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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.
*/
#include "rate_limiting.h"
namespace mongo {
RateLimiting::RateLimiting(RequestCount samplingRate, Milliseconds timePeriod)
: _clockSource(SystemClockSource::get()),
_samplingRate(samplingRate),
_timePeriod(timePeriod),
_windowStart(_clockSource->now()),
_prevCount(0),
_currentCount(0) {}
Date_t RateLimiting::tickWindow() {
Date_t currentTime = _clockSource->now();
// Elapsed time since window start exceeds the time period. Start a new window.
if (currentTime - _windowStart > _timePeriod) {
_windowStart = currentTime;
_prevCount = _currentCount;
_currentCount = 0;
}
return currentTime;
}
bool RateLimiting::handleRequestFixedWindow() {
stdx::unique_lock windowLock{_windowMutex};
tickWindow();
if (_currentCount < _samplingRate.load()) {
_currentCount += 1;
return true;
}
return false;
}
bool RateLimiting::handleRequestSlidingWindow() {
stdx::unique_lock windowLock{_windowMutex};
Date_t currentTime = tickWindow();
auto windowStart = _windowStart;
auto prevCount = _prevCount;
// Sliding window is implemented over fixed size time periods/blocks as follows. Instead of
// making the decision to limit the rate using only the current time period, we look to the rate
// of the previous period to predicate the rate of the current. This smooths the "sampling" of
// the events by predicting a constant rate and limiting accordingly.
// Percentage of time remaining in current window.
double percentRemainingOfCurrentWindow =
((double)(_timePeriod.count() - (currentTime - windowStart).count())) / _timePeriod.count();
// Estimate the number of requests remaining in the current period. We assume the requests in
// the previous time block occurred at a constant rate. We multiply the total number of requests
// in the previous period by the percentage of time remaining in the current period.
double estimatedRemaining = prevCount * percentRemainingOfCurrentWindow;
// Add this estimate to the requests we know have taken place within the current time block.
double estimatedCount = _currentCount + estimatedRemaining;
if (estimatedCount < _samplingRate.load()) {
_currentCount += 1;
return true;
}
return false;
}
} // namespace mongo
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