpoint to another. From the other side RIA service is a "new layer" on top of WCF that simplifies the development. The role of RIA service is to simplify even more an integration between client side and server code in Silverlight applications, so functionality like business rules, data validation, access to data for queries and any other custom operations could be executed in the client code in almost the same manner as if they were executed on server side.