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Run job once at a specific time in future using queue_classic

is there a simple way to run a job once at a specific time in the future using queue_classic and clockwork, something like delayed_jobs

class.delay(run_at: 5.hours.from_now).method(param)

Simple queue system with scheduling support

I would need a queue system with scheduling support for a Ruby on Rails application. This is scheduling a job to be run at a certain time. Also it could be modified to be run sooner or later, or removed from the queue.

I'm thinking of using Sidekiq, but as I'm using a PostgreSQL database I was looking at queue_classic, but it seems not to have scheduling support.

Which one is a simple system that can handle timers?

ActionMailer send email in background, queue_classic

I am using queue_classic,
How can I send the following email using queue_classic?

Notifier.welcome(david).deliver # sends the email

UPDATE

I have added a new class method into User class

def.say_hello
  puts "hello!"
  Notifier.welcome(@david).deliver
end

QC.enqueue("puts", "hello world") works fine,
but QC.enqueue("User.say_hello") doesn't send email"
What could I do?

queue classic returns PG::UndefinedColumn: ERROR: column "pid" does not exist

When I do

bundle exec rake qc:work

to run queue_calssic, I get this error

rake aborted!
PG::UndefinedColumn: ERROR:  column "pid" does not exist
LINE 1: ..., locked_by = NULL WHERE locked_by NOT IN (SELECT pid FROM p...
                                                             ^
/home/andreas/.rvm/gems/ruby-2.1.1/gems/queue_classic-3.0.3/lib/queue_classic/conn_adapter.rb:18:in `exec'
/home/andreas/.rvm/gems/ruby-2.1.1/gems/queue_classic-3.0.3/lib/queue_classic/conn_adapter.rb:18:in `block in execute'
/home/andreas/.rvm/gems/ruby-2.1.1/gems/queue_classic-3.0.3/lib/queue_classic/conn_adapter.rb:14:in `synchronize'
/home/andreas/.rvm/gems/ruby-2.1.1/gems/queue_classic-3.0.3/lib/queue_classic/conn_adapter.rb:14:in `execute'
/home/andreas/.rvm/gems/ruby-2.1.1/gems/queue_classic-3.0.3/lib/queue_classic.rb:104:in `unlock_jobs_of_dead_workers'
/home/andreas/.rvm/gems/ruby-2.1.1/gems/queue_classic-3.0.3/lib/queue_classic/worker.rb:42:in `start'
/home/andreas/.rvm/gems/ruby-2.1.1/gems/queue_classic-3.0.3/lib/queue_classic/tasks.rb:27:in `block (2 levels) in <top (required)>'
/home/andreas/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `eval'
/home/andreas/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => qc:work

When I do psql --version i get psql (PostgreSQL) 9.3.5

Any ideas on what might be wrong?

How to monitor background jobs in production, queue_classic

I am using queue_classic for background jobs,
I need to monitor background jobs in production ie start, stop etc.
I found the similar question but it didn't help me
Also I found the god code:
but how would I stop, restart workers?

number_queues.times do |queue_num|
  God.watch do |w|
    w.name = "QC-#{queue_num}"
    w.group = "QC"
    w.interval = 5.minutes
    w.start = "bundle exec rake queue:work" # This is your rake task to start QC listening
    w.gid = 'nginx' 
    w.uid = 'nginx'
    w.dir = rails_root
    w.keepalive
    w.env = {"RAILS_ENV" => rails_env} 
    w.log = "#{log_dir}/qc.stdout.log"  # Or....    "#{log_dir}//qc-#{queue_num}.stdout.log"

    # determine the state on startup
    w.transition(:init, { true => :up, false => :start }) do |on|
      on.condition(:process_running) do |c|
        c.running = true
      end
    end
  end
end

UPDATE
This code seems doesn't work

namespace :queue_classic do
  desc "Start QC worker"
  task :start, roles: :web do
    run "cd #{release_path} && RAILS_ENV=production bundle exec rake qc:work"
  end
  after "deploy:restart", "queue_classic:restart"
end