public abstract class UpdateRequestProcessor
This is a good place for subclassed update handlers to process the document before it is
indexed. You may wish to add/remove fields or check if the requested user is allowed to
update the given document...
Perhaps you continue adding an error message (without indexing the document)...
perhaps you throw an error and halt indexing (remove anything already indexed??)
By default, this just passes the request to the next processor in the chain.