createConnection does not reconnect when initializing the connection while the DB is down

[tomgrossman]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
I'm initializing the DB connection with the createConnection function.
If I'm doing so while the DB is down, the readyState of the connection stays at 2 (connecting), and when starting the DB there is no re-connection until process restart.

If the current behavior is a bug, please provide the steps to reproduce.
using the following script:

const Promise       = require('bluebird');
const mongoose      = require('mongoose');
mongoose.Promise = Promise;

const DB_URL = 'mongodb://localhost:27017/test_database';

const DbConnection = mongoose.createConnection(DB_URL, {
    promiseLibrary: Promise,
    poolSize: 20,
    socketTimeoutMS: 0,
    connectTimeoutMS: 0,
    reconnectTries: Number.MAX_VALUE
});

let currentState = DbConnection.readyState;
let counter = 0 ;
console.log(DbConnection.readyState);
setInterval(() => {
    if (DbConnection.readyState === currentState) {
        counter++;
    } else {
        counter = 0;
        currentState = DbConnection.readyState;
    }
    console.log(DbConnection.readyState + ' - ' + counter);

}, 3000);

DbConnection.on('error', function (error) {
    console.log('error: ' + error.message)
});

DbConnection.on('disconnected', function () {
    console.log('disconnected');
});

DbConnection.on('close', function () {
    console.log('close');
});

DbConnection.on('open', function () {
    console.log('open');
});

DbConnection.on('connected', function () {
    console.log('connected');
});

DbConnection.on('connecting', function () {
    console.log('connecting');
});

DbConnection.on('reconnected', function () {
    console.log('reconnected');
});

process.on('unhandledRejection', function (err) {
    if ('MongoError' === err.name) {
        console.log('unhandledRejection: ' + err.message);
    }
});

module.exports = DbConnection;

while the DB is down from the beginning, the console output is:

2
error: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]
2 - 1
2 - 2
2 - 3
2 - 4

and the counter continues to go up on the 2 readyState even when starting the DB in the middle.

What is the expected behavior?
The connection should re-connect after starting the DB. The re-connection works fine if the DB goes down and up in the middle of the running process.
For example, using the same script as above when the DB is up at the beginning, and goes down and up in the middle of the run, the console output is:

2
connected
open
1 - 0
1 - 1
disconnected
0 - 0
0 - 1
connected
reconnected
1 - 0
1 - 1

Please mention your node.js, mongoose and MongoDB version.
node.js - 8.10.0
mongoose - 5.0.10
MongoDB - 3.4

[ghost]

Hi @tomgrossman

tl;dr reconnect only applies to previously established connections.

I was a little perplexed by this the first time I encountered it. This isn't a bug with mongoose but rather the intended behavior of the mongodb driver. There is at least one issue in mongodb's native driver bug tracker that proves we're not alone. Here is the link to the first one I found.

[tomgrossman]

the readyState stays on 2 which is wrong, since it is not "connecting", so maybe it can be solved from mongoose side if the native driver ppl says it's as designed.

[ghost]

the current callback to the driver's MongoClient.connect method only emits an error and rejects the promise without changing the readyState.

@vkarpov15 would it be a backwards breaking change to set the connection's readyState to disconnected in the initial error condition of the callback to .connect()?

something like:

        if (error) {
          if (_this.listeners('error').length > 0) {
            _this.emit('error', error);
          }
          _this.readyState = STATES.disconnected 
          return reject(error);
        }

example:

#!/usr/bin/env node
'use strict'
const noMongoOnPort = 'mongodb://localhost:27999/test'
const mongoose = require('mongoose')

const conn = mongoose.createConnection(noMongoOnPort, (e) => {
  console.log(`readyState: ${conn.readyState}\nerror.name ${e.name}`)
  process.exit(1)
})

output with .connect callback as it is now:

mongoose5: ./6244.js 
readyState: 2
error.name MongoNetworkError
mongoose5: 

output with proposed change

mongoose5: ./6244.js 
readyState: 0
error.name MongoNetworkError
mongoose5: 

[vkarpov15]

@tomgrossman thanks for reporting, fixed in master and fix will be in 5.0.12