Customizable Web Applications

Question

At my company we develop prefabricated web applications. While our applications work as-is in many cases, often we receive complex customization requests. We are having a problem in trying to perform this in a structured way. Generic functionality should not be influenced by customizations. At the moment we are looking into Spring Web Flow and it looks like it can handle a part of what we need.

For example, we have an Online Shopping and we have a request from a client that in a moment of checking out the Shopping Basket order has to be written to a proprietary logging system. With SWF, it is possible to inherit our Generic Checkout Flow with ClientX Checkout Flow and to extend it with states necessary to perform a custom log write. This scenario seems to be handled well. This means we can keep our Generic Checkout Flow as is and extend it with custom functionality, according to Open/Closed principle. Our team in time can add functionality to the Generic Checkout Flow and this can be distributed to a client without modifying the extension. However, sometimes clients request our pages to be customized. For example, in our Online Shopping app a client requests a multiple currencies feature. In this case, you need to modify the View as well as the Flow (Controller). Is there a technology that would let me extend the Generic View and not modify it? So far, only two solutions with majority of template-based views (JSP, Struts, Velocity etc.) seems to be

• to have a specific version of view for each client. This obviously leads to implementation explosion
• to make application configurable depending on parameter (if multipleCurrency then) that leads to code explosion - a number of configuration conditions that have to be checked in each page

What would be the best solution in this case? There are probably some other customization cases I am not able to recall. Is there maybe a component based view technology that would let me extend certain base view and does that makes sense. What are typical solutions to a problem of configurable web applications?

Answer 1

each customization point implies some level of conditionality.

Where possible folks tend to use style sheets to control some aspects. For example display of a currency selector perhaps could be done like that.

Another thought for that currency example: 1 is the limiting case of many. So the model provides the list of currencies. The view displays a selector if there are many, and a fixed field if only one. Quite well-defined behaviour - easy to test reusable for other scenarios.