Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

{
    
    
    
    
    
    std::lock_guard lock(current_zookeeper_mutex);
    if (zookeeper_name == default_zookeeper_name)
    {
        current_zookeeper = getContext()->getZooKeeper();
    }
    else
    {
        current_zookeeper = getContext()->getAuxiliaryZooKeeper(zookeeper_name);
    }
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::tryGetZooKeeper() const
{
    std::lock_guard lock(current_zookeeper_mutex);
    return current_zookeeper;
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::getZooKeeper() const
{
    auto res = tryGetZooKeeper();
    if (!res)
        throw Exception("Cannot get ZooKeeper", ErrorCodes::NO_ZOOKEEPER);
    return res;
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::getZooKeeperAndAssertNotReadonly() const