Algorithm::TokenBucket - Token bucket rate limiting algorithm
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)
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.
AND
OR
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.
conform
count
information rate
burst size
The constructor takes as parameters at least rate of information in items per second and burst size in items.
rate of information
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.
This sub removes N (or all if there are less than N available) tokens from the bucket. Does not return a meaningful value.
Fills the bucket.
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); }
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).
Ingo Blechschmidt, <iblech@web.de> (port to Perl 6)
Alex Kapranoff, <kappa@rambler-co.ru>
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.
To install Perl6::Pugs, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Perl6::Pugs
CPAN shell
perl -MCPAN -e shell install Perl6::Pugs
For more information on module installation, please visit the detailed CPAN module installation guide.