com.google.common.util.concurrent
Class RateLimiter

java.lang.Object
  com.google.common.util.concurrent.RateLimiter

@ThreadSafe
@Beta
public abstract class RateLimiter
extends Object

A rate limiter. Conceptually, a rate limiter distributes permits at a configurable rate. Each acquire() blocks if necessary until a permit is available, and then takes it. Once acquired, permits need not be released.

Rate limiters are often used to restrict the rate at which some physical or logical resource is accessed. This is in contrast to Semaphore which restricts the number of concurrent accesses instead of the rate (note though that concurrency and rate are closely related, e.g. see Little's Law).

A 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.

It is possible to configure a RateLimiter to have a warmup period during which time the permits issued each second steadily increases until it hits the stable rate.

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:

  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);
    }
  }

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:

  final RateLimiter rateLimiter = RateLimiter.create(5000.0); // rate = 5000 permits per second
  void submitPacket(byte[] packet) {
    rateLimiter.acquire(packet.length);
    networkService.send(packet);
  }

It is important to note that the number of permits requested never affect the throttling of the request itself (an invocation to acquire(1) and an invocation to acquire(1000) will result in exactly the same throttling, if any), but it affects the throttling of the next request. I.e., if an expensive task arrives at an idle RateLimiter, it will be granted immediately, but it is the next request that will experience extra throttling, thus paying for the cost of the expensive task.

Note: RateLimiter does not provide fairness guarantees.

Since:

13.0

Author:

Dimitris Andreou

Method Summary

void	acquire() Acquires a permit from this RateLimiter, blocking until the request can be granted.
void	acquire(int permits) Acquires the given number of permits from this RateLimiter, blocking until the request be granted.
static RateLimiter	create(double permitsPerSecond) Creates a RateLimiter with the specified stable throughput, given as "permits per second" (commonly referred to as QPS, queries per second).
static RateLimiter	create(double permitsPerSecond, long warmupPeriod, TimeUnit unit) Creates a RateLimiter with the specified stable throughput, given as "permits per second" (commonly referred to as QPS, queries per second), and a warmup period, during which the 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).
double	getRate() Returns the stable rate (as permits per seconds) with which this RateLimiter is configured with.
void	setRate(double permitsPerSecond) Updates the stable rate of this RateLimiter, that is, the permitsPerSecond argument provided in the factory method that constructed the RateLimiter.
String	toString()
boolean	tryAcquire(int permits, long timeout, TimeUnit unit) Acquires the given number of permits from this RateLimiter if it can be obtained without exceeding the specified timeout, or returns false immediately (without waiting) if the permits would not have been granted before the timeout expired.
boolean	tryAcquire(long timeout, TimeUnit unit) Acquires a permit from this RateLimiter if it can be obtained without exceeding the specified timeout, or returns false immediately (without waiting) if the permit would not have been granted before the timeout expired.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Method Detail

create

public static RateLimiter create(double permitsPerSecond)

Creates a RateLimiter with the specified stable throughput, given as "permits per second" (commonly referred to as QPS, queries per second).

The returned RateLimiter ensures that on average no more than permitsPerSecond are issued during any given second, with sustained requests being smoothly spread over each second. When the incoming request rate exceeds permitsPerSecond the rate limiter will release one permit every (1.0 / permitsPerSecond) seconds. When the rate limiter is unused, bursts of up to permitsPerSecond permits will be allowed, with subsequent requests being smoothly limited at the stable rate of permitsPerSecond.

Parameters:

permitsPerSecond - the rate of the returned RateLimiter, measured in how many permits become available per second.

create

public static RateLimiter create(double permitsPerSecond,
                                 long warmupPeriod,
                                 TimeUnit unit)

Creates a RateLimiter with the specified stable throughput, given as "permits per second" (commonly referred to as QPS, queries per second), and a warmup period, during which the 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 RateLimiter is left unused for a duration of 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.

The returned RateLimiter is intended for cases where the resource that actually fulfils the requests (e.g., a remote server) needs "warmup" time, rather than being immediately accessed at the stable (maximum) rate.

The returned 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.

Parameters:

permitsPerSecond - the rate of the returned RateLimiter, measured in how many permits become available per second

warmupPeriod - the duration of the period where the RateLimiter ramps up its rate, before reaching its stable (maximum) rate

unit - the time unit of the warmupPeriod argument

setRate

public final void setRate(double permitsPerSecond)

Updates the stable rate of this RateLimiter, that is, the permitsPerSecond argument provided in the factory method that constructed the RateLimiter. Currently throttled threads will not be awakened as a result of this invocation, thus they do not observe the new rate; only subsequent requests will.

Note though that, since each request repays (by waiting, if necessary) the cost of the previous request, this means that the very next request after an invocation to 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.

The behavior of the RateLimiter is not modified in any other way, e.g. if the RateLimiter was configured with a warmup period of 20 seconds, it still has a warmup period of 20 seconds after this method invocation.

Parameters:

permitsPerSecond - the new stable rate of this RateLimiter.

getRate

public final double getRate()

Returns the stable rate (as permits per seconds) with which this RateLimiter is configured with. The initial value of this is the same as the permitsPerSecond argument passed in the factory method that produced this RateLimiter, and it is only updated after invocations to setRate(double).

acquire

public void acquire()

Acquires a permit from this RateLimiter, blocking until the request can be granted.

This method is equivalent to acquire(1).

acquire

public void acquire(int permits)

Acquires the given number of permits from this RateLimiter, blocking until the request be granted.

Parameters:

permits - the number of permits to acquire

tryAcquire

public boolean tryAcquire(long timeout,
                          TimeUnit unit)

Acquires a permit from this RateLimiter if it can be obtained without exceeding the specified timeout, or returns false immediately (without waiting) if the permit would not have been granted before the timeout expired.

This method is equivalent to tryAcquire(1, timeout, unit).

Parameters:

timeout - the maximum time to wait for the permit

unit - the time unit of the timeout argument

Returns:

true if the permit was acquired, false otherwise

tryAcquire

public boolean tryAcquire(int permits,
                          long timeout,
                          TimeUnit unit)

Acquires the given number of permits from this RateLimiter if it can be obtained without exceeding the specified timeout, or returns false immediately (without waiting) if the permits would not have been granted before the timeout expired.

Parameters:

permits - the number of permits to acquire

timeout - the maximum time to wait for the permits

unit - the time unit of the timeout argument

Returns:

true if the permits were acquired, false otherwise

toString

public String toString()

Overrides:

toString in class Object