> (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.

#pragma once
#include <Backups/BackupStatus.h>
#include <Common/ThreadPool.h>
#include <Core/UUID.h>
#include <Parsers/IAST_fwd.h>
#include <unordered_map>
namespace Poco::Util { class AbstractConfiguration; }
namespace DB
{
class ASTBackupQuery;
struct BackupSettings;
struct RestoreSettings;
struct BackupInfo;
class IBackupCoordination;
class IRestoreCoordination;
class BackupsWorker
{
public:
    BackupsWorker(size_t num_backup_threads, size_t num_restore_threads);
    
    void shutdown();
    
    using OperationID = String;
    
    OperationID start(const ASTPtr & backup_or_restore_query, ContextMutablePtr context);
    
    
    void wait(const OperationID & backup_or_restore_id, bool rethrow_exception = true);
    
    struct Info