_how_to_build_workflow_engine_part_2.md

Workflow Engine をつくろう！ Part 2

記事は Qiita に書いています。

http://qiita.com/hakobera/items/478f55c9ddf682e5caca

dag.rb

require 'tsort'

class DAG
  include TSort

  def initialize
    @tasks = {}
  end

  def tsort_each_node(&block)
    @tasks.each_key(&block)
  end

  def tsort_each_child(node, &block)
    @tasks.fetch(node).each(&block)
  end

  def add_task(task)
    _add_task(nil, task)
  end

  private

  def _add_task(parent, task)
    unless @tasks[task]    
      @tasks[task] = task.requires
      task.requires.each do |r|
        _add_task(task, r)
      end
    end
    task
  end
end

example.rb

require_relative 'workflow'
require_relative 'task'
require_relative 'file_target'

class EchoTask < Task
  def output
    FileTarget.new("/tmp/#{self.class.name}.txt")
  end

  def run
    puts "#{self.class.name}#run"
    File.write(output.path, "done")
  end
end

class TaskA < EchoTask
  def requires
    [ TaskB.new, TaskC.new ]
  end
end

class TaskB < EchoTask
  def requires
    [ TaskD.new ]
  end
end

class TaskC < EchoTask
  def requires
    [ TaskD.new ]
  end
end

class TaskD < EchoTask
end

Workflow.new.run(TaskA.new)

file_target.rb

require_relative 'target'

class FileTarget < Target
  attr_reader :path

  def initialize(path)
    @path = path
  end

  def exists?
    File.exist?(path)
  end
end

target.rb

class Target
  def exists?
    raise NotImplementedError, "You must implement #{self.class}##{__method__}"
  end
end

task.rb

class Task
  def output
    raise NotImplementedError, "You must implement #{self.class}##{__method__}"
  end
  
  def run
    raise NotImplementedError, "You must implement #{self.class}##{__method__}"
  end

  def requires
    [] # If you need to define task dependencies, override in subclass
  end  

  def eql?(other)
    self.hash == other.hash
  end

  def hash
    self.class.name.hash
  end
end

workflow.rb

require_relative 'dag'

class Workflow
  def run(task)
    dag = DAG.new
    dag.add_task(task)
    dag.tsort.each do |t|
      t.run unless t.output.exists?
    end
  end
end