Mongoose.js creates multiple connections to MongoDB from one connect() call

Question

I am making a single connection to MongoDB via Mongoose in Node.js Express app:

var express = require('express');
var mongoose = require('mongoose');
mongoose.connect('localhost', 'test');

I then define schema, followed by models, and lastly the controller that pulls all users from the database:

app.get('/users', function (req, res) {
  return User.find(function (err, users) {
    if (!err) {
      return res.send(users);
    }
  });
});

These DB connections open during the application start and they stay open for the duration of the node.js application.

What bothers me is why do I have 5 connections open? As soon as I close the node.js app, all 5 connections are closed.

Related note: For a REST API server is it better to have MongoDB connection always open. Or is it better to manually open/close connections per each user request? d

Answer 1

That's because Mongoose uses a pool of 5 connections (by default) that are shared throughout your application. For best performance, it's best to just leave them open.

You can alter the default behavior via the options parameter to mongoose.connect. For example:

mongoose.connect('localhost', 'test', { server: { poolSize: 3 }}); // Use 3 connections