When MongoDB fail to connect it crashes the app although it's surrounded with a Promise

Question

When i run it with a correct password it works fine, only with an incorrect password it throws an uncuaght exception which is not caught by the promise for some reason, and then the app crash

Here is the controller tryCtrl.js:

const connection = require('./tryDB');

connection
  .then(() => console.log('success'))
  .catch((error) => {
    console.error(error);
  });

Here is the model file that access the db tryDB.js

const mongoose = require('mongoose');

require('dotenv').config();

// create connection to db
const uri = process.env.MONGO_URI;

const connection = new Promise((resolve, reject) => {
  mongoose.connect(uri);
  const db = mongoose.connection;
  db.on('error', (error) => {
    reject(error);
  });
  db.once('open', () => {
    resolve(db);
  });
});

module.exports = connection;

when i run it with a false password i expect the expection to be caught, but it breaks the app Here is the error:

node:internal/process/promises:288
        triggerUncaughtException(err, true /* fromPromise */);
        ^

MongoServerError: bad auth : authentication failed
    at Connection.onMessage (C:\Users\israe\Documents\GitHub\my-mongo\node_modules\mongodb\lib\cmap\connection.js:201:30)
    at MessageStream.<anonymous> (C:\Users\israe\Documents\GitHub\my-mongo\node_modules\mongodb\lib\cmap\connection.js:59:60)
    at MessageStream.emit (node:events:513:28)
    at processIncomingData (C:\Users\israe\Documents\GitHub\my-mongo\node_modules\mongodb\lib\cmap\message_stream.js:124:16)
    at MessageStream._write (C:\Users\israe\Documents\GitHub\my-mongo\node_modules\mongodb\lib\cmap\message_stream.js:33:9)
    at writeOrBuffer (node:internal/streams/writable:392:12)
    at _write (node:internal/streams/writable:333:10)
    at Writable.write (node:internal/streams/writable:337:10)
    at TLSSocket.ondata (node:internal/streams/readable:766:22)
    at TLSSocket.emit (node:events:513:28) {   ok: 0,   code: 8000,   codeName: 'AtlasError',   connectionGeneration: 0,  
[Symbol(errorLabels)]: Set(2) { 'HandshakeError', 'ResetPool' } }

Any idea is appreciated

Maybe this one: https://stackoverflow.com/questions/63754742/authentication-failure-while-trying-to-save-to-mongodb/63755470#63755470 — Wernfried Domscheit, Apr 25 '23 at 07:12
@WernfriedDomscheit when i run it with a correct password it works smoothly — forstack overflowizi, Apr 25 '23 at 07:25

Charchit Kapoor · Answer 1 · 2023-04-26T14:15:43.597

Ideally, you shouldn't export variables that refer to Promises. Exports are meant for functionality or constants which you intend to reuse later. You should restructure your connection function, to return a Promise instead, like this:

const mongoose = require('mongoose');

require('dotenv').config();

// create connection to db
const uri = process.env.MONGO_URI;

const connection = () => new Promise((resolve, reject) => {
  mongoose.connect(uri);
  const db = mongoose.connection;
  db.on('error', (error) => {
    reject(error);
  });
  db.once('open', () => {
    resolve(db);
  });
});

module.exports = connection;

Now your controller becomes like this:

const connection = require('./tryDB');

connection()
  .then(() => console.log('success'))
  .catch((error) => {
    console.error(error);
  });

Note, that I have called the connection function first, then the promise functions are chained.

UPDATE: This is an uncaught exception. To catch it, you can add process.on event handlers. In your app.js file, like this:

process
  .on('unhandledRejection', (reason, p) => {
    console.error(reason, 'Unhandled Rejection at Promise', p);
  })
  .on('uncaughtException', err => {
    console.error(err, 'Uncaught Exception thrown');
  });

Note this is not the recommended way, ideally, you should let the application stop on such errors. Also, you can try setting mongoose.Promise = global.Promise; as suggested by the creator of mongoose here.

Thank you, your logic sound right, but i have tested it, and it still throws the same uncuaught exeption — forstack overflowizi, Apr 26 '23 at 11:23
is this snipper suppose to be added to the controller after the call to connection() ? — forstack overflowizi, Apr 26 '23 at 12:05
no, it should be in the main file, where the server is created — Charchit Kapoor, Apr 26 '23 at 12:13
Ok thank you, that's a nice bypass, but i still don't understand why the reject don't catch it, i thought it was the point of reject — forstack overflowizi, Apr 26 '23 at 13:26
Yeah, I have added a minor update as well. Please check it out. — Charchit Kapoor, Apr 26 '23 at 14:16

When MongoDB fail to connect it crashes the app although it's surrounded with a Promise

1 Answers1