> (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.

return true;
    }
    catch (...)
    {
        return false;
    }
}
UInt32 Context::getZooKeeperSessionUptime() const
{
    std::lock_guard lock(shared->zookeeper_mutex);
    if (!shared->zookeeper || shared->zookeeper->expired())
        return 0;
    return shared->zookeeper->getSessionUptime();
}
void Context::setSystemZooKeeperLogAfterInitializationIfNeeded()
{
    
    
    
    
    
    std::lock_guard lock(shared->zookeeper_mutex);
    if (!shared->system_logs || !shared->system_logs->zookeeper_log)
        return;
    if (shared->zookeeper)
        shared->zookeeper->setZooKeeperLog(shared->system_logs->zookeeper_log);
    for (auto & zk : shared->auxiliary_zookeepers)
        zk.second->setZooKeeperLog(shared->system_logs->zookeeper_log);
}
void Context::initializeKeeperDispatcher([[maybe_unused]] bool start_async) const