Getting started with Distributed Ruby (DRb)

14 Dec 2015

The Ruby stdlib contains a little known library called dRuby, which allows multiple Ruby processes to talk to each other over the network. It lets you call methods on objects from another Ruby process as if it is just another object initialized in the same program. This is called Remote Method Invocation (RMI) (or Remote Procedure Call, RPC, in non-OOP languages).

Hello, dRuby!

Let’s start off with a very simple dRuby program. We will write server.rb which exposes a very simple object over the network. The object responds to a #greet method and returns the string “Hello, world!”.

# server.rb
require 'drb/drb'

class MyApp
  def greet
    'Hello, world!'
  end
end

object = MyApp.new

DRb.start_service('druby://localhost:9999', object)
DRb.thread.join

Let’s focus on the last two lines of the code above. DRb.start_service starts a local dRuby server on port 9999. The second argument to the method is the object that we want to be able to call from other processes.

The final line DRb.thread.join is so that the program doesn’t exit immediately. This keeps the program running until the DRb thread stops. Start this program in one tab in the terminal.

Next, we will write another program that talks to the server. Save the following code and run it.

# client.rb
require 'drb/drb'
DRb.start_service
remote_object = DRbObject.new_with_uri('druby://localhost:9999')

remote_object.greet   #=> 'Hello, world!'

Here, we create an instance of DRbObject which proxies all method calls to the object we shared in server.rb. Now we can call greet method on remote_object and the method call is sent across the network to the process running server.rb.

Under the hood, DRbObject uses method_missing to delegate all method calls over the network.

Accessing a queue via DRb

Let’s try something more interesting now. We will set up a queue service that will be accessed by two other processes. One of the processes will push data to the queue, while the other will pull the data and process it.

# queue.rb
require 'drb/drb'

queue = Queue.new

DRb.start_service('druby://localhost:9999', queue)
DRb.thread.join

Here we’re using Ruby’s built in Queue data structure, which allows thread-safe push and pop operations. Run this program in one tab in your terminal.

Next we will define a producer, which pushes data to the queue. We could run this in the terminal using ruby producer.rb but let’s run this interactively in an irb session. Start an irb session and copy the lines below.

# producer.rb - or type this in an irb session
require 'drb/drb'

DRb.start_service
queue = DRbObject.new_with_uri('druby://localhost:9999')

#> queue.push(42)
#> queue.push(99)

Finally we need a consumer for our queue that takes the items from the queue and processes them. We’ll write that in consumer.rb and run it in another tab.

# consumer.rb
require 'drb/drb'

DRb.start_service
queue = DRbObject.new_with_uri('druby://localhost:9999')

loop do
  data = queue.pop

  # Process the data
  puts "Processing #{data}"
end

If you’re pushing data to the queue through irb, you will see that the consumer waits for new data to be added when the queue becomes empty.

This is because we’re using Queue to store our data rather than Array. When the queue is empty, it suspends the calling thread (ie. consumer) until new data is available. If we had used Array instead of Queue, it would continuously return nil every time we call queue.pop.

Wrapping up

I’m not sure how widely dRuby is used, but it seems like a nice option for smaller projects. For instance, you could set up a rudimentary key-value store for a toy project with a few lines of Ruby code instead of running Redis.

One thing to keep in mind is that you need to make your code thread-safe if you’re using DRb, when many processes are working with the same object.

Getting started with Distributed Ruby (DRb)

Hello, dRuby!

Accessing a queue via DRb

Wrapping up

Further reading