StoreKey getFirstKeyInSealedSegment() throws StoreException {
    rwLock.readLock().lock();
    try {
      if (!sealed.get()) {
        throw new IllegalStateException("Index segment {} is not sealed, not able to determine the first key.");
      }
      return sealedIndex.getFirstKeyInSealedSegment();
    } finally {
      rwLock.readLock().unlock();
    }
  }
@return the first key (in lexicographical order) of sealed index segment
ambry-store/src/main/java/com/github/ambry/store/IndexSegment.java