Parallel progress output from concurrent threads

Question

Having this code:

workers = (1..3).map do |n|
    Thread.new do
        puts
        print "Worker_#{n}..."
        sleep rand
        print "done (#{n})"
    end
end.each &:join
puts
puts '- ready -'

how can we get a correct output like this (lines updated dynamically):

Worker_1...done (1)
Worker_2...done (2)
Worker_3...done (3)
- ready -

and not like this:

Worker_1...
Worker_2...
Worker_3...done (3)done (1)done (2)
- ready -

Advanced option: if we redirect stdout to file — it should look fine too.

Answer 1

I'd introduce some class to manage output. It will also move the carriage 3 lines above to update the output every time (at the print method).

class StatusPrint
  def initialize
    @hash = {}
  end

  def declare_worker(name)
    @hash[name] = "Worker_#{name}"
  end

  def complete_worker(name)
    @hash[name] += "...done (#{name})"
  end

  def print(move_carriage: true)
    puts "\033[#{@hash.size + 1}A" if move_carriage
    puts @hash.values.join("\n")
  end

  def done!
    @done = true
  end

  def done?
    @done
  end
end

So your code will look like this

status_print = StatusPrint.new

workers = (1..3).map do |n|
  Thread.new do
    status_print.declare_worker(n)
    sleep rand
    status_print.complete_worker(n)
  end
end

print_loop = Thread.new do
  status_print.print(move_carriage: false)

  until status_print.done?
    sleep 0.1
    status_print.print
  end
end

workers.each &:join
status_print.done!
print_loop.join

puts '- ready -'

Also it won't work if you want to pipe your output so you'll have to add some command line option to disable updating the output and print this static text:

Worker_1...done (1)
Worker_2...done (2)
Worker_3...done (3)
- ready -

Also see https://blog.stevenocchipinti.com/2013/06/removing-previously-printed-lines.html/ for more about updating the output.