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. 

continue;
        }
        else
        {
            zkutil::KeeperMultiException::check(rc, ops, responses);
        }
    }
    ::sort(source_queue_names.begin(), source_queue_names.end());
    struct QueueEntryInfo
    {
        String data = {};
        Coordination::Stat stat = {};
        LogEntryPtr parsed_entry = {};
    };
    
    
    
    
    
    
    
    std::vector<QueueEntryInfo> source_queue;
    ActiveDataPartSet get_part_set{format_version};
    ActiveDataPartSet drop_range_set{format_version};
    {
        std::vector<zkutil::ZooKeeper::FutureGet> queue_get_futures;
        queue_get_futures.reserve(source_queue_names.size());
        for (const String & entry_name : source_queue_names)
            queue_get_futures.push_back(zookeeper->asyncTryGet(fs::path(source_path) / "queue" / entry_name));
        source_queue.reserve(source_queue_names.size());