@Override
      public InternalScanner call(RegionObserver observer) throws IOException {
        InternalScanner scanner = observer.preFlush(this, store, getResult(), tracker);
        if (scanner == null) {
          throw new CoprocessorException("Null Scanner return disallowed!");
        }
        return scanner;
      }
Invoked before a memstore flush
@return Scanner to use (cannot be null!)
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.java