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.