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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/concurrency/mutex.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},
+                 ClockSource* clockSource = nullptr);
+
+    /*
+     * 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.
+     */
+    SimpleMutex _windowMutex;
+};
+}  // namespace mongo