Sample Fibers+EventMachine usage with NATS

nats_sinatra.rb

require 'thin'
require 'sinatra/base'
require 'nats/client'
require 'fiber'
require 'json'

class App < Sinatra::Base

  def initialize
    @nats = NATS.start(:uri => "nats://localhost:4222")
    super
  end

  # Expected number of components that should be running
  # It fails if we underestimate the number of components
  # with a "resume dead fiber called" error
  # Taken from NATS#timed_request in cloud_controller
  def get_components_info
    expected = 20 # e.g. Max number of components running at once
    timeout  = 1

    f = Fiber.current
    results = []
    sid = @nats.request("vcap.component.discover") do |msg|
      results << msg
      begin
        f.resume if results.length >= expected
      rescue
        puts "More components running than expected"
      end
    end
    @nats.timeout(sid, timeout, :expected => expected) { f.resume }
    Fiber.yield

    return results.slice(0, expected)
  end

  def start_monitoring_task
    EM.next_tick do
      Fiber.new do
        components_info = get_components_info
        components = []
        components_info.each do |c|
          components << JSON.parse(c)
        end
        puts "Number of components:"
        p components.count
      end.resume
    end
  end

  get '/' do
    EM.add_periodic_timer(1) { start_monitoring_task }
    "Started the timer"
  end

end

# Sample run:
#
# >> Thin web server (v1.3.1 codename Triple Espresso)
# >> Maximum connections set to 1024
# >> Listening on 0.0.0.0:9292, CTRL+C to stop
# 127.0.0.1 - - [04/Feb/2012 09:13:38] "GET / HTTP/1.1" 200 22 5.7494
#
# Number of components:
# 6
# Number of components:
# 7
# Number of components:
# 6
# Number of components:
# 0
# Number of components:
# 8

# Occasional error during dev:
#
# [2012-02-04 09:18:58] hm - 11126 3776 fe29  ERROR -- Eventmachine problem, SQLite3::BusyException: database is locked: SELECT  "apps".* FROM "apps" WHERE "apps"."id" = 33 LIMIT 1