NAME

Algorithm::Backoff::LIMD - Linear Increment, Multiplicative Decrement (LIMD) backoff

VERSION

This document describes version 0.009 of Algorithm::Backoff::LIMD (from Perl distribution Algorithm-Backoff), released on 2019-06-20.

SYNOPSIS

 use Algorithm::Backoff::LIMD;

 # 1. instantiate

 my $ab = Algorithm::Backoff::LIMD->new(
     #consider_actual_delay => 1, # optional, default 0
     #max_actual_duration   => 0, # optional, default 0 (retry endlessly)
     #max_attempts          => 0, # optional, default 0 (retry endlessly)
     #jitter_factor         => 0.25, # optional, default 0
     min_delay              => 1, # optional, default 0
     #max_delay             => 100, # optional
     initial_delay              => 2,   # required
     delay_increment_on_failure => 4,   # required
     delay_multiple_on_success  => 0.2, # required
 );

 # 2. log success/failure and get a new number of seconds to delay, timestamp is
 # optional but must be monotonically increasing.

 # for example, using the parameters initial_delay=2,
 # delay_increment_on_failure=4, delay_multiple_on_success=0.2, min_delay=1:

 my $secs;
 $secs = $ab->failure();   # =>  2   (= initial_delay)
 $secs = $ab->failure();   # =>  6   (2 + 4)
 $secs = $ab->failure();   # => 10   (2 + 4)
 $secs = $ab->success();   # =>  2   (10 * 0.2)
 $secs = $ab->success();   # =>  1   (max(2 * 0.2, 1))
 $secs = $ab->failure();   # =>  5   (1 + 4)

Illustration using CLI show-backoff-delays (3 failures followed by 3 successes, followed by 3 failures):

 % show-backoff-delays -a LILD --initial-delay 2 --min-delay 1 \
     --delay-increment-on-failure 4 --delay-multiple-on-success 0.2 \
     0 0 0   1 1 1   0 0 0
 2
 6
 10
 2
 1
 1
 5
 9
 13

DESCRIPTION

Upon failure, this backoff algorithm calculates the next delay as:

 D1 = initial_delay
 D2 = min(D1 + delay_increment_on_failure, max_delay)
 ...

Upon success, the next delay is calculated as:

 D1 = initial_delay
 D2 = max(D1 * delay_multiple_on_success, min_delay)
 ...

initial_delay, delay_increment_on_failure, and delay_multiple_on_success are required.

There are limits on the number of attempts (`max_attempts`) and total duration (`max_actual_duration`).

It is recommended to add a jitter factor, e.g. 0.25 to add some randomness to avoid "thundering herd problem".

METHODS

new

Usage:

 new(%args) -> obj

This function is not exported.

Arguments ('*' denotes required arguments):

  • consider_actual_delay => bool (default: 0)

    Whether to consider actual delay.

    If set to true, will take into account the actual delay (timestamp difference). For example, when using the Constant strategy of delay=2, you log failure() again right after the previous failure() (i.e. specify the same timestamp). failure() will then return ~2+2 = 4 seconds. On the other hand, if you waited 2 seconds before calling failure() again (i.e. specify the timestamp that is 2 seconds larger than the previous timestamp), failure() will return 2 seconds. And if you waited 4 seconds or more, failure() will return 0.

  • delay_increment_on_failure* => float

    How much to add to previous delay, in seconds, upon failure (e.g. 5).

  • delay_multiple_on_success* => ufloat

    How much to multiple previous delay, upon success (e.g. 0.5).

  • initial_delay* => ufloat

    Initial delay for the first attempt after failure, in seconds.

  • jitter_factor => float

    How much to add randomness.

    If you set this to a value larger than 0, the actual delay will be between a random number between original_delay * (1-jitter_factor) and original_delay * (1+jitter_factor). Jitters are usually added to avoid so-called "thundering herd" problem.

    The jitter will be applied to delay on failure as well as on success.

  • max_actual_duration => ufloat (default: 0)

    Maximum number of seconds for all of the attempts (0 means unlimited).

    If set to a positive number, will limit the number of seconds for all of the attempts. This setting is used to limit the amount of time you are willing to spend on a task. For example, when using the Exponential strategy of initial_delay=3 and max_attempts=10, the delays will be 3, 6, 12, 24, ... If failures are logged according to the suggested delays, and max_actual_duration is set to 21 seconds, then the third failure() will return -1 instead of 24 because 3+6+12 >= 21, even though max_attempts has not been exceeded.

  • max_attempts => uint (default: 0)

    Maximum number consecutive failures before giving up.

    0 means to retry endlessly without ever giving up. 1 means to give up after a single failure (i.e. no retry attempts). 2 means to retry once after a failure. Note that after a success, the number of attempts is reset (as expected). So if max_attempts is 3, and if you fail twice then succeed, then on the next failure the algorithm will retry again for a maximum of 3 times.

  • max_delay => ufloat

    Maximum delay time, in seconds.

  • min_delay => ufloat (default: 0)

    Maximum delay time, in seconds.

Return value: (obj)

HOMEPAGE

Please visit the project's homepage at https://metacpan.org/release/Algorithm-Backoff.

SOURCE

Source repository is at https://github.com/perlancar/perl-Algorithm-Backoff.

BUGS

Please report any bugs or feature requests on the bugtracker website https://rt.cpan.org/Public/Dist/Display.html?Name=Algorithm-Backoff

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.

SEE ALSO

Algorithm::Backoff::LILD

Algorithm::Backoff::MILD

Algorithm::Backoff::MIMD

Algorithm::Backoff

Other Algorithm::Backoff::* classes.

AUTHOR

perlancar <perlancar@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2019 by perlancar@cpan.org.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.