001/*
002 * Copyright (C) 2012 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import static com.google.common.base.Preconditions.checkArgument;
018import static com.google.common.base.Preconditions.checkNotNull;
019import static java.lang.Math.max;
020import static java.util.concurrent.TimeUnit.MICROSECONDS;
021import static java.util.concurrent.TimeUnit.SECONDS;
022
023import com.google.common.annotations.Beta;
024import com.google.common.annotations.GwtIncompatible;
025import com.google.common.annotations.VisibleForTesting;
026import com.google.common.base.Stopwatch;
027import com.google.common.util.concurrent.SmoothRateLimiter.SmoothBursty;
028import com.google.common.util.concurrent.SmoothRateLimiter.SmoothWarmingUp;
029import com.google.errorprone.annotations.CanIgnoreReturnValue;
030import java.util.Locale;
031import java.util.concurrent.TimeUnit;
032import org.checkerframework.checker.nullness.compatqual.MonotonicNonNullDecl;
033
034/**
035 * A rate limiter. Conceptually, a rate limiter distributes permits at a configurable rate. Each
036 * {@link #acquire()} blocks if necessary until a permit is available, and then takes it. Once
037 * acquired, permits need not be released.
038 *
039 * <p>{@code RateLimiter} is safe for concurrent use: It will restrict the total rate of calls from
040 * all threads. Note, however, that it does not guarantee fairness.
041 *
042 * <p>Rate limiters are often used to restrict the rate at which some physical or logical resource
043 * is accessed. This is in contrast to {@link java.util.concurrent.Semaphore} which restricts the
044 * number of concurrent accesses instead of the rate (note though that concurrency and rate are
045 * closely related, e.g. see <a href="http://en.wikipedia.org/wiki/Little%27s_law">Little's
046 * Law</a>).
047 *
048 * <p>A {@code RateLimiter} is defined primarily by the rate at which permits are issued. Absent
049 * additional configuration, permits will be distributed at a fixed rate, defined in terms of
050 * permits per second. Permits will be distributed smoothly, with the delay between individual
051 * permits being adjusted to ensure that the configured rate is maintained.
052 *
053 * <p>It is possible to configure a {@code RateLimiter} to have a warmup period during which time
054 * the permits issued each second steadily increases until it hits the stable rate.
055 *
056 * <p>As an example, imagine that we have a list of tasks to execute, but we don't want to submit
057 * more than 2 per second:
058 *
059 * <pre>{@code
060 * final RateLimiter rateLimiter = RateLimiter.create(2.0); // rate is "2 permits per second"
061 * void submitTasks(List<Runnable> tasks, Executor executor) {
062 *   for (Runnable task : tasks) {
063 *     rateLimiter.acquire(); // may wait
064 *     executor.execute(task);
065 *   }
066 * }
067 * }</pre>
068 *
069 * <p>As another example, imagine that we produce a stream of data, and we want to cap it at 5kb per
070 * second. This could be accomplished by requiring a permit per byte, and specifying a rate of 5000
071 * permits per second:
072 *
073 * <pre>{@code
074 * final RateLimiter rateLimiter = RateLimiter.create(5000.0); // rate = 5000 permits per second
075 * void submitPacket(byte[] packet) {
076 *   rateLimiter.acquire(packet.length);
077 *   networkService.send(packet);
078 * }
079 * }</pre>
080 *
081 * <p>It is important to note that the number of permits requested <i>never</i> affects the
082 * throttling of the request itself (an invocation to {@code acquire(1)} and an invocation to {@code
083 * acquire(1000)} will result in exactly the same throttling, if any), but it affects the throttling
084 * of the <i>next</i> request. I.e., if an expensive task arrives at an idle RateLimiter, it will be
085 * granted immediately, but it is the <i>next</i> request that will experience extra throttling,
086 * thus paying for the cost of the expensive task.
087 *
088 * @author Dimitris Andreou
089 * @since 13.0
090 */
091// TODO(user): switch to nano precision. A natural unit of cost is "bytes", and a micro precision
092// would mean a maximum rate of "1MB/s", which might be small in some cases.
093@Beta
094@GwtIncompatible
095public abstract class RateLimiter {
096  /**
097   * Creates a {@code RateLimiter} with the specified stable throughput, given as "permits per
098   * second" (commonly referred to as <i>QPS</i>, queries per second).
099   *
100   * <p>The returned {@code RateLimiter} ensures that on average no more than {@code
101   * permitsPerSecond} are issued during any given second, with sustained requests being smoothly
102   * spread over each second. When the incoming request rate exceeds {@code permitsPerSecond} the
103   * rate limiter will release one permit every {@code (1.0 / permitsPerSecond)} seconds. When the
104   * rate limiter is unused, bursts of up to {@code permitsPerSecond} permits will be allowed, with
105   * subsequent requests being smoothly limited at the stable rate of {@code permitsPerSecond}.
106   *
107   * @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in how many
108   *     permits become available per second
109   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
110   */
111  // TODO(user): "This is equivalent to
112  // {@code createWithCapacity(permitsPerSecond, 1, TimeUnit.SECONDS)}".
113  public static RateLimiter create(double permitsPerSecond) {
114    /*
115     * The default RateLimiter configuration can save the unused permits of up to one second. This
116     * is to avoid unnecessary stalls in situations like this: A RateLimiter of 1qps, and 4 threads,
117     * all calling acquire() at these moments:
118     *
119     * T0 at 0 seconds
120     * T1 at 1.05 seconds
121     * T2 at 2 seconds
122     * T3 at 3 seconds
123     *
124     * Due to the slight delay of T1, T2 would have to sleep till 2.05 seconds, and T3 would also
125     * have to sleep till 3.05 seconds.
126     */
127    return create(permitsPerSecond, SleepingStopwatch.createFromSystemTimer());
128  }
129
130  @VisibleForTesting
131  static RateLimiter create(double permitsPerSecond, SleepingStopwatch stopwatch) {
132    RateLimiter rateLimiter = new SmoothBursty(stopwatch, 1.0 /* maxBurstSeconds */);
133    rateLimiter.setRate(permitsPerSecond);
134    return rateLimiter;
135  }
136
137  /**
138   * Creates a {@code RateLimiter} with the specified stable throughput, given as "permits per
139   * second" (commonly referred to as <i>QPS</i>, queries per second), and a <i>warmup period</i>,
140   * during which the {@code RateLimiter} smoothly ramps up its rate, until it reaches its maximum
141   * rate at the end of the period (as long as there are enough requests to saturate it). Similarly,
142   * if the {@code RateLimiter} is left <i>unused</i> for a duration of {@code warmupPeriod}, it
143   * will gradually return to its "cold" state, i.e. it will go through the same warming up process
144   * as when it was first created.
145   *
146   * <p>The returned {@code RateLimiter} is intended for cases where the resource that actually
147   * fulfills the requests (e.g., a remote server) needs "warmup" time, rather than being
148   * immediately accessed at the stable (maximum) rate.
149   *
150   * <p>The returned {@code RateLimiter} starts in a "cold" state (i.e. the warmup period will
151   * follow), and if it is left unused for long enough, it will return to that state.
152   *
153   * @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in how many
154   *     permits become available per second
155   * @param warmupPeriod the duration of the period where the {@code RateLimiter} ramps up its rate,
156   *     before reaching its stable (maximum) rate
157   * @param unit the time unit of the warmupPeriod argument
158   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero or {@code
159   *     warmupPeriod} is negative
160   */
161  public static RateLimiter create(double permitsPerSecond, long warmupPeriod, TimeUnit unit) {
162    checkArgument(warmupPeriod >= 0, "warmupPeriod must not be negative: %s", warmupPeriod);
163    return create(
164        permitsPerSecond, warmupPeriod, unit, 3.0, SleepingStopwatch.createFromSystemTimer());
165  }
166
167  @VisibleForTesting
168  static RateLimiter create(
169      double permitsPerSecond,
170      long warmupPeriod,
171      TimeUnit unit,
172      double coldFactor,
173      SleepingStopwatch stopwatch) {
174    RateLimiter rateLimiter = new SmoothWarmingUp(stopwatch, warmupPeriod, unit, coldFactor);
175    rateLimiter.setRate(permitsPerSecond);
176    return rateLimiter;
177  }
178
179  /**
180   * The underlying timer; used both to measure elapsed time and sleep as necessary. A separate
181   * object to facilitate testing.
182   */
183  private final SleepingStopwatch stopwatch;
184
185  // Can't be initialized in the constructor because mocks don't call the constructor.
186  @MonotonicNonNullDecl private volatile Object mutexDoNotUseDirectly;
187
188  private Object mutex() {
189    Object mutex = mutexDoNotUseDirectly;
190    if (mutex == null) {
191      synchronized (this) {
192        mutex = mutexDoNotUseDirectly;
193        if (mutex == null) {
194          mutexDoNotUseDirectly = mutex = new Object();
195        }
196      }
197    }
198    return mutex;
199  }
200
201  RateLimiter(SleepingStopwatch stopwatch) {
202    this.stopwatch = checkNotNull(stopwatch);
203  }
204
205  /**
206   * Updates the stable rate of this {@code RateLimiter}, that is, the {@code permitsPerSecond}
207   * argument provided in the factory method that constructed the {@code RateLimiter}. Currently
208   * throttled threads will <b>not</b> be awakened as a result of this invocation, thus they do not
209   * observe the new rate; only subsequent requests will.
210   *
211   * <p>Note though that, since each request repays (by waiting, if necessary) the cost of the
212   * <i>previous</i> request, this means that the very next request after an invocation to {@code
213   * setRate} will not be affected by the new rate; it will pay the cost of the previous request,
214   * which is in terms of the previous rate.
215   *
216   * <p>The behavior of the {@code RateLimiter} is not modified in any other way, e.g. if the {@code
217   * RateLimiter} was configured with a warmup period of 20 seconds, it still has a warmup period of
218   * 20 seconds after this method invocation.
219   *
220   * @param permitsPerSecond the new stable rate of this {@code RateLimiter}
221   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
222   */
223  public final void setRate(double permitsPerSecond) {
224    checkArgument(
225        permitsPerSecond > 0.0 && !Double.isNaN(permitsPerSecond), "rate must be positive");
226    synchronized (mutex()) {
227      doSetRate(permitsPerSecond, stopwatch.readMicros());
228    }
229  }
230
231  abstract void doSetRate(double permitsPerSecond, long nowMicros);
232
233  /**
234   * Returns the stable rate (as {@code permits per seconds}) with which this {@code RateLimiter} is
235   * configured with. The initial value of this is the same as the {@code permitsPerSecond} argument
236   * passed in the factory method that produced this {@code RateLimiter}, and it is only updated
237   * after invocations to {@linkplain #setRate}.
238   */
239  public final double getRate() {
240    synchronized (mutex()) {
241      return doGetRate();
242    }
243  }
244
245  abstract double doGetRate();
246
247  /**
248   * Acquires a single permit from this {@code RateLimiter}, blocking until the request can be
249   * granted. Tells the amount of time slept, if any.
250   *
251   * <p>This method is equivalent to {@code acquire(1)}.
252   *
253   * @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
254   * @since 16.0 (present in 13.0 with {@code void} return type})
255   */
256  @CanIgnoreReturnValue
257  public double acquire() {
258    return acquire(1);
259  }
260
261  /**
262   * Acquires the given number of permits from this {@code RateLimiter}, blocking until the request
263   * can be granted. Tells the amount of time slept, if any.
264   *
265   * @param permits the number of permits to acquire
266   * @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
267   * @throws IllegalArgumentException if the requested number of permits is negative or zero
268   * @since 16.0 (present in 13.0 with {@code void} return type})
269   */
270  @CanIgnoreReturnValue
271  public double acquire(int permits) {
272    long microsToWait = reserve(permits);
273    stopwatch.sleepMicrosUninterruptibly(microsToWait);
274    return 1.0 * microsToWait / SECONDS.toMicros(1L);
275  }
276
277  /**
278   * Reserves the given number of permits from this {@code RateLimiter} for future use, returning
279   * the number of microseconds until the reservation can be consumed.
280   *
281   * @return time in microseconds to wait until the resource can be acquired, never negative
282   */
283  final long reserve(int permits) {
284    checkPermits(permits);
285    synchronized (mutex()) {
286      return reserveAndGetWaitLength(permits, stopwatch.readMicros());
287    }
288  }
289
290  /**
291   * Acquires a permit from this {@code RateLimiter} if it can be obtained without exceeding the
292   * specified {@code timeout}, or returns {@code false} immediately (without waiting) if the permit
293   * would not have been granted before the timeout expired.
294   *
295   * <p>This method is equivalent to {@code tryAcquire(1, timeout, unit)}.
296   *
297   * @param timeout the maximum time to wait for the permit. Negative values are treated as zero.
298   * @param unit the time unit of the timeout argument
299   * @return {@code true} if the permit was acquired, {@code false} otherwise
300   * @throws IllegalArgumentException if the requested number of permits is negative or zero
301   */
302  public boolean tryAcquire(long timeout, TimeUnit unit) {
303    return tryAcquire(1, timeout, unit);
304  }
305
306  /**
307   * Acquires permits from this {@link RateLimiter} if it can be acquired immediately without delay.
308   *
309   * <p>This method is equivalent to {@code tryAcquire(permits, 0, anyUnit)}.
310   *
311   * @param permits the number of permits to acquire
312   * @return {@code true} if the permits were acquired, {@code false} otherwise
313   * @throws IllegalArgumentException if the requested number of permits is negative or zero
314   * @since 14.0
315   */
316  public boolean tryAcquire(int permits) {
317    return tryAcquire(permits, 0, MICROSECONDS);
318  }
319
320  /**
321   * Acquires a permit from this {@link RateLimiter} if it can be acquired immediately without
322   * delay.
323   *
324   * <p>This method is equivalent to {@code tryAcquire(1)}.
325   *
326   * @return {@code true} if the permit was acquired, {@code false} otherwise
327   * @since 14.0
328   */
329  public boolean tryAcquire() {
330    return tryAcquire(1, 0, MICROSECONDS);
331  }
332
333  /**
334   * Acquires the given number of permits from this {@code RateLimiter} if it can be obtained
335   * without exceeding the specified {@code timeout}, or returns {@code false} immediately (without
336   * waiting) if the permits would not have been granted before the timeout expired.
337   *
338   * @param permits the number of permits to acquire
339   * @param timeout the maximum time to wait for the permits. Negative values are treated as zero.
340   * @param unit the time unit of the timeout argument
341   * @return {@code true} if the permits were acquired, {@code false} otherwise
342   * @throws IllegalArgumentException if the requested number of permits is negative or zero
343   */
344  public boolean tryAcquire(int permits, long timeout, TimeUnit unit) {
345    long timeoutMicros = max(unit.toMicros(timeout), 0);
346    checkPermits(permits);
347    long microsToWait;
348    synchronized (mutex()) {
349      long nowMicros = stopwatch.readMicros();
350      if (!canAcquire(nowMicros, timeoutMicros)) {
351        return false;
352      } else {
353        microsToWait = reserveAndGetWaitLength(permits, nowMicros);
354      }
355    }
356    stopwatch.sleepMicrosUninterruptibly(microsToWait);
357    return true;
358  }
359
360  private boolean canAcquire(long nowMicros, long timeoutMicros) {
361    return queryEarliestAvailable(nowMicros) - timeoutMicros <= nowMicros;
362  }
363
364  /**
365   * Reserves next ticket and returns the wait time that the caller must wait for.
366   *
367   * @return the required wait time, never negative
368   */
369  final long reserveAndGetWaitLength(int permits, long nowMicros) {
370    long momentAvailable = reserveEarliestAvailable(permits, nowMicros);
371    return max(momentAvailable - nowMicros, 0);
372  }
373
374  /**
375   * Returns the earliest time that permits are available (with one caveat).
376   *
377   * @return the time that permits are available, or, if permits are available immediately, an
378   *     arbitrary past or present time
379   */
380  abstract long queryEarliestAvailable(long nowMicros);
381
382  /**
383   * Reserves the requested number of permits and returns the time that those permits can be used
384   * (with one caveat).
385   *
386   * @return the time that the permits may be used, or, if the permits may be used immediately, an
387   *     arbitrary past or present time
388   */
389  abstract long reserveEarliestAvailable(int permits, long nowMicros);
390
391  @Override
392  public String toString() {
393    return String.format(Locale.ROOT, "RateLimiter[stableRate=%3.1fqps]", getRate());
394  }
395
396  abstract static class SleepingStopwatch {
397    /** Constructor for use by subclasses. */
398    protected SleepingStopwatch() {}
399
400    /*
401     * We always hold the mutex when calling this. TODO(cpovirk): Is that important? Perhaps we need
402     * to guarantee that each call to reserveEarliestAvailable, etc. sees a value >= the previous?
403     * Also, is it OK that we don't hold the mutex when sleeping?
404     */
405    protected abstract long readMicros();
406
407    protected abstract void sleepMicrosUninterruptibly(long micros);
408
409    public static SleepingStopwatch createFromSystemTimer() {
410      return new SleepingStopwatch() {
411        final Stopwatch stopwatch = Stopwatch.createStarted();
412
413        @Override
414        protected long readMicros() {
415          return stopwatch.elapsed(MICROSECONDS);
416        }
417
418        @Override
419        protected void sleepMicrosUninterruptibly(long micros) {
420          if (micros > 0) {
421            Uninterruptibles.sleepUninterruptibly(micros, MICROSECONDS);
422          }
423        }
424      };
425    }
426  }
427
428  private static void checkPermits(int permits) {
429    checkArgument(permits > 0, "Requested permits (%s) must be positive", permits);
430  }
431}