GlassFish Server is designed to provide its functionality in a modular form so that you can choose to include the functionality that you need and leave out the functionality that is not needed. OSGi modules, also called bundles, provide add-on functionality for your deployed GlassFish Server. As new add-on components are developed and existing components are modified, you can extend and update GlassFish Server by installing these components. You can add components during runtime, without stopping the server. But you must stop the server before updating or removing an installed component.