example of how to make calls in parallel

server.js

var http = require("http")

// expensive call
// takes 2 sec
function name(cb){
  setTimeout(function(){ cb("Brock") }, 2000)
}
 
// expensive call
// takes 2 sec
function place(cb){
  setTimeout(function(){ cb("Canada") }, 2000)
}

/* ------------------------------------ 
   Example 1
   - make calls sequentially
   - takes 4 sec to complete 
*/

http.createServer(function(req, rsp){
  name(function(name){
    place(function(place){
      rsp.end(name + place)
    })
  })
}).listen(8001)


/* ------------------------------------ 
   Example 2a
   - make calls in parallel
   - takes 2 sec to complete 
*/

http.createServer(function(req, rsp){
    var counter = 2
    var results = {}

    name(function(name){
      counter -- 
      results["name"] = name

      if(counter == 0) 
        rsp.end(results["name"] + results["place"])
    })

    place(function(place){
      counter --
      results["place"] = place

      if(counter == 0) 
        rsp.end(results["name"] + results["place"])
    })
    
}).listen(8002)

/* ------------------------------------ 
   Example 2b
   - make calls in parallel using generic helper
   - takes 2 sec to complete 
   - http://stackoverflow.com/questions/4631774/coordinating-parallel-execution-in-node-js
*/

function parallel(calls, cb){
  var counter = calls.length
  var results = []
  
  function makeCb(index){
    return function(){
      counter --
      var result = []

      for (var i=0;i<arguments.length;i++)
        result.push(arguments[i])

      results[index] = result

      if(counter == 0)
        cb(results)
    }
  }

  for (var i=0;i<calls.length;i++)
    calls[i](makeCb(i))
}

http.createServer(function(req, rsp){
  
  parallel([name, place], function(results){
    rsp.end(results[0] + results[1])
  })
  
}).listen(8003)