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. 

{
    const int sleep_max_ms = 1000 * seconds_to_wait;
    const int sleep_one_ms = 100;
    int sleep_current_ms = 0;
    int current_connections = 0;
    for (;;)
    {
        current_connections = 0;
        for (auto & server : servers)
        {