Robert Grimes
and 1 contributors

NAME

Pg::Queue - Simple SKIP LOCKED based Queue for Postgresql

VERSION

This document describes Pg::Queue version 1.0

SYNOPSIS

        use Pg::Queue;
        use DBI;

        my $dbh = DBI->connect("dbi:Pg:dbname=database", "", "", {AutoCommit=>1, RaiseError=>1})
                or die "DBI FAILURE $DBI::errstr";
        
        my $queue = Pg::Queue->new( dbh => $dbh, queuename => "myqueue" );

        $queue->create_queue_table;

        for( 0 .. 10 ) {
                $queue->add_work_item( "work item $_" );
        }

        while( 1 ) { # We don't know how many queue items are remaining
                my $processed = $queue->pull_work_item(sub{
                        my($id,$item) =  @_;
                        print "$item has id $id\n";
                        return 1;
                });

                last unless $processed; #Exit when there's no more queue items
                                        # Or just sleep and try again
        }

  

DESCRIPTION

This module provides an OO interface to a Postgresql table which implements a simple yet highly concurrent queue via the new 9.5 feature "SKIP LOCKED". For further details on how SKIP LOCKED works, see https://www.postgresql.org/docs/current/static/sql-select.html#SQL-FOR-UPDATE-SHARE. For information using it to implement a queue, see https://blog.2ndquadrant.com/what-is-select-skip-locked-for-in-postgresql-9-5/.

INTERFACE

new( dbh => $dbh, queuename => NAME );
$dbh

A connected DBI database handle to a postgresql database.

queuename

The name of the table in the database to use as a queue.

This can be the name of an existing table or you can call c<create_queue_table> to create it.

create_queue_table

Attempts to create a table for the queue using the c<queuename> set during construction. WILL DROP TABLE IF IT EXISTS

The queue table created will have the following columns:

id (serial)

Auto-incrementing integer id

available (boolean)

Used to tell if the queue item is available to be fetched by a worker. This will only actually be set to false if the callback returns true. Must default to false. Note that the locking mechanism employed prevents concurrent workers from fetching the same item while it is being processed.

processed (timestamp)

Automatically set to NOW() when the callback returns true and available is set to false.

item (text)

An opaque field containing data to be passed to the worker's callback.

add_work_item($data)

Takes a single scalar to be used as the value of the item column in the queue table and inserts a new row into the configured queue table.

pull_work_item($callback)

Takes a callback that will be called with two items, the id of the row and the contents of the item column. The callback must return true to signal that it has successfully processed the queue item and to prevent other workers from attempting to process it.

The callback operates inside the transaction used to fetch the queue item so any database changes made will be lost if you return false or throw an exception unless you manually commit.

count_total

Returns the count(*) from the queue table

count_available

Returns the count(*) WHERE available from the queue table

The created table will also have an index on id WHERE available; which utilizes PostgreSQL "partial" indexes to speed up finding an available queue item.

DEPENDENCIES

Requires PostgreSQL 9.5 for the SKIP LOCKED feature.

BUGS AND LIMITATIONS

No bugs have been reported.

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

AUTHOR

Robert Grimes rmzgrimes@gmail.com

LICENCE AND COPYRIGHT

Copyright (c) 2017, Robert Grimes rmzgrimes@gmail.com. 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.