Are bluebird promises blocking in nature

Question

I may be a bit new to understand the underlying functioning of promises esp bluebird. What i am trying to accomplish is an API server that processes bulk write of 250K+ rows into a database. It takes around 30s to complete the request. I want the API server to return ok or error based on the success of the bulk write. When i don't use promises i don't get an opportunity to bubble up the error because the request is not waiting for the bulk write to complete. However, if I use promise the error and success works properly. But the server becomes unresponsive till the action is complete. In nutshell, using the promise library to handle the bulk write blocks the API server.

function chunk(arr, chunkSize) {
  var R = [];

  for (var i=0,len=arr.length; i<len; i+=chunkSize)
    {
        R.push(arr.slice(i,i+chunkSize));
    }
  return promise.resolve(R);
}


exports.add = function(req, res) {
var PO_STD_LT_TIME = 90; //DAYS
    Parts.sync()
        .then(function() {
            return Parts.destroy({
                where: {}
            });
        })
        .then(function() {
            var workbook = XLSX.readFileSync(__dirname + '/dbo_tblCPLParts_.xlsx');
            var sheet_name_list = workbook.SheetNames;
            var JSON_S = XLSX.utils.sheet_to_json(workbook.Sheets[sheet_name_list[0]]);

            var size = 40000;
            chunk(JSON_S, size).then(function(JSON_Small) {
                promise.each(JSON_Small, function (JSON_small_){
                    Parts.bulkCreate(JSON_small_)
                    .catch(function(err) {
                        res.json(500, {
                            Error: "Error : " + err
                        });
                    })

                }).finally(ext_fns.handleResult(res,200))
            })
            })

}

What is the best way to handle this? Am I using the promise the right way?

Answer 1

It's because you're invoking synchronous behavior along the way. The culprit is

var workbook = XLSX.readFileSync(__dirname + '/dbo_tblCPLParts_.xlsx');

You should instead use the XLSX equivalent of fs.readFile, which is asynchronous. Alternatively, since you're using bluebird, you can use promisifyAll on the XLSX module:

var Promise = require('bluebird')
var fs = Promise.promisifyAll(/* the XLSX module */)

which will allow you to treat all the XLSX modules as promises.

Answer 2

Here is a protip.

In io.js (NodeJS) recently a synchronous io flag was added. Fetch the most recent version and run it with --trace-sync-io. It wil figure out all your synchronous blocking issues - I suspect they are not promises related.

As for bluebird - it is not blocking.