@Override
    public void append(ExtendedCell cell) throws IOException {
      if (currentWriter == null && existingWriters.isEmpty()) {
        // First append ever, do a sanity check.
        sanityCheckLeft(this.boundaries.get(0), cell);
      }
      prepareWriterFor(cell);
      currentWriter.append(cell);
      lastCell = cell; // for the sanity check
      ++cellsInCurrentWriter;
    }
@param targetBoundaries The boundaries on which writers/files are separated.
@param majorRangeFrom   Major range is the range for which at least one file should be
                        written (because all files are included in compaction).
                        majorRangeFrom is the left boundary.
@param majorRangeTo     The right boundary of majorRange (see majorRangeFrom).
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StripeMultiFileWriter.java