Multiple http requests on the same long running operation with Sinatra and EventMachine - asynchronous

Question

I'm trying to understand how to use evented web servers with a combination of async sinatra and EventMachine.
In the code below each request on '/' will generate a new async http request to google. Is there an elegant solution for detecting that a request is already ongoing and waiting for its execution ?
If I have 100 concurrent requests on '/', this will generate 100 requests to the google backend. It would be much better to have a way to detect there is already an ongoing backend request and wait for its execution.
thanks for the answer.
require 'sinatra'
require 'json'
require 'eventmachine'
require 'em-http-request'
require 'sinatra/async'
Sinatra.register Sinatra::Async
def get_data
puts "Start request"
http = EventMachine::HttpRequest.new("http://www.google.com").get
http.callback {
puts "Request completed"
yield http.response
}
end
aget '/' do
get_data {|data| body data}
end
Update
I actually discovered you can add several callbacks to the same http request. So, it's easy to implement:
class Request
def get_data
if !#http || #http.response_header.status != 0
#puts "Creating new request"
#http = EventMachine::HttpRequest.new("http://www.bbc.com").get
end
#puts "Adding callback"
#http.callback do
#puts "Request completed"
yield #http.response
end
end
end
$req = Request.new
aget '/' do
$req.get_data {|data| body data}
end
This gives a very high number of requests per second. Cool!

Answer 1

You don't have to use sinatra/async at all to make it evented, just run it with an evented server (Thin, Rainbows!, Goliath).
Take a look at em-synchrony for an example of making multiple parallel requests without introducing spaghetti callback code:
require "em-synchrony"
require "em-synchrony/em-http"
EventMachine.synchrony do
multi = EventMachine::Synchrony::Multi.new
multi.add :a, EventMachine::HttpRequest.new("http://www.postrank.com").aget
multi.add :b, EventMachine::HttpRequest.new("http://www.postrank.com").apost
res = multi.perform
p "Look ma, no callbacks, and parallel HTTP requests!"
p res
EventMachine.stop
end
And yes, you can run this inside your Sinatra action.
Also take a look at Faraday, specifically with EM adapter.