BBC (British Broadcasting Corporation)

NAME

Sub::Slice::Backend - API for Sub::Slice backends

SYNOPSIS

        use Sub::Slice;
        use Sub::Slice::Backend::MyBackend;
        our %Options = ( ...options for your backend... );
        
        sub create_token {
                my $job = new Sub::Slice (
                        backend => 'MyBackend',
                        storage_options => \%Options,
                );
                return $job->token;
        }
        
        sub do_work {
                my $job = new Sub::Slice (
                        backend => 'MyBackend',
                        storage_options => \%Options,
                        token => shift()
                );
                
                ...
        }

DESCRIPTION

This is the API which storage backends for Sub::Slice must implement. Sub::Slice comes with a default Filesystem backend using Storable to serialise data. If you'd rather store the Sub::Slice data somewhere else or in another format, you can create an adaptor class satisfying this API to plug Sub::Slice into your storage system.

Module naming conventions

If a string matching \w+ is passed to Sub::Slice for the backend, the module is assumed to be in the Sub::Slice::Backend namespace (such as the example in the SYNOPSIS). If you have a module in a namespace other than the top-level one, e.g. MySystem::StorageModule, that exposes this API, Sub::Slice will happily use this as a backend module without you needing to create a wrapper in the Sub::Slice::Backend namespace.

        my $job = new Sub::Slice (
                backend => 'MySystem::StorageModule',
                storage_options => \%Options,
        );

METHODS

$backend = new Sub::Slice::Backend::YOURCLASS(\%options)

Contructor takes a hash of options

$id = $backend->new_id()

Generates a new ID.

$job = $backend->load_job($id)

Loads an existing job from persistent storage

$backend->save_job($job)

Saves a job to persistent storage

$backend->delete_job($id)

Removes a job from persistent storage

$backend->store($job, $key, $data)

Store data for job

$data = $backend->fetch($job, $key)

Fetch data for job

$backend->store_blob($job, $key, $data)

Store BLOB data for job

$data = $backend->fetch_blob($job, $key)

Fetch BLOB data for job

$count = $backend->cleanup($age)

Deletes any leftover data which has been unmodified for at least $age days (default 1 day). $count will equal the number of files or items deleted, or may be undef if cleanup wasn't able to scan for leftover data (eg. because a directory didn't exist). Should die if it doesn't manage to clean up (eg. because of a permissions problem).

Normally, tasks should clean up after themselves, so unless you are experiencing errors, cleanup will have nothing to do.

Strategies for issuing IDs

The storage API requires that you can issue an ID for a new record before storing data into it. In an Oracle database you normally use a sequence to generate a unique ID in advance of storing a record so this isn't a problem. In MySQL, if you are planning on using an AUTO_INCREMENT column to generate ids, you're probably best off inserting an empty row. Alternatively in most database engines you can simulate an Oracle sequence using a counter table (see the example in the MySQL manual for a neat example using LAST_INSERT_ID). If your storage engine doesn't have a unique ID mechanism, a couple of strategies are open to you: you can use a perl GUID generator (such as Data::UUID), or you can roll your own sequence generator as in the example below.

Worked Example

Here's a sample implementation. It's not a particularly good one in that it doesn't handle concurrent processes writing to the DBM file, but it does illustrate the steps required to write a backend.

        package Sub::Slice::Backend::MLDBM;
        
        use GDBM_File;
        use Storable();
        use MLDBM qw(GDBM_File Storable);

The constructor takes a hashref of storage options, which you're free to define the meaning of. These are what the caller will pass in as the storage_options to Sub::Slice.

        sub new {
                my ($class, $options) = @_;
                my $dbm = $options->{DBM} or die("You must supply a DBM");
                my %db;
                tie(%db, 'MLDBM', $dbm, GDBM_WRCREAT, 0666) or die("unable to tie to $dbm");
                my $self = {$db => \%db};
                return bless($self, $class);
        }

        sub DESTROY {
                my $self = shift;
                untie %{$self->{db}} if tied %{$self->{db}};
        }

The new_id routine should create a unique ID which can be used to store the job against. Here we use a sentinel key to store a sequence counter against:

        sub new_id {
                my ($self) = @_;
                my $id = ++$self->{db}->{__COUNT__};
                return $id;
        }

The delete_job routine takes an ID rather than a job:

        sub delete_job {
                my ($self, $id) = @_;
                delete $self->{db}->{$id};
        }
        

This allows it to be used in cleaner processes without loading the job first. The load_job routine should return the job given the ID:

        sub load_job {
                my ($self, $id) = @_;
                return $self->{db}->{$id};
        }

The save_job should persist the job (against the ID):

        sub save_job {
                my ($self, $job) = @_;
                $self->{db}->{$job->{id}} = $job;
        }

Store and fetch methods are passed the job object so they can hang data on it, if the backend desires:

        sub store {
                my ($self, $job, $key, $value) = @_;
                $job->{data}{$key} = $value;
        }
        
        sub fetch {
                my ($self, $job, $key) = @_;
                return $job->{data}{$key};
        }

If there is no optimisation you want to perform for BLOB data, the following implementation is legitimate:

        sub store_blob {
                shift()->store(@_);
        }
        
        sub fetch_blob {
                shift()->fetch(@_);
        }

VERSION

$Revision: 1.8 $ on $Date: 2004/12/17 14:41:21 $ by $Author: johna $

AUTHOR

John Alden <cpan _at_ bbc _dot_ co _dot_ uk>

COPYRIGHT

(c) BBC 2004. This program is free software; you can redistribute it and/or modify it under the GNU GPL.

See the file COPYING in this distribution, or http://www.gnu.org/licenses/gpl.txt