Token Bucket 算法

最近在看celery代码，看到worker的代码，发现了tocket bucket算法，自己之前还一直 在想像有些API对调用次数有限制是怎么做到的，看完才发现，原来是这么朴素的算法。 真是无知，无知啊。嗯，以后要多读读源码。

celery外链里但是写上了注释的代码(稍微做了一些改动)：

.. code:: python

# coding: utf-8 import time

class TokenBucket(object):
    def __init__(self, tokens, fill_rate):
        """tokens is the total tokens in the bucket. fill_rate is the
        rate in tokens/second that the bucket will be refilled."""
        self.capacity = tokens  # 桶的容量
        self._tokens = tokens  # 令牌们
        self.fill_rate = fill_rate  # 每秒放入的令牌数量
        self.timestamp = int(time.time())  # 上次请求令牌的时间

    def consume(self, tokens):
        """Consume tokens from the bucket. Returns True if there were
        sufficient tokens otherwise False."""
        if tokens <= self.__get_tokens():
            self._tokens -= tokens
        else:
            return False
        return True

    def __get_tokens(self):
        now = int(time.time())
        if self._tokens < self.capacity:
            delta = self.fill_rate * (now - self.timestamp)
            self._tokens = min(self.capacity, self._tokens + delta)
            print("delta: %s" % delta)
        self.timestamp = now
        return self._tokens

if __name__ == "__main__":
    bucket = TokenBucket(80, 1)
    print("tokens = %s" % bucket._tokens)
    print("consume(10) = %s" % bucket.consume(10))
    print("consume(10) = %s" % bucket.consume(10))
    time.sleep(1)
    print("tokens = %s" % bucket._tokens)
    time.sleep(1)
    print("tokens = %s" % bucket._tokens)
    print("consume(90) = %s" % bucket.consume(90))
    print("tokens = %s" % bucket._tokens)
    print("consume(90) = %s" % bucket.consume(90))
    print("tokens = %s" % bucket._tokens)

我们看一下测试结果:

.. code:: bash

$ python token_bucket.py
tokens = 80
consume(10) = True
delta: 0
consume(10) = True
tokens = 60
tokens = 60
delta: 2
consume(90) = False
tokens = 62
delta: 0
consume(90) = False
tokens = 62

最后我们用大白话来描述一下上面的代码：

1，初始化的时候，指定了桶的大小和每秒钟放入令牌的速率

2，每次消耗令牌的时候，都会计算，上次消耗到本次消耗之间产生了多少令牌，如果产生 令牌的数量超过了容量，则丢弃多余的令牌。

3，如果要消耗的令牌数量大于现有的令牌数量，则返回失败。

.. [#] https://en.wikipedia.org/wiki/Token_bucket

.. [#] http://code.activestate.com/recipes/511490/