protected function addIdentifierToTags(string $entryIdentifier, array $tags)
    {
        foreach ($tags as $tag) {
            // Update tag-to-identifier index
            $identifiers = $this->findIdentifiersByTag($tag);
            if (array_search($entryIdentifier, $identifiers) === false) {
                $identifiers[] = $entryIdentifier;
                $this->memcache->set($this->identifierPrefix . 'tag_' . $tag, $identifiers);
            }

            // Update identifier-to-tag index
            $existingTags = $this->findTagsByIdentifier($entryIdentifier);
            if (array_search($tag, $existingTags) === false) {
                $this->memcache->set($this->identifierPrefix . 'ident_' . $entryIdentifier, array_merge($existingTags, $tags));
            }
        }
    }