NAME

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

VERSION

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

SYNOPSIS

 use Algorithm::Backoff::MILD;
 # 1. instantiate
 my $ab = Algorithm::Backoff::MILD->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             => 2, # optional, default 0
     #max_delay             => 100, # optional
     initial_delay              => 1,   # required
     delay_multiple_on_failure  => 1.5, # required
     delay_increment_on_success => -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=1,
 # delay_multiple_on_failure=1.5, delay_increment_on_success=-2, min_delay=0.5:
 my $secs;
 $secs = $ab->failure();   # => 1    (= initial_delay)
 $secs = $ab->failure();   # => 1.5  (1 * 1.5)
 $secs = $ab->failure();   # => 2.25 (1.5 * 1.5)
 $secs = $ab->success();   # => 0.25 (2.25 - 2)
 $secs = $ab->success();   # => 1 (max(0.25 - 2, 0.5, 1))
 $secs = $ab->failure();   # => 1.5 (1 * 1.5)

Illustration using CLI show-backoff-delays (4 failures followed by 5 successes, followed by 3 failures):

 % show-backoff-delays -a MILD --initial-delay 3 --min-delay 1 \
     --delay-multiple-on-failure 2 --delay-increment-on-success -5 \
     0 0 0 0   1 1 1 1 1   0 0 0
 3
 6
 12
 24
 19
 14
 9
 4
 1
 2
 4

DESCRIPTION

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

 D1 = initial_delay
 D2 = max(min(D1 * delay_multiple_on_failure, max_delay), min_delay)
 ...

Upon success, the next delay is calculated as:

 D1 = initial_delay
 D2 = max(D1 + delay_increment_on_success, min_delay, initial_delay)
 ...

"initial_delay", "delay_multiple_on_failure", and "delay_increment_on_success" are required. "initial_delay" and "min_delay" should be larger than zero; otherwise the next delays will all be zero.

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_success* => float
How much to add to previous delay, in seconds, upon success (e.g. -5).
delay_multiple_on_failure* => ufloat
How much to multiple previous delay, upon failure (e.g. 1.5).
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.

AUTHOR

perlancar <perlancar@cpan.org>

COPYRIGHT AND LICENSE

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