Minor Description
UniqFieldsUpdateProcessorFactory has been arround for a while, but if we change it to subclass FieldValueSubsetUpdateProcessorFactory, a lot of redundent code could be eliminated from that class, and the factory could be made more configurable by supporting all of the field matching logic in FieldMutatingUpdateProcessorFactory, not just a list of field names.
(the only new code that would be needed is handling the legacy config case currently supported by UniqFieldsUpdateProcessorFactory)
—
For users of 4.x starting with 4.5, the existing init param syntax will still be supported, but a warning will be logged recommending they switch to using <arr name="fieldName">...</arr> instead of <lst name="fields">..</lst>. Starting with 5.0, the "fields" option won't be recognized at all.