@Override
    public int compareTo(Session rhs) {
        Long lhsId = getSessionId();
        Long rhsId = rhs.getSessionId();
        return lhsId.compareTo(rhsId);
    }
Returns application id read from ZooKeeper. Will return Optional.empty() if not found
configserver/src/main/java/com/yahoo/vespa/config/server/session/Session.java