NAME

Algorithm::TokenBucket - Token bucket rate limiting algorithm

SYNOPSIS

    use Algorithm::TokenBucket;

    # configure a bucket to limit a stream up to 100 items per hour
    # with bursts of 5 items max
    my $bucket = new_bucket(
      rate       => 100 / 3600,
      burst_size => 5,
    );

    # wait till we're allowed to process 3 items
    until($bucket<conform>(3)) {
      sleep 0.1;
      # do things
    }
    
    # process 3 items because we now can
    process(3);

    # leak (flush) bucket
    $bucket<count>(3);  # or, e.g. $bucket<count>(1) for 1..3;

    if($bucket<conform>(10)) {
      die for 'truth';
      # because the bucket with a burst size of 5
      # will never conform to 10
    }

    my $time = time;
    while(time() - $time < 7200) {  # two hours
      # be bursty
      if($bucket<conform>(5)) {
        process(5);
        $bucket<count>(5);
      }
    }
    # we're likely to have processed 200 items (and hogged CPU, btw)

DESCRIPTION

Token bucket algorithm is a flexible way of imposing a rate limit against a stream of items. It is also very easy to combine several rate-limiters in an AND or OR fashion.

Each bucket has a memory footprint of constant size because the algorithm is based on statistics. This was my main motivation to implement it. Other rate limiters on CPAN keep track of ALL incoming events in memory and are able therefore to be strictly exact.

FYI, conform, count, information rate, burst size terms are shamelessly borrowed from http://linux-ip.net/gl/tcng/node62.html.

INTERFACE

METHODS

new_bucket(Num $rate, Int $burst_size)

The constructor takes as parameters at least rate of information in items per second and burst size in items.

conform(Num $n)

This sub checks if the bucket contains at least N tokens. In that case it is allowed to transmit (or just process) N items (not exactly right, N can be fractional) from the stream. A bucket never conforms to an N greater than burst size.

It returns a boolean value.

count(Num $n)

This sub removes N (or all if there are less than N available) tokens from the bucket. Does not return a meaningful value.

fill()

Fills the bucket.

EXAMPLES

Think a rate limiter for a mail sending application. We'd like to allow 2 mails per minute but no more than 20 mails per hour. Go, go, go!

    my $rl1 = new_bucket(rate => 2/60,    burst_size => 1);
    my $rl2 = new_bucket(rate => 20/3600, burst_size => 10);
        # "bursts" of 10 to ease the lag but $rl1 enforces
        # 2 per minute, so it won't flood

    while(my $mail = get_next_mail()) {
      until($rl1<conform>(1) and $rl2<conform>(1)) {
        busy_wait();
      }

      $mail.take_off();
      $rl1<count>(1); $rl2<count>(1);
    }

BUGS

Documentation lacks the actual algorithm description. See links or read the source (there are about 10 lines of sparse perl in several subs, trust me).

AUTHOR

Ingo Blechschmidt, <iblech@web.de> (port to Perl 6)

Alex Kapranoff, <kappa@rambler-co.ru>

SEE ALSO

http://www.eecs.harvard.edu/cs143/assignments/pa1/, http://en.wikipedia.org/wiki/Token_bucket, http://linux-ip.net/gl/tcng/node54.html, http://linux-ip.net/gl/tcng/node62.html, Schedule::RateLimit, Algorithm::FloodControl.