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. 

{
                if (zookeeper->exists(current_part_path))
                    throw Exception(ErrorCodes::LOGICAL_ERROR, "Part {} has empty header and does not have columns and checksums. "
                                                               "Looks like a bug.", current_part_path);
                LOG_INFO(log, "Not checking checksums of part {} with replica {} because part was removed from ZooKeeper", part_name, replica);
                continue;
            }
        }
        else
        {
            replica_part_header = ReplicatedMergeTreePartHeader::fromString(part_zk_str);
        }
        if (replica_part_header.getColumnsHash() != local_part_header.getColumnsHash())
        {
            
            
            
            
            
            
            throw Exception(ErrorCodes::CHECKSUM_DOESNT_MATCH, "Part {} from {} has different columns hash "
                            "(it may rarely happen on race condition with KILL MUTATION or ALTER COLUMN).", part_name, replica);
        }
        replica_part_header.getChecksums().checkEqual(local_part_header.getChecksums(), true);
        if (replica == replica_name)
            has_been_already_added = true;
        
        
        if (absent_replicas_paths)
        {