ClickHouse/dbms/include/DB/Interpreters/QueryPriorities.h

#pragma once

#include <map>
#include <mutex>
#include <condition_variable>
#include <memory>
#include <chrono>
#include <DB/Common/CurrentMetrics.h>


namespace CurrentMetrics
{
	extern const Metric QueryPreempted;
}


namespace DB
{

/** Implements query priorities in very primitive way.
  * Allows to freeze query execution if at least one query of higher priority is executed.
  *
  * Priority value is integer, smaller means higher priority.
  *
  * Priority 0 is special - queries with that priority is always executed,
  *  not depends on other queries and not affect other queries.
  * Thus 0 means - don't use priorities.
  *
  * NOTE Possibilities for improvement:
  * - implement limit on maximum number of running queries with same priority.
  */
class QueryPriorities
{
public:
	using Priority = int;

private:
	friend struct Handle;

	using Count = int;

	/// Number of currently running queries for each priority.
	using Container = std::map<Priority, Count>;

	std::mutex mutex;
	std::condition_variable condvar;
	Container container;


	/** If there are higher priority queries - sleep until they are finish or timeout happens.
	  * Returns true, if higher priority queries has finished at return of function, false, if timout exceeded.
	  */
	template <typename Duration>
	bool waitIfNeed(Priority priority, Duration timeout)
	{
		if (0 == priority)
			return true;

		std::unique_lock<std::mutex> lock(mutex);

		while (true)
		{
			/// Если ли хотя бы один более приоритетный запрос?
			bool found = false;
			for (const auto & value : container)
			{
				if (value.first >= priority)
					break;

				if (value.second > 0)
				{
					found = true;
					break;
				}
			}

			if (!found)
				return true;

			CurrentMetrics::Increment metric_increment{CurrentMetrics::QueryPreempted};
			if (std::cv_status::timeout == condvar.wait_for(lock, timeout))
				return false;
		}
	}

public:
	struct HandleImpl
	{
	private:
		QueryPriorities & parent;
		QueryPriorities::Container::value_type & value;

	public:
		HandleImpl(QueryPriorities & parent_, QueryPriorities::Container::value_type & value_)
			: parent(parent_), value(value_) {}

		~HandleImpl()
		{
			{
				std::lock_guard<std::mutex> lock(parent.mutex);
				--value.second;
			}
			parent.condvar.notify_all();
		}

		template <typename Duration>
		bool waitIfNeed(Duration timeout)
		{
			return parent.waitIfNeed(value.first, timeout);
		}
	};

	using Handle = std::shared_ptr<HandleImpl>;

	/** Register query with specified priority.
	  * Returns an object that remove record in destructor.
	  */
	Handle insert(Priority priority)
	{
		if (0 == priority)
			return {};

		std::lock_guard<std::mutex> lock(mutex);
		auto it = container.emplace(priority, 0).first;
		++it->second;
		return std::make_shared<HandleImpl>(*this, *it);
	}
};

}