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. 

}
        try
        {
            global_context->startClusterDiscovery();
        }
        catch (...)
        {
            tryLogCurrentException(log, "Caught exception while starting cluster discovery");
        }
        SCOPE_EXIT_SAFE({
            LOG_DEBUG(log, "Received termination signal.");
            LOG_DEBUG(log, "Waiting for current connections to close.");
            is_cancelled = true;
            int current_connections = 0;
            {
                std::lock_guard lock(servers_lock);
                for (auto & server : servers)
                {
                    server.stop();
                    current_connections += server.currentConnections();
                }
            }
            if (current_connections)
                LOG_INFO(log, "Closed all listening sockets. Waiting for {} outstanding connections.", current_connections);
            else
                LOG_INFO(log, "Closed all listening sockets.");
            
            if (!config().getBool("shutdown_wait_unfinished_queries", false))
                global_context->getProcessList().killAllQueries();
            if (current_connections)