NAME

       DBIx::Class::DeploymentHandler::HandlesVersioning - Interface for version methods

DESCRIPTION

       Typically a VersionHandler will take a "to_version" and yield an iterator of version sets.

       Typically a call to a VersionHandler's "next_version_set" with a "db_version" of 1 and a
       "to_version" of 5 will iterate over something like the following:

        [1, 2]
        [2, 3]
        [3, 4]
        [4, 5]
        undef

       or maybe just

        [1, 5]
        undef

       Really how the version sets are arranged is up to the VersionHandler being used.

       In some cases users will not want versions to have inherent "previous versions," which is
       why the version set is an "ArrayRef".  In those cases the user should opt to returning
       merely the version that the database is being upgraded to in each step.

       One idea that has been suggested to me has been to have a form of dependency management of
       the database "versions."  In this case the versions are actually more like features that
       may or may not be applied.  For example, one might start with version 1 and have a feature
       (version) "users".

       Each feature might require that the database be upgraded to another version first.  If one
       were to implement a system like this, here is how the VersionHandler's "next_version_set"
       might look.

        to_version = "users", db_version = 1
        [3]
        [5]
        ["users"]
        undef

       So what just happened there is that "users" depends on version 5, which depends on version
       3, which depends on version 1, which is already installed.  To be clear, the reason we use
       single versions instead of version pairs is because there is no inherent order for this
       type of database upgraded.

   Downgrades
       For the typical case downgrades should be easy for users to perform and understand.  That
       means that with the first two examples given above we can use the "previous_version_set"
       iterator to yield the following:

        db_version = 5, to_version=1
        [5, 4]
        [4, 3]
        [3, 2]
        [2, 1]
        undef

       or maybe just

        [5, 1]
        undef

       Note that we do not swap the version number order.  This allows us to remain consistent in
       our version set abstraction, since a version set really just describes a version change,
       and not necessarily a defined progression.

VERSION SET

       A version set could be defined as:

        subtype 'Version', as 'Str';
        subtype 'VersionSet', as 'ArrayRef[Str]';

       A version set should uniquely identify a migration.

KNOWN IMPLEMENTATIONS

       •   DBIx::Class::DeploymentHandler::VersionHandler::Monotonic

       •   DBIx::Class::DeploymentHandler::VersionHandler::DatabaseToSchemaVersions

       •   DBIx::Class::DeploymentHandler::VersionHandler::ExplicitVersions

METHODS

   next_version_set
        print 'versions to install: ';
        while (my $vs = $dh->next_version_set) {
          print join q(, ), @{$vs}
        }
        print qq(\n);

       Return a version set describing each version that needs to be installed to upgrade to
       "$dh->to_version".

   previous_version_set
        print 'versions to uninstall: ';
        while (my $vs = $dh->previous_version_set) {
          print join q(, ), @{$vs}
        }
        print qq(\n);

       Return a version set describing each version that needs to be "installed" to downgrade to
       "$dh->to_version".

AUTHOR

       Arthur Axel "fREW" Schmidt <frioux+cpan@gmail.com>

COPYRIGHT AND LICENSE

       This software is copyright (c) 2018 by Arthur Axel "fREW" Schmidt.

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

perl v5.26.1                               DBIx::Class::DeploymentHandler::HandlesVersioning(3pm)