> (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.

}
    
    if (in_current_database)
    {
        if (current_database.empty())
        {
            if (exception)
                exception->emplace("Default database is not selected", ErrorCodes::UNKNOWN_DATABASE);
            return StorageID::createEmpty();
        }
        storage_id.database_name = current_database;
        
        return storage_id;
    }
    if (exception)
        exception->emplace("Cannot resolve database name for table " + storage_id.getNameForLogs(), ErrorCodes::UNKNOWN_TABLE);
    return StorageID::createEmpty();
}
void Context::initZooKeeperMetadataTransaction(ZooKeeperMetadataTransactionPtr txn, [[maybe_unused]] bool attach_existing)
{
    assert(!metadata_transaction);
    assert(attach_existing || query_context.lock().get() == this);
    metadata_transaction = std::move(txn);
}
ZooKeeperMetadataTransactionPtr Context::getZooKeeperMetadataTransaction() const
{
    assert(!metadata_transaction || hasQueryContext());
    return metadata_transaction;
}
void Context::resetZooKeeperMetadataTransaction()