EventMachine: a TCP server module for Ruby

I just discovered EventMachine. EM is, at its least, an event-driven library around which you can build a custom TCP server. The nitty-gritty of listening, sending and receiving, and multiplexing connections is all done for you.

That's selling it short, though, because EventMachine can do a lot more. In its own words, EventMachine "provides event-driven I/O using the Reactor pattern." It can be used for other client/server models like serial ports or even interactive keyboard I/O. But it really stands out for abstracting away the low-level hassles of writing a TCP daemon.

Here's a simple Echo service built using EventMachine. (Borrowed from the README file)

require 'rubygems'
require 'eventmachine'

module EchoServer
  def receive_data data
    send_data ">>>you sent: #{data}"
    close_connection if data =~ /quit/i
  end
end

EventMachine::run {
  EventMachine::start_server "192.168.0.100", 8081, EchoServer
}

Here's an example I made. You can pass a class to EventMachine::start_server, and each new connection will create a new instance of your class. That allows you to use instance variables to record distinct state for each connection. This example buffers up the client input until the client sends a ".<CRLF>" a la SMTP, or a maximum of 10 lines.

require 'rubygems'
require 'eventmachine'

class DataBuffer < EM::Protocols::LineAndTextProtocol
  def initialize
    puts "init'ing new instance of #{self.class.to_s}"
    @line_ctr = 0
    @databuf = []
  end

  def receive_data(data)
    @databuf << data
    @line_ctr += 1
    if data == ".\r\n" || @line_ctr == 10
      if data == ".\r\n"
        @databuf.pop
      end
      send_data(@databuf.to_s)
      reset_databuf()
    end
  end

  private
  def reset_databuf
    @line_ctr = 0
    @databuf = []
  end
end

EventMachine::run {
  EventMachine::start_server "127.0.0.1", 8081, DataBuffer
}

And the best part is, I've only been playing with EventMachine for about an hour! I've only begun to understand what it can do.

So, yeah. EventMachine is badass. You should check it out.