---+ Package ==
*extends* <tt>CGI::Session::ErrorHandler</tt>


%TOC%
=head1 NAME

CGI::Session::Driver - CGI::Session driver specifications

=head1 SYNOPSIS

    require CGI::Session::Driver;
    @ISA = qw( CGI::Session::Driver );

=head1 DESCRIPTION

CGI::Session::Driver is a base class for all CGI::Session's native drivers. It also documents driver specifications for those willing to write drivers for different databases not currently supported by CGI::Session.

=head1 WHAT IS A DRIVER

Driver is a piece of code that helps CGI::Session library to talk to specific database engines, or storage mechanisms. To be more precise, driver is a F<.pm> file that inherits from CGI::Session::Driver and defines L<retrieve()|/"retrieve($self, $sid)">, L<store()|/"store($self, $sid, $datastr)"> and L<remove()|/"remove($self, $sid)"> methods.

=head2 BLUEPRINT

The best way of learning the specs is to look at a blueprint of a driver:

    package CGI::Session::Driver::your_driver_name;
    use strict;
    use base qw( CGI::Session::Driver CGI::Session::ErrorHandler );

    sub init {
        my ($self) = @_;
        # optional
    }

    sub DESTROY {
        my ($self) = @_;
        # optional
    }

    sub store {
        my ($self, $sid, $datastr) = @_;
        # Store $datastr, which is an already serialized string of data.
    }

    sub retrieve {
        my ($self, $sid) = @_;
        # Return $datastr, which was previously stored using above store() method.
        # Return $datastr if $sid was found. Return 0 or "" if $sid doesn't exist
        }

    sub remove {
        my ($self, $sid) = @_;
        # Remove storage associated with $sid. Return any true value indicating success,
        # or undef on failure.
    }

    sub traverse {
        my ($self, $coderef) = @_;
        # execute $coderef for each session id passing session id as the first and the only
        # argument
    }

    1;

All the attributes passed as the second argument to CGI::Session's new() or load() methods will automatically
be made driver's object attributes. For example, if session object was initialized as following:

    $s = CGI::Session->new("driver:your_driver_name", undef, {Directory=>'/tmp/sessions'});

You can access value of 'Directory' from within your driver like so:

    sub store {
        my ($self, $sid, $datastr) = @_;
        my $dir = $self->{Directory};   # <-- in this example will be '/tmp/sessions'
    }

Optionally, you can define C<init()> method within your driver to do driver specific global initialization. C<init()> method will be invoked only once during the lifecycle of your driver, which is the same as the lifecycle of a session object.

For examples of C<init()> look into the source code of native CGI::Session drivers.

=head1 METHODS

This section lists and describes all driver methods. All the driver methods will receive driver object ($self) as the first argument. Methods that pertain to an individual session (such as C<retrieve()>, C<store()> and C<remove()>) will also receive session id ($sid) as the second argument.

Following list describes every driver method, including its argument list and what step of session's life they will be invoked. Understanding this may help driver authors.

=over 4

=item retrieve($self, $sid)

Called whenever a specific session is requested either via C<< CGI::Session->new() >> or C<< CGI::Session->load() >> syntax. Method should try to retrieve data associated with C< $sid > and return it. In case no data could be retrieved for C< $sid > 0 (zero) or "" should be returned. undef must be returned only to signal error. Error message should be set via set_error(), which can be inherited from L<CGI::Session::ErrorHandler|CGI::Session::ErrorHandler>. 

Tip: set_error() always returns undef. Use it for your advantage.

=item store($self, $sid, $datastr)

Called whenever modified session data is to be stored back to disk. This happens whenever CGI::Session->flush() is called on modified session. Since CGI::Session->DESTROY() calls flush(), store() gets requested each time session object is to be terminated.

C< store() > is called both to store new sessions and to update already stored sessions. It's driver author's job to figure out which operation needs to be performed.

$datastr, which is passed as the third argument to represents B<already serialized> session data that needs to be saved.

store() can return any true value indicating success or undef on failure. Error message should be passed to set_error()

=item remove($self, $sid)

Called whenever session data is to be deleted, which is when CGI::Session->delete() is called. Should return any true value indicating success, undef on failure. Error message should be logged in set_error().

=item traverse($self, \&coderef)

Called only from within CGI::Session->find(). Job of traverse() is to call \&coderef for every single session stored in disk passing session's id as the first and only argument: C<< $coderef->( $sid ) >>

=item init($self)

Optional. Called whenever driver object is to be initialized, which happens only once during the lifecycle of CGI::Session object. Here you can do driver-wide initialization, such as to open connection to a database server.

=item DESTROY($self)

Optional. Perl automatically calls this method on objects just before they are to be terminated. This gives your driver chance to close any database connections or close any open file handles.

=back

=head2 NOTES

=over 4

=item *

All driver F<.pm> files must be lowercase!

=item *

DBI-related drivers are better off using L<CGI::Session::Driver::DBI|CGI::Session::Driver::DBI> as base, but don't have to.

=back

=head1 BACKWARDS COMPATIBILITY

Version 4.0 of CGI::Session's driver specification is B<NOT> backward
compatible with the previous specification. If you already have a driver
developed to work with the previous version you're highly encouraged to upgrade
your driver code to make it compatible with the current version. Fortunately,
current driver specs are a lot easier to adapt to.

For support information see L<CGI::Session|CGI::Session>

=head1 LICENSING

For support and licensing see L<CGI::Session|CGI::Session>.

