Are you a Ruby developer? If you are, I’m pretty sure you have a very vague idea of what concurrency and thread safety are. No offense, but this is what I’ve figured out after dealing with Ruby code and speaking with Ruby programmers over the last half a year. I’ve been writing in Ruby pretty actively recently and I do like the language and the ecosystem around it. Zold, the experimental cryptocurrency we are creating, is written almost entirely in Ruby. What does that tell you? I like Ruby. But when it comes to concurrency, there are blank spots. Big time.

Look at this Ruby class:

require 'sinatra'
class Front < Sinatra::Base
  configure do
    IO.write('idx.txt', '0')
  end
  get '/' do
    idx = IO.read('idx.txt').to_i + 1
    IO.write('idx.txt', idx.to_s)
    idx.to_s
  end
end
Front.run!

It’s a simple web server. It does work—try to run it like this (you will need Ruby 2.3+ installed):

$ gem install sinatra
$ ruby server.rb

Then, open http://localhost:4567 and you will see the counter. Refresh the page and the counter will increment. Try again. It works. The counter is in the file idx.txt and it’s essentially a global variable, which we increment on every HTTP request.

Let’s create a unit test for it, to make sure it is automatically tested:

require 'minitest/autorun'
require 'net/http'
require 'uri'
class FrontTest < Minitest::Test
  def test_works
    front = Thread.start do
      Front.run!
    end
    sleep 1
    count = Net::HTTP.get(URI('http://localhost:4567/')).to_i
    assert_equal(1, count)
    Front.stop!
  end
end

OK, it’s not a unit test, but more like an integration test. First we start a web server in a background thread. Then we wait for a second, to give that thread enough time to bootstrap the server. I know, it’s a very ugly approach, but I don’t have anything better for this small example. Next we make an HTTP request and compare it with the expected number 1. Finally, we stop the web server.

So far so good. Now, the question is, what will happen when many requests are sent to the server? Will it still return the correct, consecutive numbers? Let’s try:

def test_works
  front = Thread.start do
    Front.run!
  end
  sleep 1
  numbers = []
  1000.times do
    numbers << Net::HTTP.get(URI('http://localhost:4567/')).to_i
  end
  assert_equal(1000, numbers.uniq.count)
  Front.stop!
end

Here we make a thousand requests and put all the returned numbers into an array. Then we uniq the array and count its elements. If there is a thousand of them—everything worked fine, we received a correct list of consecutive, unique numbers. I just tested it, and it works.

But we are making them one by one, that’s why our server doesn’t have any problems. We aren’t making them concurrently. They go strictly one after another. Let’s try to use a few additional threads to simulate parallel execution of HTTP requests:

require 'concurrent/set'
def test_works
  front = Thread.start do
    Front.run!
  end
  sleep 1
  numbers = Concurrent::Set.new
  threads = []
  5.times do
    threads << Thread.start do
      200.times do
        numbers << Net::HTTP.get(URI('http://localhost:4567/')).to_i
      end
    end
  end
  threads.each { |t| t.join }
  assert_equal(1000, numbers.to_a.count)
  Front.stop!
end

First of all, we keep the list of numbers in a Concurrent::Set, which is a thread-safe version of Ruby Set. Second, we start five background threads, each of which makes 200 HTTP requests. They all run in parallel and we wait for them to finish by calling join on each of them. Finally, we take the numbers out of the Set and validate the correctness of the list.

No surprise, it fails.

Of course, you know why. Because the implementation is not thread-safe. When one thread is reading the file, another one is writing it. Eventually, and very soon, they clash and the contents of the file is broken. The more threads we put into the test, the less accurate will be the result.

In order to make this type of testing easier I created threads, a simple Ruby gem. Here is how it works:

require 'threads'
def test_works
  front = Thread.start do
    Front.run!
  end
  sleep 1
  numbers = Concurrent::Set.new
  Threads.new(5).assert(1000) do
    numbers << Net::HTTP.get(URI('http://localhost:4567/')).to_i
  end
  assert_equal(1000, numbers.to_a.count)
  Front.stop!
end

That’s it. This single line with Threads.new() replaces all other lines, where we have to create threads, make sure they start at the same time, and then collect their results and make sure their stack traces are visible in the console if they crash (by default, the error log of a background thread is not visible).

Try this gem in your projects, it’s pretty well tested already and I use it in all my concurrency tests.