Set<StoreKey> findMissingKeys(List<StoreKey> keys) throws StoreException {
    Set<StoreKey> missingKeys = new HashSet<StoreKey>();
    for (StoreKey key : keys) {
      if (findKey(key) == null) {
        missingKeys.add(key);
      }
    }
    return missingKeys;
  }
Returns the list of keys that are not found in the index from the given input keys. This also checks
keys that are marked for deletion and those that have an expired ttl
@param keys The list of keys that needs to be tested against the index
@return The list of keys that are not found in the index
@throws StoreException
ambry-store/src/main/java/com/github/ambry/store/PersistentIndex.java