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I am working in the logical architecture of a project that receives some info from users and processes it. One of the requirements is to expose an interface for external developers to add further functionalities. So far I have proposed a MVC 2-tier architecture, where the View and Controller run in the user's machine, and the Model is hosted in an Application server and remotely invoked. The requirement on functionalities suggests me to use a plugin pattern.

Additional steps selected by the user might be executed when processing the information, so I wanted to model them as plugins that will already exist when the application is released. This means that this plugins would affect the same resource (the processing flow), and I am uncertain about how to deal with this when both plugins are enabled.

Since I am not as familiar with the plugin pattern as with other patterns, the reading I did before asking made me try something similar to the Abstract Factory pattern. The problem is that, when two or more plugins are enabled, I would need mutiple inheritance. I also thought of the Builder pattern to model steps of the processing separately, but then an order among plugins would have to be defined and this would affect the independence of plugin's developers.

dvaccam
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If I understand correctly, you want to be able to extend the same variation point with multiple independent plugins. If so, the pipes and filters pattern is an appropriate mechanism.

With this approach, plugins represent filters and you can design a plugin container that loads and then chains them. If no plugin is loaded for a given variation point, then you either short-circuit the variation point or provide some form of default filter.

Also, giving the plugins a mechanism to specify their position in the filter chain will be helpful, so think about that when designing the plugin-interface.

theDmi
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