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 javax.annotation.CheckForNull;
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
095@ElementTypesAreNonnullByDefault
096public abstract class RateLimiter {
097  /**
098   * Creates a {@code RateLimiter} with the specified stable throughput, given as "permits per
099   * second" (commonly referred to as <i>QPS</i>, queries per second).
100   *
101   * <p>The returned {@code RateLimiter} ensures that on average no more than {@code
102   * permitsPerSecond} are issued during any given second, with sustained requests being smoothly
103   * spread over each second. When the incoming request rate exceeds {@code permitsPerSecond} the
104   * rate limiter will release one permit every {@code (1.0 / permitsPerSecond)} seconds. When the
105   * rate limiter is unused, bursts of up to {@code permitsPerSecond} permits will be allowed, with
106   * subsequent requests being smoothly limited at the stable rate of {@code permitsPerSecond}.
107   *
108   * @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in how many
109   *     permits become available per second
110   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
111   */
112  // TODO(user): "This is equivalent to
113  // {@code createWithCapacity(permitsPerSecond, 1, TimeUnit.SECONDS)}".
114  public static RateLimiter create(double permitsPerSecond) {
115    /*
116     * The default RateLimiter configuration can save the unused permits of up to one second. This
117     * is to avoid unnecessary stalls in situations like this: A RateLimiter of 1qps, and 4 threads,
118     * all calling acquire() at these moments:
119     *
120     * T0 at 0 seconds
121     * T1 at 1.05 seconds
122     * T2 at 2 seconds
123     * T3 at 3 seconds
124     *
125     * Due to the slight delay of T1, T2 would have to sleep till 2.05 seconds, and T3 would also
126     * have to sleep till 3.05 seconds.
127     */
128    return create(permitsPerSecond, SleepingStopwatch.createFromSystemTimer());
129  }
130
131  @VisibleForTesting
132  static RateLimiter create(double permitsPerSecond, SleepingStopwatch stopwatch) {
133    RateLimiter rateLimiter = new SmoothBursty(stopwatch, 1.0 /* maxBurstSeconds */);
134    rateLimiter.setRate(permitsPerSecond);
135    return rateLimiter;
136  }
137
138  /**
139   * Creates a {@code RateLimiter} with the specified stable throughput, given as "permits per
140   * second" (commonly referred to as <i>QPS</i>, queries per second), and a <i>warmup period</i>,
141   * during which the {@code RateLimiter} smoothly ramps up its rate, until it reaches its maximum
142   * rate at the end of the period (as long as there are enough requests to saturate it). Similarly,
143   * if the {@code RateLimiter} is left <i>unused</i> for a duration of {@code warmupPeriod}, it
144   * will gradually return to its "cold" state, i.e. it will go through the same warming up process
145   * as when it was first created.
146   *
147   * <p>The returned {@code RateLimiter} is intended for cases where the resource that actually
148   * fulfills the requests (e.g., a remote server) needs "warmup" time, rather than being
149   * immediately accessed at the stable (maximum) rate.
150   *
151   * <p>The returned {@code RateLimiter} starts in a "cold" state (i.e. the warmup period will
152   * follow), and if it is left unused for long enough, it will return to that state.
153   *
154   * @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in how many
155   *     permits become available per second
156   * @param warmupPeriod the duration of the period where the {@code RateLimiter} ramps up its rate,
157   *     before reaching its stable (maximum) rate
158   * @param unit the time unit of the warmupPeriod argument
159   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero or {@code
160   *     warmupPeriod} is negative
161   */
162  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
163  public static RateLimiter create(double permitsPerSecond, long warmupPeriod, TimeUnit unit) {
164    checkArgument(warmupPeriod >= 0, "warmupPeriod must not be negative: %s", warmupPeriod);
165    return create(
166        permitsPerSecond, warmupPeriod, unit, 3.0, SleepingStopwatch.createFromSystemTimer());
167  }
168
169  @VisibleForTesting
170  static RateLimiter create(
171      double permitsPerSecond,
172      long warmupPeriod,
173      TimeUnit unit,
174      double coldFactor,
175      SleepingStopwatch stopwatch) {
176    RateLimiter rateLimiter = new SmoothWarmingUp(stopwatch, warmupPeriod, unit, coldFactor);
177    rateLimiter.setRate(permitsPerSecond);
178    return rateLimiter;
179  }
180
181  /**
182   * The underlying timer; used both to measure elapsed time and sleep as necessary. A separate
183   * object to facilitate testing.
184   */
185  private final SleepingStopwatch stopwatch;
186
187  // Can't be initialized in the constructor because mocks don't call the constructor.
188  @CheckForNull private volatile Object mutexDoNotUseDirectly;
189
190  private Object mutex() {
191    Object mutex = mutexDoNotUseDirectly;
192    if (mutex == null) {
193      synchronized (this) {
194        mutex = mutexDoNotUseDirectly;
195        if (mutex == null) {
196          mutexDoNotUseDirectly = mutex = new Object();
197        }
198      }
199    }
200    return mutex;
201  }
202
203  RateLimiter(SleepingStopwatch stopwatch) {
204    this.stopwatch = checkNotNull(stopwatch);
205  }
206
207  /**
208   * Updates the stable rate of this {@code RateLimiter}, that is, the {@code permitsPerSecond}
209   * argument provided in the factory method that constructed the {@code RateLimiter}. Currently
210   * throttled threads will <b>not</b> be awakened as a result of this invocation, thus they do not
211   * observe the new rate; only subsequent requests will.
212   *
213   * <p>Note though that, since each request repays (by waiting, if necessary) the cost of the
214   * <i>previous</i> request, this means that the very next request after an invocation to {@code
215   * setRate} will not be affected by the new rate; it will pay the cost of the previous request,
216   * which is in terms of the previous rate.
217   *
218   * <p>The behavior of the {@code RateLimiter} is not modified in any other way, e.g. if the {@code
219   * RateLimiter} was configured with a warmup period of 20 seconds, it still has a warmup period of
220   * 20 seconds after this method invocation.
221   *
222   * @param permitsPerSecond the new stable rate of this {@code RateLimiter}
223   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
224   */
225  public final void setRate(double permitsPerSecond) {
226    checkArgument(
227        permitsPerSecond > 0.0 && !Double.isNaN(permitsPerSecond), "rate must be positive");
228    synchronized (mutex()) {
229      doSetRate(permitsPerSecond, stopwatch.readMicros());
230    }
231  }
232
233  abstract void doSetRate(double permitsPerSecond, long nowMicros);
234
235  /**
236   * Returns the stable rate (as {@code permits per seconds}) with which this {@code RateLimiter} is
237   * configured with. The initial value of this is the same as the {@code permitsPerSecond} argument
238   * passed in the factory method that produced this {@code RateLimiter}, and it is only updated
239   * after invocations to {@linkplain #setRate}.
240   */
241  public final double getRate() {
242    synchronized (mutex()) {
243      return doGetRate();
244    }
245  }
246
247  abstract double doGetRate();
248
249  /**
250   * Acquires a single permit from this {@code RateLimiter}, blocking until the request can be
251   * granted. Tells the amount of time slept, if any.
252   *
253   * <p>This method is equivalent to {@code acquire(1)}.
254   *
255   * @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
256   * @since 16.0 (present in 13.0 with {@code void} return type})
257   */
258  @CanIgnoreReturnValue
259  public double acquire() {
260    return acquire(1);
261  }
262
263  /**
264   * Acquires the given number of permits from this {@code RateLimiter}, blocking until the request
265   * can be granted. Tells the amount of time slept, if any.
266   *
267   * @param permits the number of permits to acquire
268   * @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
269   * @throws IllegalArgumentException if the requested number of permits is negative or zero
270   * @since 16.0 (present in 13.0 with {@code void} return type})
271   */
272  @CanIgnoreReturnValue
273  public double acquire(int permits) {
274    long microsToWait = reserve(permits);
275    stopwatch.sleepMicrosUninterruptibly(microsToWait);
276    return 1.0 * microsToWait / SECONDS.toMicros(1L);
277  }
278
279  /**
280   * Reserves the given number of permits from this {@code RateLimiter} for future use, returning
281   * the number of microseconds until the reservation can be consumed.
282   *
283   * @return time in microseconds to wait until the resource can be acquired, never negative
284   */
285  final long reserve(int permits) {
286    checkPermits(permits);
287    synchronized (mutex()) {
288      return reserveAndGetWaitLength(permits, stopwatch.readMicros());
289    }
290  }
291
292  /**
293   * Acquires a permit from this {@code RateLimiter} if it can be obtained without exceeding the
294   * specified {@code timeout}, or returns {@code false} immediately (without waiting) if the permit
295   * would not have been granted before the timeout expired.
296   *
297   * <p>This method is equivalent to {@code tryAcquire(1, timeout, unit)}.
298   *
299   * @param timeout the maximum time to wait for the permit. Negative values are treated as zero.
300   * @param unit the time unit of the timeout argument
301   * @return {@code true} if the permit was acquired, {@code false} otherwise
302   * @throws IllegalArgumentException if the requested number of permits is negative or zero
303   */
304  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
305  public boolean tryAcquire(long timeout, TimeUnit unit) {
306    return tryAcquire(1, timeout, unit);
307  }
308
309  /**
310   * Acquires permits from this {@link RateLimiter} if it can be acquired immediately without delay.
311   *
312   * <p>This method is equivalent to {@code tryAcquire(permits, 0, anyUnit)}.
313   *
314   * @param permits the number of permits to acquire
315   * @return {@code true} if the permits were acquired, {@code false} otherwise
316   * @throws IllegalArgumentException if the requested number of permits is negative or zero
317   * @since 14.0
318   */
319  public boolean tryAcquire(int permits) {
320    return tryAcquire(permits, 0, MICROSECONDS);
321  }
322
323  /**
324   * Acquires a permit from this {@link RateLimiter} if it can be acquired immediately without
325   * delay.
326   *
327   * <p>This method is equivalent to {@code tryAcquire(1)}.
328   *
329   * @return {@code true} if the permit was acquired, {@code false} otherwise
330   * @since 14.0
331   */
332  public boolean tryAcquire() {
333    return tryAcquire(1, 0, MICROSECONDS);
334  }
335
336  /**
337   * Acquires the given number of permits from this {@code RateLimiter} if it can be obtained
338   * without exceeding the specified {@code timeout}, or returns {@code false} immediately (without
339   * waiting) if the permits would not have been granted before the timeout expired.
340   *
341   * @param permits the number of permits to acquire
342   * @param timeout the maximum time to wait for the permits. Negative values are treated as zero.
343   * @param unit the time unit of the timeout argument
344   * @return {@code true} if the permits were acquired, {@code false} otherwise
345   * @throws IllegalArgumentException if the requested number of permits is negative or zero
346   */
347  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
348  public boolean tryAcquire(int permits, long timeout, TimeUnit unit) {
349    long timeoutMicros = max(unit.toMicros(timeout), 0);
350    checkPermits(permits);
351    long microsToWait;
352    synchronized (mutex()) {
353      long nowMicros = stopwatch.readMicros();
354      if (!canAcquire(nowMicros, timeoutMicros)) {
355        return false;
356      } else {
357        microsToWait = reserveAndGetWaitLength(permits, nowMicros);
358      }
359    }
360    stopwatch.sleepMicrosUninterruptibly(microsToWait);
361    return true;
362  }
363
364  private boolean canAcquire(long nowMicros, long timeoutMicros) {
365    return queryEarliestAvailable(nowMicros) - timeoutMicros <= nowMicros;
366  }
367
368  /**
369   * Reserves next ticket and returns the wait time that the caller must wait for.
370   *
371   * @return the required wait time, never negative
372   */
373  final long reserveAndGetWaitLength(int permits, long nowMicros) {
374    long momentAvailable = reserveEarliestAvailable(permits, nowMicros);
375    return max(momentAvailable - nowMicros, 0);
376  }
377
378  /**
379   * Returns the earliest time that permits are available (with one caveat).
380   *
381   * @return the time that permits are available, or, if permits are available immediately, an
382   *     arbitrary past or present time
383   */
384  abstract long queryEarliestAvailable(long nowMicros);
385
386  /**
387   * Reserves the requested number of permits and returns the time that those permits can be used
388   * (with one caveat).
389   *
390   * @return the time that the permits may be used, or, if the permits may be used immediately, an
391   *     arbitrary past or present time
392   */
393  abstract long reserveEarliestAvailable(int permits, long nowMicros);
394
395  @Override
396  public String toString() {
397    return String.format(Locale.ROOT, "RateLimiter[stableRate=%3.1fqps]", getRate());
398  }
399
400  abstract static class SleepingStopwatch {
401    /** Constructor for use by subclasses. */
402    protected SleepingStopwatch() {}
403
404    /*
405     * We always hold the mutex when calling this. TODO(cpovirk): Is that important? Perhaps we need
406     * to guarantee that each call to reserveEarliestAvailable, etc. sees a value >= the previous?
407     * Also, is it OK that we don't hold the mutex when sleeping?
408     */
409    protected abstract long readMicros();
410
411    protected abstract void sleepMicrosUninterruptibly(long micros);
412
413    public static SleepingStopwatch createFromSystemTimer() {
414      return new SleepingStopwatch() {
415        final Stopwatch stopwatch = Stopwatch.createStarted();
416
417        @Override
418        protected long readMicros() {
419          return stopwatch.elapsed(MICROSECONDS);
420        }
421
422        @Override
423        protected void sleepMicrosUninterruptibly(long micros) {
424          if (micros > 0) {
425            Uninterruptibles.sleepUninterruptibly(micros, MICROSECONDS);
426          }
427        }
428      };
429    }
430  }
431
432  private static void checkPermits(int permits) {
433    checkArgument(permits > 0, "Requested permits (%s) must be positive", permits);
434  }
435}