queue_classic

v2.1.4

queue_classic provides a simple interface to a PostgreSQL-backed message queue. queue_classic specializes in concurrent locking and minimizing database load while providing a simple, intuitive developer experience. queue_classic assumes that you are already using PostgreSQL in your production environment and that adding another dependency (e.g. redis, beanstalkd, 0mq) is undesirable.

Features:

• Leverage of PostgreSQL's listen/notify & row locking.
• Support for multiple queues with heterogeneous workers.
• JSON data format.
• Forking workers.
• Fuzzy-FIFO support.

Contents:

• Documentation
• Usage
• Setup
• Configuration
• Hacking
• License

Usage

There are 2 ways to use queue_classic.

• Producing Jobs
• Working Jobs

Producing Jobs

# This method has no arguments.
QC.enqueue("Time.now")

Enqueue a job with priority 2 (default is 1)
QC.enqueue_with_priority(2, "Kernel.puts", "I go first")

# This method has 1 argument.
QC.enqueue("Kernel.puts", "hello world")

# This method has 2 arguments.
QC.enqueue("Kernel.printf", "hello %s", "world")

# This method has a hash argument.
QC.enqueue("Kernel.puts", {"hello" => "world"})

# This method has an array argument.
QC.enqueue("Kernel.puts", ["hello", "world"])

# This method uses a non-default queue.
p_queue = QC::Queue.new("priority_queue")
p_queue.enqueue("Kernel.puts", ["hello", "world"])

QueueClassic uses MultiJSON to encode the job's payload.

MultiJson.dump({"test" => "test"})

Working Jobs

There are two ways to work jobs. The first approach is to use the Rake task. The second approach is to use a custom executable.

Rake Task

Require queue_classic in your Rakefile.

require 'queue_classic'
require 'queue_classic/tasks'

Start the worker via the Rakefile.

$ bundle exec rake qc:work

Setup a worker to work a non-default queue.

$ QUEUE="priority_queue" bundle exec rake qc:work

Custom Worker

This example is probably not production ready; however, it serves as an example of how to leverage the code in the Worker class to fit your non-default requirements.

require 'timeout'
require 'queue_classic'

trap('INT') {exit}
trap('TERM') {worker.stop}

FailedQueue = QC::Queue.new("failed_jobs")

class MyWorker < QC::Worker
 	def handle_failure(job, e)
		FailedQueue.enqueue(job)
 	end
end

worker = MyWorker.new(max_attempts: 10, listening_worker: true)

loop do
	job = worker.lock_job
	Thread.new do
	  Timeout::timeout(5) { worker.process(job) }
	end
end

Setup

In addition to installing the rubygem, you will need to prepare your database. Database preparation includes creating a table and loading PL/pgSQL functions. You can issue the database preparation commands using PSQL(1) or use a database migration script.

Quick Start

$ gem install queue_classic
$ createdb queue_classic_test
$ export QC_DATABASE_URL="postgres://username:password@localhost/queue_classic_test"
$ ruby -r queue_classic -e "QC::Setup.create"
$ ruby -r queue_classic -e "QC.enqueue('Kernel.puts', 'hello world')"
$ ruby -r queue_classic -e "QC::Worker.new.work"

Ruby on Rails Setup

Declare dependencies in Gemfile.

source "http://rubygems.org"
gem "queue_classic", "2.1.4"

Require these files in your Rakefile so that you can run rake qc:work.

require "queue_classic"
require "queue_classic/tasks"

By default, queue_classic will use the QC_DATABASE_URL falling back on DATABASE_URL. The URL must be in the following format: postgres://username:password@localhost/database_name. If you use Heroku's PostgreSQL service, this will already be set. If you don't want to set this variable, you can set the connection in an initializer. QueueClassic will maintain its own connection to the database. This may double the number of connections to your database. Set QC::Conn.connection to share the connection between Rails & QueueClassic

require 'queue_classic'
QC::Conn.connection = ActiveRecord::Base.connection.raw_connection

Note on using ActiveRecord migrations: If you use the migration, and you wish to use commands that reset the database from the stored schema (e.g. rake db:reset), your application must be configured with config.active_record.schema_format = :sql in config/application.rb. If you don't do this, the PL/pgSQL function that queue_classic creates will be lost when you reset the database.

require 'queue_classic'

class AddQueueClassic < ActiveRecord::Migration
  def self.up
    QC::Setup.create
  end

  def self.down
    QC::Setup.drop
  end
end

Rake Task Setup

Alternatively, you can use the Rake task to prepare your database.

# Creating the table and functions
$ bundle exec rake qc:create

# Dropping the table and functions
$ bundle exec rake qc:drop

Configuration

All configuration takes place in the form of environment vars. See queue_classic.rb for a list of options.

Logging

By default queue_classic does not talk very much. If you find yourself in a situation where you need to know what's happening inside QC, you can enable the debug output by setting the DEBUG environment variable:

export DEBUG="true"

Hacking on queue_classic

Dependencies

• Ruby 1.9.2 (tests work in 1.8.7 but compatibility is not guaranteed or supported)
• Postgres ~> 9.0
• Rubygem: pg ~> 0.11.0
• For JRuby, see queue_classic_java

Running Tests

$ bundle
$ createdb queue_classic_test
$ export QC_DATABASE_URL="postgres://username:pass@localhost/queue_classic_test"
$ rake

License

Copyright (C) 2010 Ryan Smith

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.