code: @Override
      public boolean keyExists(String spaceId, String key) throws IOException {
          /*
           * Call get(spaceId, key)!=null or delegate to kvStorage.keyExists(spaceId, key)?
           * Since DELETE and PUT operations can be async, delegating to kvStorage.keyExists(spaceId,
           * key) would be preferred to avoid out-of-date data.
           */
          return kvStorage.keyExists(spaceId, key);
      }
docs: {@inheritDoc}