by Alexandre Bergel, Stéphane Ducasse and Lukas Renggli

Page-centric Web application frameworks fail to offer adequate solutions to model composition and control flow. Seaside allows Web applications to be developed in the same way as desktop applications. Control flow is modelled as a continuous piece of code, and components may be composed, configured and nested as one would expect from traditional user interface frameworks.

Seaside is a framework for building dynamic Web applications, uniquely combining object-oriented and continuation-based approaches. Seaside applications are built by composing stateful components, each encapsulating a small portion of the page. Programmers are freed from the concern of providing unique names, since Seaside automates this by associating callback functions with links and form fields. Control flow is expressed as a continuous piece of code and in addition, Seaside offers a rich application programming interface (API) to integrate with the latest Web 2.0 technology, such as AJAX (Asynchronous JavaScript) and Comet (Server Push).

Seaside is implemented in Smalltalk, a dynamically typed programming language. Seaside inherits powerful reflective capabilities from the underlying language. Web applications may be debugged while the application is running. Inspection and modification may occur on objects on the fly. Source code is changeable and recompilable without interrupting the running application, and there is no need to restart a session.

Whereas most other Web application frameworks work in a page-centric fashion, Seaside makes use of stateful components that encapsulate a small portion of a page. Developers can compose the user interface as a tree of individual components, and often these components are reused over and over again, within and between applications. A basic set of ready-made widgets is also provided to handle user interactions.

Seaside offers a mechanism by which objects may be registered to be backtracked. With every response sent to the client, Seaside takes a snapshot of and caches registered objects. This allows previous application states to be restored in a controlled fashion, for example when the user is using the 'back' button in the Web browser.

Seaside uses programmatic XHTML generation. Instead of repeatedly pasting the same sequence of tags into templates, it provides a rich API to generate XHTML. This approach not only avoids common problems with invalid markup, but also allows markup patterns to be easily abstracted into convenient reusable methods. CSS (Cascading Style Sheets) are used to give the application a professional look.

Seaside also provides callback-based request handling. This allows developers to associate a piece of code with anchors and form fields, which are then automatically performed when the link is clicked or the form is submitted. This feature makes it almost trivial to connect the view with its model, as Seaside abstracts all serialization and parsing of query parameters away.
Callbacks are used in a natural way to define a control flow, for example to temporarily delegate control to a sequence of other components. The flow is defined by writing plain source code. Control statements, loops and method calls are mixed with messages to display components. Whenever a new view is generated, the control flow is suspended and the response is sent back to the client. Upon a new user interaction the flow is resumed.

In order that the definition of control flows as part of a Web application be as seamless as possible, Seaside internally stores a 'continuation' whenever a new component is displayed. This suspends the current control flow and allows one to resume it later on. Since continuations may be resumed multiple times, Seaside supports the use of Web browsers' 'back' and 'forward' buttons at any time. The execution state is automatically restored to the requested point and everything behaves as the developer expects.

To complement the expressiveness of the Smalltalk programming language, a set of tools including a memory analyser, a speed profiler, a code editor and an object inspector are included. The debugger supports incremental code recompilation, and enables highly interactive Web applications to be built quickly and in a reusable and maintainable fashion.

Seaside is open-source software distributed under the MIT licence. It is under constant development by an international community using Squeak Smalltalk. Due to the platform independence of Smalltalk, Seaside can be run on almost any platform. The Seaside application server can be bridged with industrial scale servers, such as Apache. Seaside is supported by major commercial Smalltalk vendors, GemStone Smalltalk and Cincom VisualWorks, as part of their product strategy, and is widely used in an industrial context.

The official Seaside Web site uses Pier, an open-source content management system written on top of Seaside.

Link:
http://www.seaside.st

Please contact:
Alexandre Bergel
INRIA Futurs, Lille, France
E-mail: alexandre.bergel@inria.fr

Next issue: April 2019
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