/*
* Copyright (C) 2012 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.util.concurrent;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static java.lang.Math.max;
import static java.util.concurrent.TimeUnit.MICROSECONDS;
import static java.util.concurrent.TimeUnit.SECONDS;
import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Stopwatch;
import com.google.common.util.concurrent.SmoothRateLimiter.SmoothBursty;
import com.google.common.util.concurrent.SmoothRateLimiter.SmoothWarmingUp;
import java.util.concurrent.TimeUnit;
import javax.annotation.concurrent.ThreadSafe;
/**
* A rate limiter. Conceptually, a rate limiter distributes permits at a
* configurable rate. Each {@link #acquire()} blocks if necessary until a permit is
* available, and then takes it. Once acquired, permits need not be released.
*
* <p>Rate limiters are often used to restrict the rate at which some
* physical or logical resource is accessed. This is in contrast to {@link
* java.util.concurrent.Semaphore} which restricts the number of concurrent
* accesses instead of the rate (note though that concurrency and rate are closely related,
* e.g. see <a href="http://en.wikipedia.org/wiki/Little's_law">Little's Law</a>).
*
* <p>A {@code RateLimiter} is defined primarily by the rate at which permits
* are issued. Absent additional configuration, permits will be distributed at a
* fixed rate, defined in terms of permits per second. Permits will be distributed
* smoothly, with the delay between individual permits being adjusted to ensure
* that the configured rate is maintained.
*
* <p>It is possible to configure a {@code RateLimiter} to have a warmup
* period during which time the permits issued each second steadily increases until
* it hits the stable rate.
*
* <p>As an example, imagine that we have a list of tasks to execute, but we don't want to
* submit more than 2 per second:
*<pre> {@code
* final RateLimiter rateLimiter = RateLimiter.create(2.0); // rate is "2 permits per second"
* void submitTasks(List<Runnable> tasks, Executor executor) {
* for (Runnable task : tasks) {
* rateLimiter.acquire(); // may wait
* executor.execute(task);
* }
* }
*}</pre>
*
* <p>As another example, imagine that we produce a stream of data, and we want to cap it
* at 5kb per second. This could be accomplished by requiring a permit per byte, and specifying
* a rate of 5000 permits per second:
*<pre> {@code
* final RateLimiter rateLimiter = RateLimiter.create(5000.0); // rate = 5000 permits per second
* void submitPacket(byte[] packet) {
* rateLimiter.acquire(packet.length);
* networkService.send(packet);
* }
*}</pre>
*
* <p>It is important to note that the number of permits requested <i>never</i>
* affect the throttling of the request itself (an invocation to {@code acquire(1)}
* and an invocation to {@code acquire(1000)} will result in exactly the same throttling, if any),
* but it affects the throttling of the <i>next</i> request. I.e., if an expensive task
* arrives at an idle RateLimiter, it will be granted immediately, but it is the <i>next</i>
* request that will experience extra throttling, thus paying for the cost of the expensive
* task.
*
* <p>Note: {@code RateLimiter} does not provide fairness guarantees.
*
* @author Dimitris Andreou
* @since 13.0
*/
// TODO(user): switch to nano precision. A natural unit of cost is "bytes", and a micro precision
// would mean a maximum rate of "1MB/s", which might be small in some cases.
@ThreadSafe
@Beta
public abstract class RateLimiter {
/**
* Creates a {@code RateLimiter} with the specified stable throughput, given as
* "permits per second" (commonly referred to as <i>QPS</i>, queries per second).
*
* <p>The returned {@code RateLimiter} ensures that on average no more than {@code
* permitsPerSecond} are issued during any given second, with sustained requests
* being smoothly spread over each second. When the incoming request rate exceeds
* {@code permitsPerSecond} the rate limiter will release one permit every {@code
* (1.0 / permitsPerSecond)} seconds. When the rate limiter is unused,
* bursts of up to {@code permitsPerSecond} permits will be allowed, with subsequent
* requests being smoothly limited at the stable rate of {@code permitsPerSecond}.
*
* @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in
* how many permits become available per second
* @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
*/
// TODO(user): "This is equivalent to
// {@code createWithCapacity(permitsPerSecond, 1, TimeUnit.SECONDS)}".
public static RateLimiter create(double permitsPerSecond) {
/*
* The default RateLimiter configuration can save the unused permits of up to one second.
* This is to avoid unnecessary stalls in situations like this: A RateLimiter of 1qps,
* and 4 threads, all calling acquire() at these moments:
*
* T0 at 0 seconds
* T1 at 1.05 seconds
* T2 at 2 seconds
* T3 at 3 seconds
*
* Due to the slight delay of T1, T2 would have to sleep till 2.05 seconds,
* and T3 would also have to sleep till 3.05 seconds.
*/
return create(SleepingStopwatch.createFromSystemTimer(), permitsPerSecond);
}
/*
* TODO(cpovirk): make SleepingStopwatch the last parameter throughout the class so that the
* overloads follow the usual convention: Foo(int), Foo(int, SleepingStopwatch)
*/
@VisibleForTesting
static RateLimiter create(SleepingStopwatch stopwatch, double permitsPerSecond) {
RateLimiter rateLimiter = new SmoothBursty(stopwatch, 1.0 /* maxBurstSeconds */);
rateLimiter.setRate(permitsPerSecond);
return rateLimiter;
}
/**
* Creates a {@code RateLimiter} with the specified stable throughput, given as
* "permits per second" (commonly referred to as <i>QPS</i>, queries per second), and a
* <i>warmup period</i>, during which the {@code RateLimiter} smoothly ramps up its rate,
* until it reaches its maximum rate at the end of the period (as long as there are enough
* requests to saturate it). Similarly, if the {@code RateLimiter} is left <i>unused</i> for
* a duration of {@code warmupPeriod}, it will gradually return to its "cold" state,
* i.e. it will go through the same warming up process as when it was first created.
*
* <p>The returned {@code RateLimiter} is intended for cases where the resource that actually
* fulfills the requests (e.g., a remote server) needs "warmup" time, rather than
* being immediately accessed at the stable (maximum) rate.
*
* <p>The returned {@code RateLimiter} starts in a "cold" state (i.e. the warmup period
* will follow), and if it is left unused for long enough, it will return to that state.
*
* @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in
* how many permits become available per second
* @param warmupPeriod the duration of the period where the {@code RateLimiter} ramps up its
* rate, before reaching its stable (maximum) rate
* @param unit the time unit of the warmupPeriod argument
* @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero or
* {@code warmupPeriod} is negative
*/
public static RateLimiter create(double permitsPerSecond, long warmupPeriod, TimeUnit unit) {
checkArgument(warmupPeriod >= 0, "warmupPeriod must not be negative: %s", warmupPeriod);
return create(SleepingStopwatch.createFromSystemTimer(), permitsPerSecond, warmupPeriod, unit);
}
@VisibleForTesting
static RateLimiter create(
SleepingStopwatch stopwatch, double permitsPerSecond, long warmupPeriod, TimeUnit unit) {
RateLimiter rateLimiter = new SmoothWarmingUp(stopwatch, warmupPeriod, unit);
rateLimiter.setRate(permitsPerSecond);
return rateLimiter;
}
/**
* The underlying timer; used both to measure elapsed time and sleep as necessary. A separate
* object to facilitate testing.
*/
private final SleepingStopwatch stopwatch;
// Can't be initialized in the constructor because mocks don't call the constructor.
private volatile Object mutexDoNotUseDirectly;
private Object mutex() {
Object mutex = mutexDoNotUseDirectly;
if (mutex == null) {
synchronized (this) {
mutex = mutexDoNotUseDirectly;
if (mutex == null) {
mutexDoNotUseDirectly = mutex = new Object();
}
}
}
return mutex;
}
RateLimiter(SleepingStopwatch stopwatch) {
this.stopwatch = checkNotNull(stopwatch);
}
/**
* Updates the stable rate of this {@code RateLimiter}, that is, the
* {@code permitsPerSecond} argument provided in the factory method that
* constructed the {@code RateLimiter}. Currently throttled threads will <b>not</b>
* be awakened as a result of this invocation, thus they do not observe the new rate;
* only subsequent requests will.
*
* <p>Note though that, since each request repays (by waiting, if necessary) the cost
* of the <i>previous</i> request, this means that the very next request
* after an invocation to {@code setRate} will not be affected by the new rate;
* it will pay the cost of the previous request, which is in terms of the previous rate.
*
* <p>The behavior of the {@code RateLimiter} is not modified in any other way,
* e.g. if the {@code RateLimiter} was configured with a warmup period of 20 seconds,
* it still has a warmup period of 20 seconds after this method invocation.
*
* @param permitsPerSecond the new stable rate of this {@code RateLimiter}
* @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
*/
public final void setRate(double permitsPerSecond) {
checkArgument(
permitsPerSecond > 0.0 && !Double.isNaN(permitsPerSecond), "rate must be positive");
synchronized (mutex()) {
doSetRate(permitsPerSecond, stopwatch.readMicros());
}
}
abstract void doSetRate(double permitsPerSecond, long nowMicros);
/**
* Returns the stable rate (as {@code permits per seconds}) with which this
* {@code RateLimiter} is configured with. The initial value of this is the same as
* the {@code permitsPerSecond} argument passed in the factory method that produced
* this {@code RateLimiter}, and it is only updated after invocations
* to {@linkplain #setRate}.
*/
public final double getRate() {
synchronized (mutex()) {
return doGetRate();
}
}
abstract double doGetRate();
/**
* Acquires a single permit from this {@code RateLimiter}, blocking until the
* request can be granted. Tells the amount of time slept, if any.
*
* <p>This method is equivalent to {@code acquire(1)}.
*
* @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
* @since 16.0 (present in 13.0 with {@code void} return type})
*/
public double acquire() {
return acquire(1);
}
/**
* Acquires the given number of permits from this {@code RateLimiter}, blocking until the
* request can be granted. Tells the amount of time slept, if any.
*
* @param permits the number of permits to acquire
* @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
* @throws IllegalArgumentException if the requested number of permits is negative or zero
* @since 16.0 (present in 13.0 with {@code void} return type})
*/
public double acquire(int permits) {
long microsToWait = reserve(permits);
stopwatch.sleepMicrosUninterruptibly(microsToWait);
return 1.0 * microsToWait / SECONDS.toMicros(1L);
}
/**
* Reserves the given number of permits from this {@code RateLimiter} for future use, returning
* the number of microseconds until the reservation can be consumed.
*
* @return time in microseconds to wait until the resource can be acquired, never negative
*/
final long reserve(int permits) {
checkPermits(permits);
synchronized (mutex()) {
return reserveAndGetWaitLength(permits, stopwatch.readMicros());
}
}
/**
* Acquires a permit from this {@code RateLimiter} if it can be obtained
* without exceeding the specified {@code timeout}, or returns {@code false}
* immediately (without waiting) if the permit would not have been granted
* before the timeout expired.
*
* <p>This method is equivalent to {@code tryAcquire(1, timeout, unit)}.
*
* @param timeout the maximum time to wait for the permit. Negative values are treated as zero.
* @param unit the time unit of the timeout argument
* @return {@code true} if the permit was acquired, {@code false} otherwise
* @throws IllegalArgumentException if the requested number of permits is negative or zero
*/
public boolean tryAcquire(long timeout, TimeUnit unit) {
return tryAcquire(1, timeout, unit);
}
/**
* Acquires permits from this {@link RateLimiter} if it can be acquired immediately without delay.
*
* <p>
* This method is equivalent to {@code tryAcquire(permits, 0, anyUnit)}.
*
* @param permits the number of permits to acquire
* @return {@code true} if the permits were acquired, {@code false} otherwise
* @throws IllegalArgumentException if the requested number of permits is negative or zero
* @since 14.0
*/
public boolean tryAcquire(int permits) {
return tryAcquire(permits, 0, MICROSECONDS);
}
/**
* Acquires a permit from this {@link RateLimiter} if it can be acquired immediately without
* delay.
*
* <p>
* This method is equivalent to {@code tryAcquire(1)}.
*
* @return {@code true} if the permit was acquired, {@code false} otherwise
* @since 14.0
*/
public boolean tryAcquire() {
return tryAcquire(1, 0, MICROSECONDS);
}
/**
* Acquires the given number of permits from this {@code RateLimiter} if it can be obtained
* without exceeding the specified {@code timeout}, or returns {@code false}
* immediately (without waiting) if the permits would not have been granted
* before the timeout expired.
*
* @param permits the number of permits to acquire
* @param timeout the maximum time to wait for the permits. Negative values are treated as zero.
* @param unit the time unit of the timeout argument
* @return {@code true} if the permits were acquired, {@code false} otherwise
* @throws IllegalArgumentException if the requested number of permits is negative or zero
*/
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) {
long timeoutMicros = max(unit.toMicros(timeout), 0);
checkPermits(permits);
long microsToWait;
synchronized (mutex()) {
long nowMicros = stopwatch.readMicros();
if (!canAcquire(nowMicros, timeoutMicros)) {
return false;
} else {
microsToWait = reserveAndGetWaitLength(permits, nowMicros);
}
}
stopwatch.sleepMicrosUninterruptibly(microsToWait);
return true;
}
private boolean canAcquire(long nowMicros, long timeoutMicros) {
return queryEarliestAvailable(nowMicros) - timeoutMicros <= nowMicros;
}
/**
* Reserves next ticket and returns the wait time that the caller must wait for.
*
* @return the required wait time, never negative
*/
final long reserveAndGetWaitLength(int permits, long nowMicros) {
long momentAvailable = reserveEarliestAvailable(permits, nowMicros);
return max(momentAvailable - nowMicros, 0);
}
/**
* Returns the earliest time that permits are available (with one caveat).
*
* @return the time that permits are available, or, if permits are available immediately, an
* arbitrary past or present time
*/
abstract long queryEarliestAvailable(long nowMicros);
/**
* Reserves the requested number of permits and returns the time that those permits can be used
* (with one caveat).
*
* @return the time that the permits may be used, or, if the permits may be used immediately, an
* arbitrary past or present time
*/
abstract long reserveEarliestAvailable(int permits, long nowMicros);
@Override
public String toString() {
return String.format("RateLimiter[stableRate=%3.1fqps]", getRate());
}
@VisibleForTesting
abstract static class SleepingStopwatch {
/*
* We always hold the mutex when calling this. TODO(cpovirk): Is that important? Perhaps we need
* to guarantee that each call to reserveEarliestAvailable, etc. sees a value >= the previous?
* Also, is it OK that we don't hold the mutex when sleeping?
*/
abstract long readMicros();
abstract void sleepMicrosUninterruptibly(long micros);
static final SleepingStopwatch createFromSystemTimer() {
return new SleepingStopwatch() {
final Stopwatch stopwatch = Stopwatch.createStarted();
@Override
long readMicros() {
return stopwatch.elapsed(MICROSECONDS);
}
@Override
void sleepMicrosUninterruptibly(long micros) {
if (micros > 0) {
Uninterruptibles.sleepUninterruptibly(micros, MICROSECONDS);
}
}
};
}
}
private static int checkPermits(int permits) {
checkArgument(permits > 0, "Requested permits (%s) must be positive", permits);
return permits;
}
}