@Override
    public ClickHouseNode apply(ClickHouseNodeSelector t) {
        lock.readLock().lock();
        try {
            return policy.get(this, t);
        } finally {
            lock.readLock().unlock();
        }
    }
Checks whether it's single node or not.

@return true if it's single node; false otherwise
clickhouse-client/src/main/java/com/clickhouse/client/ClickHouseNodes.java