Custom Fetcher for Sidekiq

Feb 16 2014

Lately I have been working a bit on Sidekiq. IMHO, it should be one of the default gems for Rails projects, one of the reasons being it also includes Celluloid. When used, the combination of these two awesome libs in conjunction with JRuby can provide a big performance boost to any Rails project.

For example, doing things asynchronously with Sidekiq / Celluloid can be as simple as -

UserMailer.delay.welcome_email(@user.id)

In this blog we will look at how to write a custom fetcher for Sidekiq. I won't go into the basics of how Sidekiq works. You can see the wiki here for more info. What we want to tinker with is the fetcher, which is used to pull a job from the Redis queue so that it can be processed by the workers.

The default fetcher uses redis' BRPOP command which pops an element from the tail of the list. Elements are inserted at the head so this makes sense. We want to write a fetcher that pops elements from the head of the list or even pops an element randomly.

To test our setup we could do something like this -

class SimpleWorker
  include Sidekiq::Worker
  
  def perform(num)
    puts "==========>"
    puts num
    puts "==========>"
  end

end

Which is called by -

class AsyncHelper

  def self.submit_jobs
    args = 100.times.map do |num|
      [num]
    end
    Sidekiq::Client.push_bulk({'class' => SimpleWorker, 'args' => args})
  end

end

When the Sidekiq process is run the output is more or less sequential printing of 0 to 99 (not 100% sequential because of concurrency).

Now lets say we want to pop the jobs in reverse order i.e. from the head. For this we need to create a custom fetcher which uses BLPOP -

Let's create a fetcher using "blpop" -

require 'sidekiq'
require 'sidekiq/fetch'

module Sidekiq
  class CustomFetch < Sidekiq::BasicFetch
    
    def retrieve_work
      work = Sidekiq.redis { |conn| conn.blpop(*queues_cmd) }
      UnitOfWork.new(*work) if work
    end

  end
end

And we need to register this with Sidekiq (in config/initializers/sidekiq.rb) -

require "lib/sidekiq/custom_fetch"

Sidekiq.configure_server do |config|
  Sidekiq.options[:fetch] = Sidekiq::CustomFetch
end

That is it. We have written our fist custom Sidekiq fetcher. The output from our sample worker generates numbers from 99 to 0.

Now let's write a more complex fetcher, which fetches items randomly -

require 'sidekiq'
require 'sidekiq/fetch'

module Sidekiq
  class RandomFetch < Sidekiq::BasicFetch

    @@semaphore ||= Mutex.new
    
    def initialize(options)
      queues = options[:queues]
      raise ArgumentError, "Randomized fetch only works with exactly one queue." if queues.size != 1
      @queue =  "queue:#{queues.first}"
    end
    
    def retrieve_work
      @@semaphore.synchronize do
        work = random
        UnitOfWork.new(*work) if work
      end
    end

    def random
      work = nil
        Sidekiq.redis do |conn|
          length = conn.llen(@queue)
          if length < 2
            work = conn.brpop(@queue)
          else
            random_index = (0...length).to_a.sample
            item = conn.lindex(@queue, random_index)
            conn.lrem(@queue, 1, item)
            work = [@queue, item]
          end
        end
      work
    end

  end
end

As evident this will only work when we have one sidekiq process and one queue because of synchronization issues (threads!!). To solve this we can write a LUA script to do this random fetch since the LUA script execution by Redis is atomic (maybe I will write a gem for this some day). Anyways, hope this was useful, have a nice day.