code: private @Nullable CacheWriterException deleteAllToCacheWriter(Set<? extends K> keys) {
      if (!configuration.isWriteThrough() || keys.isEmpty()) {
        return null;
      }
      List<K> keysToDelete = new ArrayList<>(keys);
      try {
        writer.deleteAll(keysToDelete);
        return null;
      } catch (CacheWriterException e) {
        keys.removeAll(keysToDelete);
        throw e;
      } catch (RuntimeException e) {
        keys.removeAll(keysToDelete);
        return new CacheWriterException("Exception in CacheWriter", e);
      }
    }
docs: Deletes all of the entries using the cache writer, retaining only the keys that succeeded.