Mongoose opens 2 connections to DB on second request - node.js

Question

I'm writing a small Node to just monitor MongoDB by making a simple query via Mongoose. If something is returned, the service is deemed as available. Here is my script
var res;
var logPrefix = '[MongoDB]';
var counter = 0;
var mongoose = require('mongoose');
// Simple schema and model for getting a companyprofiles document
var companyProfile = new mongoose.Schema({
_id: String
});
companyProfile.virtual('Id').get(function () {return this._id});
function closeConnection() {
console.log('8');
mongoose.connection.close(function () {
console.log('9');
console.log('%s Closed connection%d', logPrefix, counter);
});
}
function connectAndCheckHealth() {
console.log('2');
mongoose.connect('mongodb://localhost:27017/testdb');
console.log('3');
mongoose.connection.on('error', function(err) {
console.log('%s Error connecting to DB:\n%s %s', logPrefix, logPrefix, err);
res.send(503, 'ERR');
});
mongoose.connection.on('open', function() {
mongoose.connection.db.serverConfig.options.auto_reconnect = false;
});
console.log('4');
mongoose.connection.on('connected', checkHealth);
console.log('5');
mongoose.connection.on('close', function() {
console.log('%s Connection to MongoDB closed %d', logPrefix, counter);
});
}
function checkHealth() {
console.log('6');
cpModel = mongoose.model('companyProfile', companyProfile, 'companyprofiles');
cpModel.findById('spin', handleModelResponse);
}
function handleModelResponse(error, doc) {
console.log('7');
closeConnection();
console.log('10');
if (error !== null) {
console.log('11');
console.log('%s Error handling response from model:\n%s %s', logPrefix, logPrefix, error);
res.send(503, 'ERR');
} else {
console.log('12');
if (doc.Id.match(/company1/)) {
console.log('13');
console.log('%s App status is ok', logPrefix);
res.send(200, 'OK');
} else {
console.log('14');
console.log('%s Couldn\'t find the spin company profile. Found %s', logPrefix, doc.Id);
res.send(503, 'ERR');
}
}
}
module.exports = {
health: function(response) {
counter++;
var date = new Date();
console.log('%s Retrieving health from MongoDB at %s', logPrefix, date);
res = response;
console.log(mongoose.connection);
console.log('1');
connectAndCheckHealth();
console.log('15');
}
}
As you can see, I've peppered the script with console.log lines with numbers to try and work out the control flow. Here's the output:
1
2
3
4
5
15
6
6
6
7
8
[MongoDB] Connection to MongoDB closed 3
[MongoDB] Connection to MongoDB closed 3
[MongoDB] Connection to MongoDB closed 3
9
[MongoDB] Closed connection3
10
12
13
[MongoDB] App status is ok
7
8
9
[MongoDB] Closed connection3
10
12
13
[MongoDB] App status is ok
/home/GTP/healthcheck/node_modules/mongoose/lib/utils.js:413
throw err;
^
Error: Can't set headers after they are sent.
Notice that 6 appears three times (the callback after we have connected). I can't tell why we are opening multiple connections. I've turned autoconnect off and am closing the connection after every request.
Any help would be greatly appreciated.

Answer 1

Mongoose opens a pool of 5 connections by default; if you only want a single connection you can change your connect call to be:
mongoose.connect('mongodb://localhost:27017/testdb', { server: { poolSize: 1 }});

Answer 2

db.once('open') worked for me.
While testing my page, I had noticed that double and rapid form submits were triggering errors about the headers already sent. This stopped that.