I decided to follow up on my previous post and write a series of more practical articles to illustrate various decisions that are specific to mobile enterprise applications. Being is Microsoft platform developer, I am going to stick to Windows Phone 7 in this series. This first blog post discusses general architectural issues.
Choosing Client Technology
WinPhone 7 supports two development paradigms: Silverlight and XNA. The latter targets game developers, providing them with game loop, sprites, and stuff like that. Silverlight, on the other hand, is a subset of the traditional desktop UI platform (WPF). It has a rich set of controls plus the ability to do cool visual effects. Silverlight is a natural choice for a mobile enterprise application.
Relative Merits of MVVM
Model-View-ViewModel design pattern has a widespread adoption in the XAML world, including Silverlight. The question isn't whether or not MVVM can be used in WinPhone 7 applications (it can) but whether or not it should be used. On the negative side, MVVM makes your app larger and slower, which is no small thing on the mobile device. On the positive side, it provides the separation between user interface and logic which is extremely beneficial in two scenarios: unit testing and sharing projects with a designer.
Generally, I am a big proponent of unit testing - I believe they are essential to building high-quality maintainable programs. However, WinPhone 7 unit testing is a little wobbly at the moment. Test framework built into Visual Studio doesn't support phone applications, so we are supposed to download Silverlight Unit Test framework, which is included into Silverlight Toolkit. Only problem is that latest version of the toolkit supports Silverlight 4, but not WinPhone 7... I'm sure eventually Microsoft will sort everything out, but for the time being it's probably best not to concentrate on unit tests.
Sharing Projects With Designer
If your project team has a dedicated designer who actually works with the same project as developers, MVVM provides a nice clean separation of concerns. If, on the other hand, you do not have a designer, or the one you have prefers exchanging images and screenshots with you over email, MVVM doesn't really give you much of an edge.
Choosing Server Technology
We should try to shift as much logic as possible to the server. This makes sense from several points of view. First of all, mobile client doesn't offer a lot of processing power while server side can be very scalable. Second, if you support more than one client platform, it helps to have as little code to port as possible. Third, your business logic is your intellectual property, and it is definitely more secure on the server.
SOAP or REST?
When it comes to building web services, WCF allows the choice between SOAP and REST. On the high level, SOAP is really a remote procedure call mechanism which can support sophisticated logic and advanced security requirements. REST, on the other hand, operates with a simple set of HTTP verbs and is therefore better suited for simpler CRUD-type logic.
Mobile clients communicate with web services over 3G or WiFi networks, which makes them vulnerable to security attacks (open public WiFi are especially fertile ground for packet sniffing - read more here). It is therefore essential to encrypt all messages, not just those related to authentication. Using SSL protocol for communication is a good start.
(to be continued)