Increasingly, software applications are built using web technologies and made accessible via web browsers. They are commonly referred to as web applications, or hosted applications based on a software as a service model or cloud computing. These web applications are different from more traditional web sites in that their emphasis is on allowing users to [...]

This is used to notify a list of objects with a state change. Observer allows adding features in an application in a pluggable way by de-coupling the new functionality from the existing code base. An event framework is a typical implementation of the Observer pattern and is described in the figure that follows. Every time [...]

Facade provides a high-level, simpler access to a subsystem. A Facade is nothing but a shortcut to use a functionality of the application, without having to deal with the underlying complexity of a subsystem. This can be done, for instance, by providing high-level functions at the package level. Alex Martelli (aleax@google.com) about facade gdd_pydp.pdf

In data processor programming, the procurator figure is a software package purpose pattern. A procurator, in its most world-wide form, is a course of instruction functioning as an user interface to something else. The procurator could user interface to anything : a mesh connector, a boastfully target in retentiveness, a file, or some former resourcefulness [...]

An interface is a definition of an API. It describes a list of methods and attributes a class should have to implement with the desired behavior. This description does not implement any code, but just defines an explicit contract for any class that wishes to implement the interface. Any class can then implement one or [...]

The Adapter pattern is based on this philosophy and defines a wrapping mechanism, where a class or an object is wrapped in order to make it work in a context that was not primarily intended for it. StringIO is a typical example, as it adapts the str type so it can be used as a [...]

Structural patterns are really important in big applications. They decide how the code is organized and give developers recipes on how to interact with each part of the application. The most well-known implementation of structural patterns in the Python world is the Zope Component Architecture (ZCA, see http://wiki.zope.org/zope3/ ComponentArchitectureOverview). It implements most of the patterns [...]

he Singleton pattern makes sure that a given class has always only one living instance in the application. This can be used, for example, when you want to restrict a resource access to one and only one memory context in the process. For instance, a database connector class can be a Singleton that deals with [...]

General idea Since MVC is probably the most most widely used architectural pattern for designing classical web applications it is also appealing choice for GWT. However optimal implementation of this pattern differs quite significantly from traditional web MVC and is much closer to MVC for desktop clients. One of these differences is that the Model [...]

“What did everyone else say about this?” People are social. As strong as our opinions may sometimes be, what our peers think tends to influence us. Witness the spectacular growth of online “user comments”: Amazon for books, IMDb.com for movies, photo.net and flickr for photographs, and countless retailers who offer space for user-submitted product reviews. [...]