public void preSplitRegion(final TableName tableName, final byte[] splitRow) throws IOException {
    execOperation(coprocEnvironments.isEmpty() ? null : new MasterObserverOperation() {
      @Override
      public void call(MasterObserver observer) throws IOException {
        observer.preSplitRegion(this, tableName, splitRow);
      }
    });
  }
Invoked just before calling the split region procedure
@param tableName the table where the region belongs to
@param splitRow  the split point
hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterCoprocessorHost.java