class Puma::Reactor

def initialize(server, app_pool)

it will be passed to the `app_pool`.
Once a request is fully formed (header and body are received)
The `app_pool` is an instance of `Puma::ThreadPool`.

when there is an exception inside of the reactor.
that is used to write a response for "low level errors"
The `server` argument is an instance of `Puma::Server`

Creates an instance of Puma::Reactor 
def initialize(server, app_pool)
  @server = server
  @events = server.events
  @app_pool = app_pool
  @selector = NIO::Selector.new
  @mutex = Mutex.new
  # Read / Write pipes to wake up internal while loop
  @ready, @trigger = Puma::Util.pipe
  @input = []
  @sleep_for = DefaultSleepFor
  @timeouts = []
  mon = @selector.register(@ready, :r)
  mon.value = @ready
  @monitors = [mon]
end