RateLimiter限流器使用

一、概要说明

最近在工作中涉及到类似抢购的业务，在请求并发量高的时候会阻塞到正常业务的访问。因此考虑使用guawa的rateLimiter进行限流，对超限的流量进行降级处理。保证正常的业务运行

二、RateLimiter的特性

RateLimiter使用令牌桶算法，相对于传统的根据每秒的请求数进行限流的方式，令牌桶算法会让流量更加平滑地流入。eg.QPS限制为100，假如这100个请求是在1ms内同时进来，也有可能让系统崩溃。

三、核心源码分析

/**
   * 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})
   */
  @CanIgnoreReturnValue
  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());
    }
  }
/**
   * 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);
  }

@Override
  final long reserveEarliestAvailable(int requiredPermits, long nowMicros) {
    resync(nowMicros);
    long returnValue = nextFreeTicketMicros;
    double storedPermitsToSpend = min(requiredPermits, this.storedPermits);
    double freshPermits = requiredPermits - storedPermitsToSpend;
    long waitMicros =
        storedPermitsToWaitTime(this.storedPermits, storedPermitsToSpend)
            + (long) (freshPermits * stableIntervalMicros);

    this.nextFreeTicketMicros = LongMath.saturatedAdd(nextFreeTicketMicros, waitMicros);
    this.storedPermits -= storedPermitsToSpend;
    return returnValue;
  }

我们重点关注reserveEarliestAvailable这个方法。
storedPermits为令牌生成的数量，resync这个方法会根据当前的时间和上一次生成的令牌的时间对比，计算出本次生成的令牌数量。eg.QPS设置为100，上一次请求和当前时间相隔为2ms，则当前的令牌数量为20。