How to handle a request with node/express that requires two asynchronous methods

Question

I'm learning node. I have a web application that is interfacing with bitcoind through the bitcoin package, and PostgreSQL through knex. I need to get a bit of data from each module and then pass it all to my view for rendering. My code looks like this so far:

exports.index = function(req, res){
  var testnet='';
  bitcoin.getInfo(function(e, info){
    if(e){ return console.log(e);}
    if(info.testnet){
      testnet='bitcoin RPC is testnet';
    }else{
      testnet='nope.. you must be crazy';
    }
    var c=knex('config').select().then(function(k){
      res.render('index', { title: k[0].site_name, testnet: testnet });
    });
  });
};

The way this is structured though, it will first wait for Bitcoin to reply, and then it will issue the request to PostgreSQL, and then wait a while longer for it to reply. These two wait periods could clearly happen simultaneously. However, I don't know how to do that with promises/callbacks in Javascript. How can I manage this to happen asynchronously, rather than serially?

does your request to Postgre rely on the response from Bitcoin? — brandonscript
– brandonscript, Commented Dec 10, 2013 at 3:40
If not, then Joe's answer is definitely the way to go. If ever it does reply on the response, then you won't be able to run those async. — brandonscript
– brandonscript, Commented Dec 10, 2013 at 3:48

Joe Doyle · Accepted Answer · 2013-12-10 03:44:30Z

3

You are looking for the async module which will let you fire off both tasks then continue on.

An untested example to give you the idea:

exports.index = function(req, res){
    var testnet='', k={};

    async.parallel([
      function(){ 
        bitcoin.getInfo(function(e, info){
          //your getInfo callback logic
           },
      function(){
        knex('config').select().then(function(result) {       
         //your knex callback
         k = result;
      } ],
      //Here's the final callback when both are complete
      function() {
         res.render('index', { title: k[0].site_name, testnet: testnet });
      });
}

answered Dec 10, 2013 at 3:44

Joe Doyle

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2 Comments

Earlz Over a year ago

The actual code that would make this work is significantly more complicated, especially with error handling :/

Earlz Over a year ago

btw, for future reference, this is the relevant commit I made to my code. Notable is that each parallel task must take and call a callback function to indicate it's done. github.com/Earlz/d4btc/commit/…

srquinn · Accepted Answer · 2013-12-10 03:41:53Z

2

You can either use a library like caolan/async's #parallel() method or you could poke around its source and learn to role your own

answered Dec 10, 2013 at 3:41

srquinn

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Comments

Esailija · Accepted Answer · 2013-12-11 15:00:13Z

1

You need to promisify callback methods, not mix callbacks and promises.

var Promise = require("bluebird");
var bitcoin = require("bitcoin");
//Now the bitcoin client has promise methods with *Async postfix
Promise.promisifyAll(bitcoin.Client.prototype);

var client = new bitcoin.Client({...});

Then the actual code:

exports.index = function(req, res) {
    var info = client.getInfoAsync();
    var config = knex('config').select();

    Promise.all([info, config]).spread(function(info, config) {
        res.render('index', {
            title: config[0].site_name,
            testnet: info.testnet ? 'bitcoin RPC is testnet' : 'nope.. you must be crazy'
        });
    }).catch(function(e){
        //This will catch **both** info and config errors, which your code didn't
        //do
        console.log(e);
    });
};

Promise module bluebird is used.

answered Dec 11, 2013 at 15:00

Esailija

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2 Comments

Earlz Over a year ago

That is very interesting. I didn't know about Promisify, or how elegant the code could be when it was all promises

Esailija Over a year ago

@Earlz yep, having the results as values in normal variables is really awesome

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