Building dynamically extensible applications

Plug-in systems

Using Service Binder components is an ideal way to create dynamically extensible (i.e., "plug-in") systems. Plug-ins typically extend the functionality of an application core and do so by implementing a well-defined interface. Dynamically extensible systems require that the plugins can be incorporated into the system after the system has been installed. This is normally achieved through a late linking mechanism. Examples of this kind of applications are abundant, they range from web browsers, such as Mozilla, to platforms such as NetBeans or Eclipse. An advantage of building this kind of applications out of Service Binder components deployed in OSGi bundles is that plugins can be incorporated or removed during execution thanks to the OSGi platform deployment mechanisms. The following sections discusses briefly the approaches that can be followed to build this kind of applications.

Development approaches

There are two different approaches to implement a plug-in system. They are called extensible application core and service provider application core.

• Extensible application core: in this case, the core is a component that requires services provided by the plug-ins. All plug-ins implement a common service interface which in figure 1 is the PluginService interface. The core can declare an optional dependency towards the plug-in, so that it can function even when no plug-ins are present. This approach has the benefit that the core can discriminate among the plug-ins that it integrates (this can be achieved for example using a filter on properties associated to the plug-ins). Note that in the figure one of the plug-ins connects to another through its own service dependency.
  

Figure 1. An extensible core

• Service provider application core: in this case, the core provides services which are used by the plug-ins. In this approach, the core cannot discriminate among the plug-ins that it integrates, but plug-ins could choose a particular core in case multiple cores are present in the execution environment at a given moment. An example of this approach is depicted in figure 2; a well known example of this approach is OSGi's HTTP service that is used to register servlets.
  

Figure 2. A Service provider core

In traditional OSGi programming, the first approach would be preferable as service dependency management logic would be centralized in the core while in the second case this logic would have to be replicated inside every plug-in. This, however, is not a concern when using the Service Binder as no service dependency management logic has to be written. Finally, a combined approach can also be followed.

Application startup

The extensible system can be launched by starting an OSGi framework where the bundles containing the core, the plugins and  the Service Binder bundle are installed. The applications is dynamically assembled as services become available. As soon as the core component instance is activated, the application begins its execution. The core can be notified of its activation by implementing the Lifecycle interface.

Plug-in management

A big advantage of building extensible systems based on Service Binder components are that the OSGi platform provides mechanisms that allow plugins to be installed, removed or updated from an URL while the system is being executed. If service dependencies are defined as being dynamic, then the resulting application is capable of incorporating, releasing or updating the plugins as it is being executed.

A "real life" example

Figure 3. VersaTest client

The people at Ascert have developped an application based on the principles exposed above (see screenshot in Figure 3). Their system allows multiple visualisation tools to be incorporated into a service providing core. The core provides services that allow, among other things, the tools to add new entries in the toolbar menu. We are very grateful to them for providing us with this example, and for being active testers and contributors of OSCAR and the Service Binder.