IndexRebuildRegionScanner is the server side engine to rebuild and verify every index row pointed by the data table. IndexRebuildRegionScanner runs on data table regions and scans every data table rows locally, and then rebuilds and verifies index table rows referenced by the data table rows over server-to-server RPCs using the HBase client installed on region servers. However, IndexRebuildRegionScanner cannot clean up the index rows that are not referenced by the data table if there are such index rows. In order to do that we need another region scanner that scans index table regions and makes sure that every index row is valid. This region scanner will be called IndexRepairRegionScanner.