The concept of a BIServer Plugin has been introduced as of Pentaho BIServer version 2.1. A plugin provides developers with the ability to introduce custom behavior into the BIServer.  Plugins fall into three categories, although a plugin may provide one or more of these features:

Visualizations

these are live UIs that a user interfaces with directly.  Visualization plugins are focused on user experience and render interactive content typically over HTTP within the Pentaho User Console (PUC) or other client user interface.

Actions

a self contained module that may interface to an external library or service to fulfill a business need (you may know these as the steps within an Action Sequence).  Actions are useful in developing simple as well as non-trivial BI solutions through the use of Action Sequences and also integrate with the scheduling system, allowing batch processing.

Services

service plugins are useful when you want to expose behavior or return data from the BIServer, which is machine processable. 

User Console Customizations

In addition to these three types of plugins, you may also customize the Pentaho User Console (PUC) to make it aware of any new behavior you have introduced. PUC customizations include:

See the BIServer Plugin Developer's Guide for more detail about these features and how to create plugins to deliver them.

A sample plugin is now available, see Echo Plugin - a sample plugin for the BIServer. It is useful for demonstrating the features of the plugin architecture as well as providing a starting point for your own plugin.