Node.js: async.map getting slower

Question

Hello,

I use Node.js to provide an API for storing data on a MongoDB database.

I ran multiple tests on a read method, which takes ids and returns the corresponding documents. The point is that I must return these documents in the specified order. To ensure that, I use the following code:

// Sequentially fetch every element
function read(ids, callback) {
    var i = 0;
    var results = [];
    function next() {
        db.findOne(ids[i], function (err, doc) {
            results.push(err ? null : doc);
            if (ids.length > ++i) {
                return next();
            }
            callback(results);
        });
    }
    next();
}

This way, documents are fetched one-by-one, in the right order. It takes about 11s on my laptop to retrieve 27k documents.

However, I thought that it was possible to improve this method:

// Asynchronously map the whole array
var async = require('async');

function read(ids, callback) {
    async.map(ids, db.findOne.bind(db), callback):
}

After running a single test, I was quite satisfied seeing that the 27k documents were retrieved in only 8s using simpler code.

The problem happens when I repeat the same request: the response time keeps growing (proportionally to the number of elements retrieved): 9s 10s 11s 12s.... This problem does not happen in the sequential version.

I tried two versions of Node.js, v6.2.0 and v0.10.29. The problem is the same. What causes this latency and how could I suppress it?

Answer 1

Try to use async.mapLimit to prevent overload. You need some tests to tune limit value with your environment.

---

But find({_id: {$in: list}}) is always better, because single database request instead of multiple.

I suggest you to try to perform restore of original order client-side.
Something like this:

function read(ids, cb) {
  db.find(
    {_id: {$in: ids.map(id => mongoose.Types.ObjectId(id))}},
    process
  );

  function process(err, docs) {
    if (err) return cb(err);
    return cb(null, docs.sort(ordering))
  }
  function ordering(a, b) {
    return ids.indexOf(b._id.toString()) - ids.indexOf(a._id.toString());
  }
}

May be, find query needs to be corrected, I can't to know what exact mongodb driver you use.

This code is first-try, more manual sorting can improve performance alot. [].indexOf is heavy too(O(n)).
But I'm almost sure, even as-is now, it will work much faster.

---

Possible ordering replacement:

var idHash = {};
for(var i = 0; i < ids.length; i++)
  idHash[ids[i]] = i;
function ordering(a, b) {
  return idHash[b._id.toString()] - idHash[a._id.toString()];
}

Any sort algorithm has O(nlogn) in best case, but we already know result position of each found document, so, we can restore original order by O(n):

var idHash = ids.reduce((c, id, i) => (c[id] = i, c), {});
function process(err, docs) {
  if (err) return cb(err);
  return cb(null, 
    docs.reduce(
      (c, doc) => (c[idHash[doc._id.toString()]] = doc, c),
      ids.map(id => null))) //fill not_found docs by null
}

Functional style makes code flexier. For example this code can be easy modified to use async.reduce to be less sync-blocking.