For more details on OCC, see the paper On Optimistic Methods for Concurrency Control by Kung and Robinson available here .
To enable transactions, modify hbase-site.xml to turn on the TransactionalRegionServer. This is done by setting hbase.regionserver.class to org.apache.hadoop.hbase.ipc.TransactionalRegionInterface and hbase.regionserver.impl to org.apache.hadoop.hbase.regionserver.transactional.TransactionalRegionServer Additionally, to properly recover from the write-ahead-log, the transactional log key class must be registered by setting hbase.regionserver.hlog.keyclass to org.apache.hadoop.hbase.regionserver.transactional.THLogKey
The read set claimed by a transactional scanner is determined from the start and end keys which the scanner is opened with.