4.4.1 Plugins Architecture - OSGi

Overview:

GoCD plug-in architecture allows GoCD application to be modular, customizable, and easily extensible by creating extension points. Extension points are defined by set of java interfaces and classes which abstract specific aspect of GoCD application. Plugins can be developed against these extension points by implementation of interfaces or extending abstract classes. Every plugin is packaged as a Java Archive (jar). GoCD plug-in architecture will extract these plug-ins and converts them into OSGi bundles and load them at run time. GoCD plug-in architecture has the following modules.

  • GoCD Plugin Infrastructure:
      This module is responsible for responding to any changes in plug-ins, converting plug in into into OSGi bundles and loading them.
    
  • GoCD Plugin Activator:
      This module is responsible for loading all classes in the plug-in which are not in the dependency directory and registering services for the plug-in extensions.
    
  • GoCD Plugin Api:
      This module mainly contains API's for all extension points and annotations which are required to mark various aspects of a plug-in.
    
  • GoCD Plugin Api internal:
      This module contains API's for bunch of services which will be made available to plug-in at run time, for example PluginHealthService and LoggingService.
    

GoCD Plugin Infra:

  • GoPluginDescriptor is representation of plugin which essentially contains id, name, version, status , jar location, bundle location, OSGi bundle instance and any other information of plug-in.
  • GoPluginDescriptorBuilder builds GoPluginDescriptor from plug-in XML which is part of plug-in jar. If plug-in XML is missing GoPluginDescriptor will be built with default values.
  • DefaultPluginRegistry maintains list of plug-ins which are loaded.
  • DefaultPluginJarLocationMonitor periodically monitors plug-in jar location for any changes by spawning a java thread, whenever any changes are detected DefaultPluginJarLocationMonitor determines if plug-in jar is added/deleted/updated based on the in memory known plug-in list maintained by DefaultPluginJarLocationMonitor. DefaultPluginJarLocationMonitor can register list of PluginJarChangeListener and when plug-in is added/deleted/updated changes are accordingly notified to all the registered PluginJarChangeListener. Currently plug-in jars can be located at bundled directory or external directory.

    • Bundled directory contains plugins bundled with GoCD. Any unbundled plugins put in this directory will be removed. The directory is meant exclusively for plugins bundled with the product.

    • External directory contains all the unbundled plugins. This directory is recommended for use by plugin developers.

  • DefaultPluginJarChangeListener is the default implementation of PluginJarChangeListener. DefaultPluginJarChangeListener listens to plug-in add/delete/update events, upon listening to these events DefaultPluginJarChangeListener constructs GoPluginDescriptor using GoPluginDescriptorBuilder, after performing necessary validations plugin jar is extracted into bundle directory, GoPluginDescriptor will be added to DefaultPluginRegistry and OSGi bundle will be loaded using GoPluginOSGiFramework.

  • GoPluginOSGiFramework is responsible for loading and unloading OSGi bundles given a GoPluginDescriptor. All the communication between Go application and plug-ins will be facilitated through GoPluginOSGiFramework. Given java class service reference and plug-in id, GoPluginOSGiFramework can find service reference instance.

GoCD Plugin Activator:

  • DefaultGoPluginActivator is an implementation of OSGi BundleActivator and is responsible for loading all classes in the plug-in which are not in the dependency directory and registering services for the plug-in extensions.

  • GoCD Plugin Activator module gets packaged as separate jar and added as dependency JAR into each plug-in by copying activator jar into bundle directory of plug-in.

GoCD Plugin Api:

This module contains all the API's which are specific to extension points and also API classes which are common across all the extension. GoCD Plugin Api module also contains various java annotation classes which are required to mark various aspects of a plugin.

Every plugin needs to interact with GoCD server to capture certain information and it is represented as Configuration class. Configuration is either provided by GoCD application or by user. Configuration has list of Property. Each Property has name, value and set of Options associated with Property.

Any java interface or class inside GoCD Plugin Api module which is annotated with GoPluginApiMarker indicates to the plug-in framework that the interface or class will be implemented by a plugin. and will be treated as extension point provided by GoCD. Plugin authors are required to mark extension implementation class with Extension annotation. GoCD Plugin Api module also provides following java annotations

  • Load : Annotation that marks a method to be called when a plug-in is loaded.

  • UnLoad : Annotation that marks a method to be called when a plug-in is unloaded.

  • UsedOnGoAgent : A method mark by this annotation indicates that the method is invoked on the GoCD Agent.

  • UsedOnGoServer : A method mark by this annotation indicates that the method is invoked on the GoCD server.

GoCD Plugin Api internal:

GoCD application leaks bunch of service implementations to plug-in which can be used at various stages of plug-in execution. This module contains interfaces for such service implementations.

  • PluginHealthService: GoCD plugin infrastructure will load every plug-in through GoCD plug-in activator. If GoCD plugin activator encounters any issues while loading they will be notified to GoCD server via PluginHealthService

  • LoggingService: GoCD application provides Logger class via GoCD plug-in api which can be used by plug-in to log various events. All the logging which is performed by Logger class will be logged along with GoCD server log in a separate file which is specific to each plug-in. Logger class uses LoggingService behind the scene.

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