DripDrop was a fun experiment, and I may start using it again in the future, but it is not actively maintained. There are no known bugs however, and its reasonably well tested (both in a bunch of sample apps I built and unit tests), so it should be OK to use, but be aware that if you find any issues with it, or want features added to it, it'll be up to you to hack on it!

1. This does NOT work with 1.8.7. 1.9.2/RBX/jRuby only.
2. Build zeromq2 from git master if you haven't already.
3. Install with 'gem install dripdrop'.
4. If on jRuby, http servers will be unavailable unless you have a jruby with cext support, and manually gem install eventmachine_httpserver
5. If on 1.9.x gem install ffi

DripDrop is a library aiming to help you write better message passing apps. It's a rich toolchest, currently based on EventMachine, which provides async IO.

DripDrop helps in these ways:

1. Normalized Communication Interfaces. All protocols, regardless of their using HTTP, ZeroMQ, or WebSockets, have a unified API, with minor differences only to accomodate the reality of the underlying protocol.
2. A set of tools to break your app into composable parts--we call them nodelets in dripdrop--ideally communicating with each other with the aforementioned interfaces..
3. A simple, yet powerful messaging class, that lets you control the structure, formatting, and serialization of messages sent between nodelets.
4. Tools to break your nodelets off when it comes time to deploy, letting you scale your app by delegating roles and scaling out resources

Let's start by looking at the normalized communication interface in a simple app.

class MyApp < DripDrop::Node
  def action #Special method that gets executed on Node#start
    # Define some sockets, here we create an HTTP server, and
    # a client to it. :my_hts and :my_htc are custom names
    # that will be available after definition
    route :my_server, :http_server, 'http://127.0.0.1:2201'
    route :my_client, :http_client, 'http://127.0.0.1:2201'
    
    # Our http server is a simple time server
    my_server.on_receive do |message,response|
      response.send_message(:name => 'time', :body => {'time' => Time.now.to_s})
    end
    
    # Here, we setup a timer, and periodically poll the http server
    EM::PeriodicTimer.new(1) do
      # Messages must have a :name. They can optionally have a :body.
      # Additionally, they can set custom :head properties.
      my_client.send_message(:name => 'time_request') do |response_message|
        puts "The time is: #{response_message.body['time']}"
      end
    end
  end
end
 
#Start the app and block
MyApp.new.start!

What we've done here is use HTTP as a simple messaging protocol. Yes, we've thrown out a good chunk of what HTTP does, but consider this, that exact same code would work if we replaced the top two lines with:

    route :my_server, :zmq_xrep, 'http://127.0.0.1:2201', :bind
    route :my_client, :zmq_xreq, 'http://127.0.0.1:2201', :connect

That replaces the HTTP server and client with ultra-high performance zeromq sockets. Now, protocols have varying strengths and weaknesses, and ZeroMQ is not HTTP necessarily, for instance, given a :zmq_pub socket, you can only send_messages, but there is no response message, because :zmq_pub is the publishing end of a request/reply pattern. The messaging API attempts to reduce all methods on sockets to the following set:

• on_receive (sometimes takes a block with |message,response| if it can send a response)
• send_message
• on_open (Websockets only)
• on_close (Websockets only)

The tools mentioned above are useful, but if you try and build a larger app you'll quickly find them lacking. The callbacks get tricky, and mixing your logic up in a single #action method becomes messy. That's why we have nodelets in DripDrop. Here's a trivial example.

class MyApp < DripDrop::Node
  def initialize(mode=:all)
    super()
    @mode = mode
  end
  
  def action
    # This will instantiate a new StatsCollector object, and define the
    # stats_raw and stats_filtered methods inside it.
    nodelet :stats_producer, StatsProducer do |n|
      n.route :stats_output, :zmq_push, 'tcp://127.0.0.1:2301', :bind
    end

    nodelet :stats_collector, StatsCollector do |n|
      n.route :stats_raw, :zmq_pull, 'tcp://127.0.0.1:2301', :connect
      n.route :stats_filtered, :zmq_push, 'tcp://127.0.0.1:2302', :bind
    end

    nodelet :stats_processor, StatsProcessor do |n|
      n.route :stats_ingress, :zmq_pull, 'tcp://127.0.0.1:2302', :connect
    end

    # The nodelets method gives you access to all defined nodelets as a hash
    # We created a #run method on each nodelet we call here.
    nodelets.each_value { |n| n.run }
  end
end

# You must subclass Nodelet
# The method #run here is merely a convention
class StatsProducer < DripDrop::Node::Nodelet
  def run
    EM::PeriodicTimer.new(1) do
      stats_output.send_message :name => 'stat', :body => Time.now.to_s
    end
  end
end

class StatsCollector < DripDrop::Node::Nodelet
  def run
    stats_raw.on_receive do |raw_stat_msg|
      stats_filtered.send_message(raw_stat_msg)
    end
  end
end

class StatsProcessor < DripDrop::Node::Nodelet
  # Initialize shouldn't be subclassed on a Nodelet, this gets called
  # After the nodelet is instantiated
  def configure
    @name_counts = Hash.new(0)
  end

  def run
    stats_ingress.on_receive do |message|
      @name_counts[message.name] += 1
      puts @name_counts.inspect
      puts "received message.body: " + message.body
    end
  end
end

MyApp.new.start!

DripDrop::Message is the parent class of all messages in dripdrop, it's a flexible and freeform way to send data. In more complex apps you'll want to both define custom behaviour on messages, and restrict the data they carry. This is possible by subclassing DripDrop::Message. Before we look at that though, lets see what makes a DripDrop::Message.

The simplest DripDrop::Message you could create would look something like this if dumped into JSON:

{name: 'msgname', head: {}, body: null}

In other words, a dripdrop message must provide a name, it must also be able to store arbitrary, nested, keys and values in its head, and it may use the body for any data it wishes.

If you'd like to create your own Message format, simply Subclass DripDrop::Message. If you want to restrict your handlers to using a specific message type, it's easily done by passing in the :message_class option. For instance

class MyMessageClass < DripDrop::Message
  # Custom code
end
class MyApp < DripDrop::Node
  def action
    route :myhandler, :zmq_publish,   'tcp://127.0.0.1:2200', :bind,    :message_class => MyMessageClass 
    route :myhandler, :zmq_subscribe, 'tcp://127.0.0.1:2200', :connect, :message_class => MyMessageClass
  end
end

One of the core ideas behind dripdrop, is that if your application is composed of a bunch of separate parts, that in production deployment, will run on separate physical servers, it should still be possible for you to develop and test with ease. If you structure your app into separate nodelets, and only communicate between them via message passing, you can accomplish this easily.

While you will have to write your own executable wrappers suitable for your own deployment, one convenenience feature built in is the notion of a +run_list+. By setting the #run_list you can restrict which nodelets actually get initialized. For example:

class MyApp < DripDrop::Node
  nodelet :service_one, ServiceOneClass do
    #nodelet setup
  end
  nodelet :service_two, ServiceTwoClass do
    #nodelet setup
  end
end

# Only starts :service_two, the setup for :service_one
# is skipped as well
MyApp.new(:run_list => [:service_two]).start!

#RDocs

RDocs can be found here. Most of the interesting stuff is in the Node and Message classes.

#Contributors

• Andrew Cholakian: andrewvc
• John W Higgins: wishdev
• Nick Recobra: oruen