I’ve had a bit of opportunity to play with Ruby EventMachine over the last few weeks at work. While the RDoc is very helpful it’s still a bit of work to figure things out. To that end, I figured I might try to write some stuff down.

The first question is typically, what is this EventMachine thing and how come when I use it the rest of my application doesn’t execute. Well, simply put, EM (EventMachine) is an event loop. Once you kick off EventMachine it takes over the execution of your main Ruby thread and does its own thing. EM will listen on sockets, on file descriptors, set timers and various other procedures. When activity happens on the things EM is listening too it will trigger event callbacks to your code. You handle the event then return control to EventMachine.

It’s also possible to have EventMachine execute a block of long running code on another thread so you don’t hold up the entire run loop with your work. When the process is done a callback can be executed on the main thread to handle any responses to the original connection.

Hopefully this will all become more clear as we look at some code.

Timer Example

#!/usr/bin/ruby

require 'rubygems'
require 'eventmachine'

EventMachine.run do
    EM.add_periodic_timer(1) { puts "Tick ..." }

    EM.add_timer(3) do
        puts "I waited 3 seconds"
        EM.stop_event_loop
    end
end

puts "All done."

Nothing too complicated here. Let’s, as they say, start at the start. I’ve installed Eventmachine through rubygems (gem install eventmachine) so I need the proper requires to get everything available. You’ll notice I’m using both EventMachine and EM, they’re interchangeable. I prefer EM as it’s shorter but sometimes use EventMachine anyway. The call to EventMachine#run is what kicks off the main event loop. Before the event loop is started the block provided to EM#run will be executed. This is where you can setup your various servers, timers and other handlers as needed.

In this example I creating two timers. Using EM#add_timer I’ve created a timer that will execute once in at least three seconds. The EM#add_periodic_timer creates a timer that will execute every one second. In both cases when the event is fired it will execute the block attached to the timer creation call.

For the timer which fires each second we do a simple puts call. In the single shot timer we have a call to EM#stop_event_loop. EM#stop_event_loop will cause the EventMachine event loop to stop executing. Once the event loop is terminated the control flow of the application will pickup again after the EM#run block and execute the final puts statement.

So, as you can see, any code we have after the call to EM#run will not get executed until the event loop is shutdown. Typically when your application is terminating.

Server Example

#!/usr/bin/ruby

require 'rubygems'
require 'eventmachine'

module Server
    def receive_data(data)
        puts data
        send_data("helo\n")
    end
end

EM.run { EM.start_server 'localhost', 8080, Server }

Here we’re calling EM#start_server in our run block. EM#start_server will tell EventMachine to start listening on IP address defined by localhost, port 8080 and will will create an anonymous class and include the module Server for each connection. You could also define Server as a class which inherits from EventMachine::Connection.

It’s worth pointing out, the instantiated class will only exist for a single connection. You won’t be able to store information between requests in the object.

What you can do is pass data into the object when it’s created.

Server Example 2

#!/usr/bin/ruby

require 'rubygems'
require 'eventmachine'

class Server < EventMachine::Connection
    attr_accessor :options, :status

    def receive_data(data)
        puts "#{@status} -- #{data}"
        send_data("helo\n")
    end
end

EM.run do
    EM.start_server 'localhost', 8080, Server do |conn|
        conn.options = {:my => 'options'}
        conn.status = :OK
    end
end

The EM#start_server method will yield to a block passing in the object that will handle the connection. You can then assign any values into that object that you wish.

That’s it for now, my very quick introduction to EventMachine. Hopefully it was useful.