> (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.

{
    
    
    
    
    union
    {
        char bytes[16];
        struct
        {
            UInt64 a;
            UInt64 b;
        } words;
        UUID uuid{};
    } random;
    random.words.a = thread_local_rng(); 
    random.words.b = thread_local_rng(); 
    String query_id_to_set = query_id;
    if (query_id_to_set.empty())    
    {
        
        struct QueryUUID : Poco::UUID
        {
            QueryUUID(const char * bytes, Poco::UUID::Version version)