Node.js - Synchronize N functions with one

Question

Node.js - Synchronize N functions with one

Assuming a pseudo-code like this:

function get_data_and_reply(response) {
  // process 1: 
  db_connect.query(..., function(...){
    // get A from db 
  });

  // process 2:
  db_connect.query(..., function(...){
    // get B from db 
  });

  // process 3: aggregate A + B and reply
}

A possible solution is using callback (hell ;) ), like this

function get_data_and_reply(response) {
  // process 1: 
  db_connect.query(..., function(...){
    // get A from db 

    if (err) {} else {
      // process 2:
      db_connect.query(..., function(...){
        // get B from db 

        if (err) {} else {
          // process 3: aggregate A + B and reply
        }
      });
    }
  });
}

but I would like to maintain the parallelism with processes 1 and 2 (can take time, and then would be better if they go in parallel) and then "serialise" with process 3.

How can I synchronise N steps (in parallel) with one?

I have thought about playing with variables and looping until the other method is ended, but sounds a little intricate, or not?

function get_data_and_reply(response) {
  // process 1: 
  db_connect.query(..., function(...){
    // get A from db 
  });

  // process 2:
  db_connect.query(..., function(...){
    // get B from db 
  });

  // process 3: aggregate A + B and reply
  while (A_is_not_ready && B_is_not_ready) {}
}

node.js
synchronization

Answer 1

If you do not mind using libraries, async is an awesome one. You would use it like this:

function get_data_and_reply(response) {
  async.parallel({
    A: function (next) {
      // process 1: 
      db_connect.query(..., next); // Assuming the db returns the result like (err, result)
    },
    B: function (next) {
      // process 2:
      db_connect.query(..., next);
    }
  }, function (err, results) {
    // This is called when both parallel functions are done. Use results.A and .B
    // process 3: aggregate A + B and reply
  })
}

To do it without any helper is also pretty simple, thought not as pretty:

function get_data_and_reply(response) {
  var aResult, bResult;
  // process 1: 
  db_connect.query(..., function(..., result){
    // get A from db 
    aResult = result;
    done();
  });

  // process 2:
  db_connect.query(..., function(..., result){
    // get B from db 
    bResult = result;
    done();
  });

  function done() {
    if(aResult && bResult) {
      // process 3: aggregate A + B and reply
    }
  }
}

Don't forget to handle errors should you use the latter way.

Answer 2

Read up on promises pattern, and specially the Q library for node: http://strongloop.com/strongblog/promises-in-node-js-with-q-an-alternative-to-callbacks/