compute index numbers for Grandfather-Father-Son backup rotation schemes
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NAME
    Rotation::Indexer::GFS - compute index numbers for
    Grandfather-Father-Son backup rotation schemes

VERSION
    This document describes Rotation::Indexer::GFS version 0.1

SYNOPSIS
    Given a definition of a cyclic rotation scheme (A.K.A. a "Grandfather
    Father Son" rotation scheme), and an integral sequence number
    "compute_index" will return an index for it.

        use Rotation::Indexer::GFS;

        my $indexer = Rotation::Indexer::GFS->new(7, 4, 3);

        my $start_day = 0; # or whatever
        my @index = $indexer->index(today - $start_day); 

        my $backup_file = join("-", @index) . ".backup"; # 0-0-0.backup

DESCRIPTION
  Why use this module?
    Suppose you have a file for which you want a rotating back-up: you want
    a backed-up copy of the file from each day in the last week.

    To keep the backup script simple you might name the copy after the day
    of the week: "$dow.backup". Then when Monday rolls around again, the
    script simply copies the latest version over the file "Monday.backup".
    In other words, you don't need to explicitly prune old files because
    they get overwritten by newer ones.

    Now imagine a more complicated use case: you want a nested backup
    rotation scheme: one copy for each day of the last week, one from each
    week in the last month, and one for each of the last three months. You
    might attempt to invent a naming scheme like "$month-$week-$day", but it
    gets tricky because you can't simply use names like before (unless
    you're happy with the schedule that calendar names impose on you).

    This is what Rotation::Indexer::GFS is for: naming backups so that you
    don't need to explicitly prune old files because they get overwritten by
    newer ones. Except it supports arbitrarily nested rotations (so long as
    they're regular).

    For example, given a nested list of rotation periods like this:

        7, 4, 3

    Which means, keep a copy for:

    *   each of the last 7 days (i.e. 1 week),

    *   each week of the 4x7 days (i.e. 1 month), and

    *   each month of the last 3x4x7 days (i.e. 3 months).

    Rotation::Indexer::GFS can calculate an index for any day after (or
    before) the start date which maps the day into a "slot" in your rotation
    scheme. Anything in that slot can be overwritten. The scheme guarantees
    you have at least one copy from each hierarchy in the cycle.

        my $indexer = Rotation::Indexer::GFS->new(7, 4, 3);

        my $start_day = 0; # or whatever
        my @index = $indexer->index(today - $start_day); 

        my $backup_file = join("-", @index) . ".backup"; # 0-0-0.backup

    Using an index to implement your backup script makes your (log, backup
    or otherwise) script simpler and less error prone: if your backups are
    named like this example, you don't even need to query your backup store
    to see what to prune.

    Saying that, you might prefer to insert additional data into your file
    name, like this:

        my $backup_file = sprintf "%s-%s-%s.%d-%d-%d.backup", $year, $month, $day, @index;

    In this case you *do* need to prune the files, but having a unique index
    for rotations like this makes it relatively easy to find files to prune
    - simply delete any file with the same index as a newer one.

  Cycle example
    For example, given

        $ixr = Rotation::Indexer::GFS->new(3, 2, 2);

    And assuming you generate an index like this:

        $ID = join '-', $ixr->index($day)

    Then the sequence of indexes would be:

        # $day  $ID
            0   0-0-0
            1   1
            2   2
            3   0-1
            4   1
            5   2
            6   0-0-1
            7   1
            8   2
            9   0-1
           10   1
           11   2

    The slots are therefore:

          1
          2
          0-1
          0-0-0
          0-0-1

    i.e You will be keeping a maximum of 5 copies.

INTERFACE
  "$obj = $class->new(@cycles)"
    Creates a new instance which an be used to generate indexes for the
    given nested cycle periods.

  "@index = $obj->index($offset)"
    Generates the index number for a given offset from the start. $offset
    can be any positive or negative number/ Floating point values will be
    truncated using "int" to get an integer offset.

    It returns a list of one or more positive integers. The maximum length
    of this list is the number of cycles.

  "@cycles = $obj->cycles"
    Returns the cycle periods supplied to the constructor.

DIAGNOSTICS
    "there must be at least one cycle"
        You called the constructor with no arguments.

    "the following cycle sizes are not positive integers: ..."
        You called the constructor with invalid arguments.

    "you must supply an offset"
        You called the index function with no arguments.

    "too many arguments"
        You called the index function with more than one argument.

CONFIGURATION AND ENVIRONMENT
    Rotation::Indexer::GFS requires no configuration files or environment
    variables.

DEPENDENCIES
    None.

INCOMPATIBILITIES
    None reported.

BUGS AND LIMITATIONS
    No bugs have been reported.

    Please report any bugs or feature requests to
    "bug-Rotation-Indexer-GFS@rt.cpan.org", or through the web interface at
    <http://rt.cpan.org>.

AUTHOR
    Nick Stokoe "<wu-lee@cpan.org>"

LICENCE AND COPYRIGHT
    Copyright (c) 2012, Nick Stokoe "<wu-lee@cpan.org>". All rights
    reserved.

    This module is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself. See perlartistic.

DISCLAIMER OF WARRANTY
    BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
    FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
    OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
    PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
    EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
    ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
    YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
    NECESSARY SERVICING, REPAIR, OR CORRECTION.

    IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
    WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
    REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
    TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR
    CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
    SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
    RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
    FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
    SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
    DAMAGES.