private TagState getTagState(CacheTag tag) {
    TagState state = tagInfo.get(tag);
    if (state == null) {
      state = new TagState(tag);
      TagState old = tagInfo.putIfAbsent(tag, state);
      state = (old == null) ? state : old;
    }
    return state;
  }
A wrapper around cache eviction policy that tracks cache contents via tags.
llap-server/src/java/org/apache/hadoop/hive/llap/cache/CacheContentsTracker.java