NAME

Algorithm::Backoff::Exponential - Backoff exponentially

VERSION

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

SYNOPSIS

 use Algorithm::Backoff::Exponential;

 # 1. instantiate

 my $ab = Algorithm::Backoff::Exponential->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
     initial_delay          => 5, # required
     #max_delay             => 100, # optional
     #exponent_base         => 2, # optional, default 2 (binary exponentiation)
     #delay_on_success      => 0, # optional, default 0
 );

 # 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=5, max_delay=100:

 my $secs;
 $secs = $ab->failure();   # =>  5 (= initial_delay)
 $secs = $ab->failure();   # => 10 (5 * 2^1)
 $secs = $ab->failure();   # => 20 (5 * 2^2)
 $secs = $ab->failure();   # => 33 (5 * 2^3 - 7)
 $secs = $ab->failure();   # => 80 (5 * 2^4)
 $secs = $ab->failure();   # => 100 ( min(5 * 2^5, 100) )
 $secs = $ab->success();   # => 0 (= delay_on_success)

Illustration using CLI show-backoff-delays (10 failures followed by 3 successes):

 % show-backoff-delays -a Exponential --initial-delay 1 --max-delay 200 \
     0 0 0 0 0   0 0 0 0 0   1 1 1
 1
 2
 4
 8
 16
 32
 64
 128
 200
 200
 0
 0
 0

DESCRIPTION

This backoff algorithm calculates the next delay as:

 initial_delay * exponent_base ** (attempts-1)

Only the initial_delay is required. exponent_base is 2 by default (binary exponential). For the first failure attempt (attempts = 1) the delay equals the initial delay. Then it is doubled, quadrupled, and so on (using the default exponent base of 2).

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_on_success => ufloat (default: 0)

    Number of seconds to wait after a success.

  • exponent_base => ufloat (default: 2)

  • 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

https://en.wikipedia.org/wiki/Exponential_backoff

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.