Most naive worker autoscaler for Heroku.

autoscaler.rb

class Autoscaler
  def initialize(app_name:, token: ENV["AUTOSCALE_TOKEN"])
    @client = PlatformAPI.connect_oauth(token)
    @app_name = app_name
  end

  def run!
    puts "[Autoscaler] We currently have #{current_number_of_workers} workers and #{number_of_queued_jobs} jobs queued."

    if number_of_queued_jobs > 0 && current_number_of_workers < 1
      puts "[Autoscaler] We have queued jobs & no workers! Scaling up!"
      scale_workers(1)
    elsif number_of_queued_jobs.zero? && current_number_of_workers > 0
      puts "[Autoscaler] No queued jobs. Removing all workers"
      scale_workers(0)
    else
      puts "[Autoscaler] No action needed!"
    end
  end

  private

  def number_of_queued_jobs
    Sidekiq::Queue.all.sum(&:size)
  end

  def current_number_of_workers
    worker_process["quantity"]
  end

  def scale_workers(count)
    if current_number_of_workers == count
      puts "[Autoscaler] No action needed!"
    end

    @client.formation.batch_update(
      @app_name,
      updates: [{quantity: count, size: worker_process["size"], type: "worker"}]
    )

    puts "[Autoscaler] Done!"
  end

  def worker_process
    @worker_process ||= processes.find { |process| process["type"] == "worker" }
  end

  def processes
    @processes ||= @client.formation.list(@app_name)
  end
end

infrastructure.rake

# Add this to Heroku scheduler to run every 10 minutes: bundle exec rails infrastructure:autoscale_workers

namespace :infrastructure do
  desc "Autoscale sidekiq workers based on currently scheduled jobs"
  task autoscale_workers: [:environment] do
    Autoscaler.new(app_name: "your_app_name").run!
  end
end