Prevent Duplicates with Delayed Jobs

add_signature_fields_to_delayed_jobs.rb

class AddFieldsToDelayedJobs < ActiveRecord::Migration
  def change
    add_column :delayed_jobs, :signature, :string, index: true
    add_column :delayed_jobs, :args, :text
  end
end

delayed_duplicate_prevention_plugin.rb

# /lib/delayed_duplicate_prevention_plugin.rb
require 'delayed_job'
class DelayedDuplicatePreventionPlugin < Delayed::Plugin
 
  module SignatureConcern
    extend ActiveSupport::Concern

    included do
      before_validation :add_signature
      validate :manage_duplicate
    end

    attr_writer :strategy
    @@strategies = [:delete_previous_duplicate, :prevent_duplicate, :allow_duplicate]
    
    private
    def add_signature
      self.signature = generate_signature
      self.args = self.payload_object.args.to_yaml
    end

    def generate_signature
      pobj = payload_object
      if pobj.object.respond_to?(:id) and pobj.object.id.present?
        sig = "#{pobj.object.class}"
        sig += ":#{pobj.object.id}" 
      else
        sig = "#{pobj.object}"
      end
      
      sig += "##{pobj.method_name}"
      return sig
    end    
   
    def manage_duplicate
      return if @strategy == :allow_duplicate
      
      @strategy ||= :prevent_duplicate # default strategy
      raise "Only the following strategies are permitted: #{@@strategies}" unless @@strategies.include?(@strategy)
     
      @checker = DuplicateChecker.new(self)
      return unless @checker.duplicate?

      send(@strategy)
    end
  
    def prevent_duplicate
      if @checker.duplicate?
        Rails.logger.warn "Found duplicate job(#{self.signature}), ignoring..."
        errors.add(:base, "This is a duplicate")
      end
    end
    
    def allow_duplicate 
    end

    def delete_previous_duplicate
      if @checker.duplicate?
        @checker.duplicates.each { |job| Delayed::Job.destroy(job.id) }
        logger.warn "__________: #{self.class} #{__method__} DELETED #{@checker.duplicates.count} duplicate jobs"        
      end
    end   
 
  end

  class DuplicateChecker
    attr_reader :job
    attr_accessor :duplicates

    def self.duplicate?(job)
      new(job).duplicate?
    end

    def initialize(job)
      @job = job
      @duplicates = []
      enumerate_duplicates
    end
    
    def enumerate_duplicates
      possible_dupes = Delayed::Job.where(signature: job.signature)
      possible_dupes = possible_dupes.where.not(id: job.id) if job.id.present?
      result = possible_dupes.each do |possible_dupe| 
        duplicates << possible_dupe if possible_dupe.args == job.args
      end
    end
    
    def duplicate?
      !duplicates.empty?
    end

    private

    def args_match?(job1, job2)
      # TODO: make this logic robust
      normalize_args(job1.args) == normalize_args(job2.args)
    end

    def normalize_args(args)
      args.kind_of?(String) ? YAML.load(args) : args
    end
  end
end

delayed_job.rb

# config/initializers/delayed_job.rb
require 'delayed_duplicate_prevention_plugin'

Delayed::Backend::ActiveRecord::Job.send(:include, DelayedDuplicatePreventionPlugin::SignatureConcern)
Delayed::Worker.plugins << DelayedDuplicatePreventionPlugin

Here's my try:

https://gist.github.com/vfonic/fe509d53929dfc70d0463c00feec2606

I've removed the strategies, as I only need one strategy for now.
I don't generate MD5 checksum, as I don't see any added value. There's also risk of two MD5 checksums ending up being the same even if the original signature is not the same. It's rare, but why add this layer of complexity.
I've fixed the payload_object.object by extending the DelayedJob ActiveJob adapter class.

@vfonic Agreed. Thanks for the update. Updated my gist based on yours as I need the same functionality with locked / failed jobs 👍 Simplified it some more using exists? instead of blank? for checking existing jobs.

Thought about using jsonb (Postgresql) for the handler column so we could dump the signature column entirely. I don't have the time to investigate how much delayed job is dependent on the YAML format and what would need to change to make that happen. There is an open issue since 2016 (collectiveidea/delayed_job#957) and maybe someone could make that happen :)

Taking some concept from here I implemented a unique job using an active job callback https://gist.github.com/channainfo/b920eeda6b20576310c1fae9780dbedc