NAME
    Sys::RunAlone::Flexible - make sure only one invocation of a script is
    active at a time

VERSION
    This document describes version 0.003 of Sys::RunAlone::Flexible (from
    Perl distribution Sys-RunAlone-Flexible), released on 2019-08-17.

SYNOPSIS
    Use like you would use Sys::RunAlone:

     use Sys::RunAlone::Flexible;
     # code of which there may only be on instance running on system

     use Sys::RunAlone::Flexible silent => 1;
     # be silent if other running instance detected

     use Sys::RunAlone::Flexible retry => 50;
     # retry execution 50 times with wait time of 1 second in between

     use Sys::RunAlone::Flexible retry => '55,60';
     # retry execution 55 times with wait time of 60 seconds in between

     use Sys::RunAlone::Flexible 'silent';
     # obsolete form of silent => 1

    Use in run-time:

     require Sys::RunAlone::Flexible;
     Sys::RunAlone::Flexible->import(retry => "55,60");

     # then, somewhere in your program
     sub run {
         Sys::RunAlone::Flexible::lock();
     }

DESCRIPTION
    Sys::RunAlone::Flexible is a fork of Sys::RunAlone 0.13. It's just like
    Sys::RunAlone but it can be used at run-time too. The main logic is
    moved from INIT block to the lock subroutine which you can invoke at
    run-time. But, if you "use Sys::RunAlone::Flexible" at compile-time like
    you would normally use Sys::RunAlone, the lock() subroutine will still
    be invoked at INIT phase.

    The rest of the documentation is Sys::RunAlone's.

    Provide a simple way to make sure the script from which this module is
    loaded, is only running once on the server. Optionally allow for
    retrying execution until the other instance of the script has finished.

THEORY OF OPERATION
    The functionality of this module depends on the availability of the
    "DATA" handle in the script from which this module is called (more
    specifically: in the "main" namespace).

    NOTE: the "__END__" tag is always found in the "main" package namespace.
    However, the "__DATA__" tag is always found in the namespace declared by
    the script. This might very well be different when writing a modulino.

    At compile/INIT time (or when you run "lock"), it is checked when there
    is a DATA handle: if not, it exits with an error message on STDERR and
    an exit value of 2.

    If the DATA handle is available, and it cannot be "flock"ed, it exits
    with an error message on STDERR and an exit value of 1. The error
    message will be surpressed when "silent =" 1> was specified in the "use"
    statement. This can be overridden with the environment variable
    "SILENT_SYS_RUNALONE".

    If there is a DATA handle, and it could be "flock"ed, execution
    continues without any further interference.

TRYING MORE THAN ONCE
    Optionally, it is possibly to specify a number of retries to be done if
    the first "flock" fails. This can be done by either specifying the retry
    value in the "use" statement as e.g. "retry =" 55>, or with the
    environment variable "RETRY_SYS_RUNALONE". There are two forms of the
    retry value:

    times
         use Sys::RunAlone retry => 55;  # retry 55 times, with 1 second intervals

        Specify the number of times to retry, with 1 second intervals.

    times,seconds
         use Sys::RunAlone retry => '55,60'; # retry 55 times, with 60 second intervals

        Specify both the number of retries as well as the number of seconds
        interval between tries.

    This is particularly useful for minutely and hourly scripts that run a
    long and sometimes run into the next period. Instead of then not doing
    anything for the next period, it will start processing again as soon as
    it is possible. This makes the chance of catching up so that the period
    after the next period everything is in sync again.

OVERRIDING CHECK
    In some cases, the same script may need to be run simultaneously with
    another incarnation (but possibly with different parameters). In order
    to simplify this type of usage, it is possible to specify the
    environment variable "SKIP_SYS_RUNALONE" with a true value.

     SKIP_SYS_RUNALONE=1 yourscript.pl

    will run the script always.

     SKIP_SYS_RUNALONE=2 yourscript.pl

    will actually be verbose about this and say:

     Skipping Sys::RunAlone check for 'yourscript.pl'

REQUIRED MODULES
     Fcntl (any)

CAVEATS
  symlinks
    Execution of scripts that are (sym)linked to another script, will all be
    seen as execution of the same script, even though the error message will
    only show the specified script name. This could be considered a bug or a
    feature.

  changing a running script
    If you change the script while it is running, the script will
    effectively lose its lock on the file. Causing any subsequent run of the
    same script to be successful, causing two instances of the same script
    to run at the same time (which is what you wanted to prevent by using
    Sys::RunAlone in the first place). Therefore, make sure that no
    instances of the script are running (and won't be started by cronjobs
    while making changes) if you really want to be 100% sure that only one
    instance of the script is running at the same time.

SYS::RUNALONE ACKNOWLEDGEMENTS
    Inspired by Randal Schwartz's mention of using the DATA handle as a
    semaphore on the London PM mailing list.

    Booking.com for using this heavily in production and allowing me to
    improve this module.

SYS::RUNALONE AUTHOR
     Elizabeth Mattijsen

SYS::RUNALONE COPYRIGHT
    Copyright (c) 2005, 2006, 2008, 2009, 2011, 2012 Elizabeth Mattijsen
    <liz@dijkmat.nl>. Copyright (c) 2017 Ben Tilly <btilly@gmail.com>.
    Copyright (c) 2019 Jim Bacon <boftx@cpan.org>. All rights reserved. This
    program is free software; you can redistribute it and/or modify it under
    the same terms as Perl itself.

FUNCTIONS
  lock
METHODS
    There are no methods.

HOMEPAGE
    Please visit the project's homepage at
    <https://metacpan.org/release/Sys-RunAlone-Flexible>.

SOURCE
    Source repository is at
    <https://github.com/perlancar/perl-Sys-RunAlone-Flexible>.

BUGS
    Please report any bugs or feature requests on the bugtracker website
    <https://rt.cpan.org/Public/Dist/Display.html?Name=Sys-RunAlone-Flexible
    >

    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
    Sys::RunAlone.

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.

