> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

String password;
    
    
    bool structure_only = false;
    
    RestoreTableCreationMode create_table = RestoreTableCreationMode::kCreateIfNotExists;
    
    RestoreDatabaseCreationMode create_database = RestoreDatabaseCreationMode::kCreateIfNotExists;
    
    
    
    bool allow_different_table_def = false;
    
    
    
    bool allow_different_database_def = false;
    
    bool async = false;
    
    
    size_t shard_num = 0;
    
    
    size_t replica_num = 0;
    
    
    
    size_t shard_num_in_backup = 0;