Architecting Silverlight LOB applications (Part 1) – Concepts and technologies

In this first part of the series I’ll focus on some open source projects in Microsoft’s Patterns & Practices and other technologies that are available and can help us achieve our goal.

First we have PRISM (Composite Application Guidance for WPF and Silverlight). PRISM is composed of a guidance and a set of libraries that are designed to help you more easily build modular WPF and Silverlight apps. It also has built-in components that let you take advantage of some known patterns like MVVM, Event Aggregator and Dependency Injection. There are others but I’ll focus on these three because I find them to be the most useful.

prism

PRISM applies to client development. I think the best feature PRISM provides is the ability to specify your application modules (XAPs in Silverlight) as a catalog and load them on demand. We’ll explore this in detail in other parts of this series when we create a module catalog to be used within the application.

Dependency injection (a specific usage of the Inversion of Control concept) is a prime technique for building loosely coupled applications because it provides ways to handle dependencies between objects. It helps you build testable code because you can easily switch concrete implementations which your objects depend on. PRISM is built using this design principle and it uses the Unity container, which implements the DI technique. PRISM also lets you use other DI containers but I’ll focus on Unity since I haven’t enough experience with others.

This pattern can be applied in both client and server development.

I didn’t find a nice and simple figure about Dependency Injection so I’ll link to this post where I’ve used this technique in a WCF Service (OrderService) which used an IOrderRepository implemented by the OrderRepository class which had dependencies being injected in its constructor.

The MVVM pattern is becoming the alpha and the omega when building WPF and Silverlight apps. It helps you separate concerns when designing your presentation layer and if it is well thought you can create a great set of reusable code for your application and even reuse it for other applications.

mvvm

With MVVM you basically think of your view as a set of classes and properties and not as a set of UI controls. This allows you to easily switch your UI while keeping the same logic that it is bound to, and at the same time enables testability. Plus it allows you to reuse a set of UI logic in lots of views with less effort. This pattern is applied to client development.

The Event Aggregator pattern allows you to build your app with an Event Driven Architecture where components communicate with each other in a decoupled manner. It provides a way to publish and subscribe to events without knowing who is subscribing or who is publishing. The only thing you need to know is the event you are publishing or subscribing and how to react to that event. Usually modules do not depend on each other and using events as contracts to communicate between them is a good approach. This pattern applies to client development.

Next we’ll focus on server development.

Client-server communication in Silverlight has been enforced and recommended to use web services. Since we are building LOB applications it makes sense to use services that are good to deal with data. WCF services sure are good but are they ideal? What’s the cost of maintaining a complete service layer that lets us expose CRUD operations on our data and also provides us the ability to do other queries (like get products by productType), or other business operations? How many times have we done this? And how many times have we done this for different applications? How about validation of the data on the server and client side? How about proxy generation? How about the amount of data sent to the wire just to change a DateTime value on the database?

We all know Visual Studio’s generated code is ugly as hell and maintaining client-side complete service agents can be troublesome, because you usually need to keep it synchronized with your service contract. In Silverlight it is slightly more complicated to share an assembly with service and data contracts than WPF, because of type incompatibilities. Also we usually like our client-side data contracts to implement INotifyPropertyChanged and use ObservableCollections so we take advantage of data binding but at the server we really do not care about this.

WCF RIA Services aims to solve all these problems and many others.. I’ll skip the “all the things RIA Services can do part”, and point you to the right place if you still don’t know. The most important features we’ll explore are query composition, data validation, change tracking, unit of work, among others.

riaservices

I’ve been using RIA Services for the last 7 months and one thing I find very useful is that the way you communicate either with the server and with your model (at the client side) is through a very well defined API which makes it easy to create very generic code that can be used to create reusable viewmodels. This is especially useful when you have multiple persons working in the same project and you want to ensure that everyone will code the same way (I will definitely explore this in later posts).

WCF RIA Services current release is the RC2, it has a go live license, so we’re good to go.

This doesn’t mean you shouldn’t use WCF as well. There are cases where WCF RIA Services isn’t well suited to work for. For example, file uploading or downloading. In this situation it’s preferable that you create a WCF Service or an HttpHandler to deal with this particular case.

For our data layer the best options nowadays are still NHibernate (using Linq to NHibernate) or Entity Framework. Nevertheless, we should encapsulate our data access through repository objects and do not use our data access technology directly in our services. I’ve got a very simple post that talks exactly about this. In this case, we can use any or both. RIA Services generates code on the client based on reflection and other metadata that is applied in our server side entities. If you use Entity Framework with RIA Services it already has a specific metadata provider that is able to understand relationships between entities and generate them correctly in the client. If you wish to use NHibernate you will have to specify additional metadata in your entities. For simplicity I’ll use Entity Framework. There are other resources on the web that explains how to use NHibernate.

Last but not least, I’d like to talk about MEF. MEF stands for Managed Extensibility Framework, has been shipped with .NET 4 (SL4 also) and is designed to help us compose our apps in a decoupled way much easier than using an IoC container for dependency injection. You simply annotate your classes with Import/Export attributes and MEF will resolve dependencies between objects for you. Of course there’s more we can do. You can provide metadata when exporting an object and import only objects that follow some criteria. MEF can be used to download XAP’s when an Import must be satisfied and the exported object is in another XAP, which helps building modular apps. It is said that the new Prism version will use MEF for its modularity features, which makes it even more interesting.

In the next post I will start with a simple business scenario and follow a top-down approach by designing an architecture built upon these technologies. I haven’t had much time lately but I’m trying to catch up so see you soon!

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