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/**
* Copyright (C) 2022-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include "mongo/util/clock_source.h"
#include "mongo/util/system_clock_source.h"
namespace mongo {
/**
* Rate limiting is used to put a bound on the number of requests to a certain resource over a fixed
* time window. This implementation is approximate in the sense that it may permit the bound to
* exceeded. The bound is approximate as a trade off to reduce contention on internal resources.
*/
class RateLimiting {
using RequestCount = uint32_t;
public:
/*
* Constructor for a rate limiter. Specify the number of requests you want to take place, as
* well as the time period in milliseconds.
*/
RateLimiting(RequestCount samplingRate, Milliseconds timePeriod = Seconds{1});
/*
* Getter for the sampling rate.
*/
RequestCount getSamplingRate() {
return _samplingRate.load();
}
/*
* Setter for the sampling rate.
*/
void setSamplingRate(RequestCount samplingRate) {
_samplingRate.store(samplingRate);
}
/*
* A simple method for rate limiting. Returns false if we have reached the request limit for the
* current time window; otherwise, returns true and adds the request to the count for the
* current window. If we have passed the end of the previous window, the slate is wiped clean.
*/
bool handleRequestFixedWindow();
/*
* A method that ensures a more steady rate of requests. Rather than only looking at the current
* time block, this method simulates a sliding window to estimate how many requests occurred in
* the last full time period. Like the above, returns whether the request should be handled, and
* resets the window if enough time has passed.
*/
bool handleRequestSlidingWindow();
private:
/*
* Resets the current window if it has ended. Returns the current time. This must be called in
* the beginning of each handleRequest...() method.
*/
Date_t tickWindow();
/*
* Clock source used to track time.
*/
ClockSource* const _clockSource;
/*
* Sampling rate is the bound on the number of requests we want to admit per window.
*/
AtomicWord<RequestCount> _samplingRate;
/*
* Time period is the window size in ms.
*/
const Milliseconds _timePeriod;
/*
* Window start.
*/
Date_t _windowStart;
/*
* Count of requests handled in the previous window.
*/
RequestCount _prevCount;
/*
* Count of requests handled in the current window.
*/
RequestCount _currentCount;
/*
* Mutex used when reading/writing the window.
*/
stdx::recursive_mutex _windowMutex;
};
} // namespace mongo
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