How to properly reuse connection to Mongodb across NodeJs application and modules

I've been reading and reading and still am confused on what is the best way to share the same database (MongoDb) connection across whole NodeJs app. As I understand connection should be open when app starts and reused between modules. My current idea of the best way is that server.js (main file where everything starts) connects to database and creates object variable that is passed to modules. Once connected this variable will be used by modules code as necessary and this connection stays open. E.g.:

    var MongoClient = require('mongodb').MongoClient;
    var mongo = {}; // this is passed to modules and code
    
    MongoClient.connect("mongodb://localhost:27017/marankings", function(err, db) {
    	if (!err) {
    		console.log("We are connected");
    
    		// these tables will be passed to modules as part of mongo object
    		mongo.dbUsers = db.collection("users");
    		mongo.dbDisciplines = db.collection("disciplines");
    
    		console.log("aaa " + users.getAll()); // displays object and this can be used from inside modules
    
    	} else
    		console.log(err);
    });
    
    var users = new(require("./models/user"))(app, mongo);
    console.log("bbb " + users.getAll()); // not connected at the very first time so displays undefined

then another module models/user looks like that:

Users = function(app, mongo) {

Users.prototype.addUser = function() {
	console.log("add user");
}

Users.prototype.getAll = function() {

	return "all users " + mongo.dbUsers;

	}
}

module.exports = Users;

Now I have horrible feeling that this is wrong so are there any obvious problems with this approach and if so how to make it better?

You can create a mongoUtil.js module that has functions to both connect to mongo and return a mongo db instance:

const MongoClient = require( 'mongodb' ).MongoClient;
const url = "mongodb://localhost:27017";

var _db;

module.exports = {

  connectToServer: function( callback ) {
    MongoClient.connect( url,  { useNewUrlParser: true }, function( err, client ) {
      _db  = client.db('test_db');
      return callback( err );
    } );
  },

  getDb: function() {
    return _db;
  }
};

To use it, you would do this in your app.js:

var mongoUtil = require( 'mongoUtil' );

mongoUtil.connectToServer( function( err, client ) {
  if (err) console.log(err);
  // start the rest of your app here
} );

And then, when you need access to mongo somewhere else, like in another .js file, you can do this:

var mongoUtil = require( 'mongoUtil' );
var db = mongoUtil.getDb();

db.collection( 'users' ).find();

The reason this works is that in node, when modules are require'd, they only get loaded/sourced once so you will only ever end up with one instance of _db and mongoUtil.getDb() will always return that same instance.

Note, code not tested.

There are many ways this could be tweaked to accept configuration objects in places, but overall it's similar to how you have your code laid out, albeit with more modern JS syntax. Could easily be rewritten to prototypes and callbacks, if that's your requirement.

mongo.js

const { MongoClient } = require('mongodb');
const config = require('./config');
const Users = require('./Users');
const conf = config.get('mongodb');

class MongoBot {
  constructor() {
    const url = `mongodb://${conf.hosts.join(',')}`;

    this.client = new MongoClient(url, conf.opts);
  }
  async init() {
    await this.client.connect();
    console.log('connected');

    this.db = this.client.db(conf.db);
    this.Users = new Users(this.db);
  }
}

module.exports = new MongoBot();

Users.js

class User {
  constructor(db) {
    this.collection = db.collection('users');
  }
  async addUser(user) {
    const newUser = await this.collection.insertOne(user);
    return newUser;
  }
}
module.exports = User;

app.js

const mongo = require('./mongo');

async function start() {
  // other app startup stuff...
  await mongo.init();
  // other app startup stuff...
}
start();

someFile.js

const { Users } = require('./mongo');

async function someFunction(userInfo) {
  const user = await Users.addUser(userInfo);
  return user;
}

If you are using Express, then you can use mongo-express-req module that allows you to get db connection in request object.

Install

npm install --save mongo-express-req

server.js

var app = require('express')();

var mongoExpressReq = require('mongo-express-req');
app.use(mongoExpressReq('mongodb://localhost/test'));

routes/users.js

app.get('/', function (req, res, next) {
	req.db // => Db object
});

---

Note: mongo-express-req is fork of not maintained express-mongo-db.

Here's how I do it with contemporary syntax, based on go-oleg's example. Mine is tested and functional.

I put some comments in the code.

> ./db/mongodb.js

 const MongoClient = require('mongodb').MongoClient
 const uri = 'mongodb://user:password@localhost:27017/dbName'
 let _db

 const connectDB = async (callback) => {
     try {
         MongoClient.connect(uri, (err, db) => {
             _db = db
             return callback(err)
         })
     } catch (e) {
         throw e
     }
 }

 const getDB = () => _db

 const disconnectDB = () => _db.close()

 module.exports = { connectDB, getDB, disconnectDB }

> ./index.js

 // Load MongoDB utils
 const MongoDB = require('./db/mongodb')
 // Load queries & mutations
 const Users = require('./users')

 // Improve debugging
 process.on('unhandledRejection', (reason, p) => {
     console.log('Unhandled Rejection at:', p, 'reason:', reason)
 })

 const seedUser = {
     name: 'Bob Alice',
     email: '[email protected]',
     bonusSetting: true
 }

 // Connect to MongoDB and put server instantiation code inside
 // because we start the connection first
 MongoDB.connectDB(async (err) => {
     if (err) throw err
     // Load db & collections
     const db = MongoDB.getDB()
     const users = db.collection('users')

     try {
         // Run some sample operations
         // and pass users collection into models
         const newUser = await Users.createUser(users, seedUser)
         const listUsers = await Users.getUsers(users)
         const findUser = await Users.findUserById(users, newUser._id)

         console.log('CREATE USER')
         console.log(newUser)
         console.log('GET ALL USERS')
         console.log(listUsers)
         console.log('FIND USER')
         console.log(findUser)
     } catch (e) {
         throw e
     }

     const desired = true
     if (desired) {
         // Use disconnectDB for clean driver disconnect
         MongoDB.disconnectDB()
         process.exit(0)
     }
     // Server code anywhere above here inside connectDB()
 })

> ./users/index.js

 const ObjectID = require('mongodb').ObjectID

 // Notice how the users collection is passed into the models
 const createUser = async (users, user) => {
     try {
         const results = await users.insertOne(user)
         return results.ops[0]
     } catch (e) {
         throw e
     }
 }

 const getUsers = async (users) => {
     try {
         const results = await users.find().toArray()
         return results
     } catch (e) {
         throw e
     }
 }

 const findUserById = async (users, id) => {
     try {
         if (!ObjectID.isValid(id)) throw 'Invalid MongoDB ID.'
         const results = await users.findOne(ObjectID(id))
         return results
     } catch (e) {
         throw e
     }
 }

 // Export garbage as methods on the Users object
 module.exports = { createUser, getUsers, findUserById }

A tested solution based on the accepted answer:

mongodbutil.js:

var MongoClient = require( 'mongodb' ).MongoClient;
var _db;
module.exports = {
  connectToServer: function( callback ) {
    MongoClient.connect( "<connection string>", function( err, client ) {
      _db = client.db("<database name>");
      return callback( err );
    } );
  },
  getDb: function() {
    return _db;
  }
};

app.js:

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');
var app = express();
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');
app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

var mongodbutil = require( './mongodbutil' );
mongodbutil.connectToServer( function( err ) {
  //app goes online once this callback occurs
  var indexRouter = require('./routes/index');
  var usersRouter = require('./routes/users');
  var companiesRouter = require('./routes/companies');
  var activitiesRouter = require('./routes/activities');
  var registerRouter = require('./routes/register');  
  app.use('/', indexRouter);
  app.use('/users', usersRouter);
  app.use('/companies', companiesRouter);
  app.use('/activities', activitiesRouter);
  app.use('/register', registerRouter);  
  // catch 404 and forward to error handler
  app.use(function(req, res, next) {
    next(createError(404));
  });
  // error handler
  app.use(function(err, req, res, next) {
    res.locals.message = err.message;
    res.locals.error = req.app.get('env') === 'development' ? err : {};
    res.status(err.status || 500);
    res.render('error');
  });
  //end of calback
});

module.exports = app;

activities.js -- a route:

var express = require('express');
var router = express.Router();
var mongodbutil = require( '../mongodbutil' );
var db = mongodbutil.getDb();

router.get('/', (req, res, next) => {  
    db.collection('activities').find().toArray((err, results) => {
        if (err) return console.log(err)
            res.render('activities', {activities: results, title: "Activities"})
    });
});

router.post('/', (req, res) => {
  db.collection('activities').save(req.body, (err, result) => {
    if (err) return console.log(err)
    res.redirect('/activities')
  })
});

module.exports = router;

Here is my setup in 2020:

./utils/database.js

const { MongoClient } = require('mongodb');

class Mongo {
    constructor () {
        this.client = new MongoClient("mongodb://127.0.0.1:27017/my-app", {
            useNewUrlParser: true,
            useUnifiedTopology: true
        });
    }

    async main () {
        await this.client.connect();
        console.log('Connected to MongoDB');

        this.db = this.client.db();
    }
}

module.exports = new Mongo();

/app.js

const mongo = require('./utils/database');
const express = require('express');

const app = express();

const boot = async () => {
    await mongo.main();
    app.listen(3000);
};

boot();

go-oleg is basically right, but in these days you (probably) dont want use "mongodb" itself, rather use some framework, which will do a lot of "dirty work" for you.

For example, mongoose is one of the most common. This is what we have in our initial server.js file :

const mongoose = require('mongoose');
const options = {server: {socketOptions: {keepAlive: 1}}};
mongoose.connect(config.db, options);

This is everything what is needed to set it up. Now use this anywhere in your code

const mongoose = require('mongoose');

And you get that instance you set up with mongoose.connect

I´m late to the party, but hopefully this answer will help someone, this is a functional code:

db.js

const MongoClient = require("mongodb").MongoClient
const urlMongo = "mongodb://localhost:27017"

var db;

function connectToServer( callback ) {
    MongoClient.connect(urlMongo,  { useUnifiedTopology: true , useNewUrlParser: true }, function( err, client ) {
        db  = client.db('auth');
        return callback( err );
    })
}

function getDb() {
    return db
}

module.exports = {connectToServer, getDb}

We export one function to connect to the mongo and another to get de instanceof the connection.

app.js

const express = require('express')
const app = express()

const mongo = require('./db.js');

mongo.connectToServer( function( err) {
  if (err) console.log(err);
  const auth = require('./modulos')

  app.post('/login', (req, res) => { auth.login(req, res)})
  app.listen(3000, function () { console.log('Corriendo en puerto 3000')})

});

We must do the require of the auth module after we initiallize the connection, otherwise the getDb function will return undefined.

module.js

const db = require('../db.js').getDb()
const usuariosCollection = db.collection('usuarios')

function login(req, res){
    usuariosCollection.find({ 'username': 'Fran' }).toArray(function (err, doc) {
        ...
    })
}

As this is tagged with Express, I thought I would mention that Express has a built in feature to share data between routes. There is an object called app.locals. We can attach properties to it and access it from inside our routes. You simply instantiate your mongo connection in your app.js file.

var app = express();

MongoClient.connect('mongodb://localhost:27017/')
.then(client =>{
  const db = client.db('your-db');
  const collection = db.collection('your-collection');
  app.locals.collection = collection;
});
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              // view engine setup
app.set('views', path.join(__dirname, 'views'));

This database connection can now be accessed within your routes as below without the need for creating and requiring additional modules.

app.get('/', (req, res) => {
  const collection = req.app.locals.collection;
  collection.find({}).toArray()
  .then(response => res.status(200).json(response))
  .catch(error => console.error(error));
});

This method ensures that you have a database connection open for the duration of your app unless you choose to close it at any time. It's easily accessible with req.app.locals.your-collection and doesn't require additional modules.

Initialize the connection as a promise:

const MongoClient = require('mongodb').MongoClient
const uri = 'mongodb://...'
const client = new MongoClient(uri)
const connection = client.connect() // initialized connection

And then call the connection whenever you wish you perform an action on the database:

    // if I want to insert into the database...
    const connect = connection
    connect.then(() => {
        const doc = { id: 3 }
        const db = client.db('database_name')
        const coll = db.collection('collection_name')
        coll.insertOne(doc, (err, result) => {
            if(err) throw err
        })
    })

Here's a suggestion using TypeScript and ES6 features and syntax:

db.ts

import { Db, MongoClient } from 'mongodb'

let client: MongoClient
let db: Db

const connectToDatabase = async () => {
    client = new MongoClient('databaseURI')
    await client.connect()
    db = client.db('dbname')
}

export {
    connectToDatabase,
    client,
    db,
}

index.ts

import express from 'express'
import { someRouter } from './routes/someRoute'
import { connectToDatabase } from './db'

connectToDatabase().then(() => {
    const app = express()
    app.use('/someRoute', someRouter)
    const port = process.env.PORT || 5000
    app.listen(port, () => {
        console.log(`Server is listening on port ${port}`)
    })
})

routes/someRoute.ts

import express from 'express'
import { db } from '../db'

const someRouter = express.Router()

someRouter.route('/')
    .get(async (req, res) => {
        const results = await db.collection('collectionName').find().toArray()
        return res.send(results)
    })

export { 
    someRouter,
}

we can create a dbconnection file like dbconnection.js

const MongoClient = require('mongodb').MongoClient
const mongo_url = process.env.MONGO_URL;

    module.exports = {
        connect: async function(callback) {
            var connection;
            await new Promise((resolve, reject) => {
                MongoClient.connect(mongo_url, {
                    useNewUrlParser: true
                }, (err, database) => {
                    if (err)
                        reject();
                    else {
                        connection = database;
                        resolve();
                    }
                });
            });
            return connection;
        }

    };

and then use this file in the your app like

var connection = require('../dbconnection');

and then use like this inside your async function

db  = await connection.connect();

hope this will work

I find this works well :)

mongoUtil.ts

import { MongoClient } from 'mongodb';
const uri =
  'MONGOSTRING';

let connPoolPromise: any = null;

const mongoPoolPromise = () => {
  if (connPoolPromise) return connPoolPromise;

  connPoolPromise = new Promise((resolve, reject) => {
    const conn = new MongoClient(uri, {
      useNewUrlParser: true,
      useUnifiedTopology: true,
    });

    if (conn.isConnected()) {
      return resolve(conn);
    } else {
      conn
        .connect()
        .then(() => {
          return resolve(conn.db('DATABASENAME'));
        })
        .catch(err => {
          console.log(err);
          reject(err);
        });
    }
  });

  return connPoolPromise;
};

export = {
  mongoPoolPromise,
};

anyFile.ts

const { mongoPoolPromise } = require('./mongoUtil');

async function getProducts() {
  const db = await mongoPoolPromise();
  const data = await db
    .collection('myCollection')
    .find({})
    .toArray();
  console.log(data);
  return data;
}

export { getProducts };

I'm a bit late for this, but I'll add my solution too. It's a much noobier approach compared to the answers here.

Anyway if you are using MongoDB version 4.0 and Node.js 3.0 (or higher versions) you can use isConnected() function from the MongoClient.

const MongoClient = require('mongodb').MongoClient;
const uri = "<your connection url>";
const client = new MongoClient(uri, { useNewUrlParser: true });

if (client.isConnected()) {
  execute();
} else {
  client.connect().then(function () {
    execute();
  });
}

function execute() {
    // Do anything here
    // Ex: client.db("mydb").collection("mycol");
}

This worked fine for me. Hope it helps.

Based on accepted answers, I use a simple approach. But use this only if you want to use db inside function which will be executed after some time. For ex: In express route functions, it is the easiest approach you can take.

mongo.js

const MongoClient = require("mongodb").MongoClient

var db

const connectDb = (callback) => {
	if (db) return callback()
	MongoClient.connect( uri, {ops}, 
		(err, database) => {
			if (err) return console.log(err)
			db = database.db("dbName") 
			console.log("Database Connected")
			callback()
		}
	)
}

const getDb = (collectionToGet) => {
	return db.collection(collectionToGet)
}

module.exports = {
	connectDb,
	getDb,
}

Now, in other files where you want the db object,

user.js

const { connectDb, getDb } = require('mongo.js')

var db // store db object in this object
connectDb(() => ( db = getDb("user") ))

app.get('/', (req, res) => {
    // do something with req 
    db.insert({})
    // do something with res
}

If you opt for using mongoose in your application edit your app.js file with the following snippet

app.js

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/Your_Data_Base_Name', {useNewUrlParser:true})
  .then((res) => {
    console.log(' ########### Connected to mongDB ###########');
  })
  .catch((err) => {
    console.log('Error in connecting to mongoDb' + err);
  });`

Next Step: Define Models for your application require them and perform CRUD operation directly for example

blogSchema.js

 const mongoose = require('mongoose');
 const Schema = mongoose.Schema;
 const blogSchema = new Schema({
     _id : mongoose.Schema.Types.ObjectId,
     title : {
        type : 'String',
        unique : true,
        required : true       
    },
    description : String,
        comments : [{type : mongoose.Schema.Types.ObjectId, ref: 'Comment'}]
 });
 module.exports = mongoose.model('Blog', blogSchema);

Usage createBlog.js

const Blog = require('../models/blogSchema');
exports.createBlog = (req, res, next) => {
const blog = new Blog({
  _id : new mongoose.Types.ObjectId,
  title : req.body.title,
  description : req.body.description,
});
blog.save((err, blog) => {
  if(err){
    console.log('Server Error save fun failed');
    res.status(500).json({
      msg : "Error occured on server side",
      err : err
    })
  }else{
    //do something....
  }

U don't need to connect to mogoDB always ....

var MongoClient = require('mongodb').MongoClient;
var url = 'mongodb://localhost:27017/';
var Pro1;

module.exports = {
    DBConnection:async function()
    {
        Pro1 = new Promise(async function(resolve,reject){
            MongoClient.connect(url, { useNewUrlParser: true },function(err, db) {
                if (err) throw err;
                resolve(db);
            });        
        });
    },
    getDB:async function(Blockchain , Context)
    {
        bc = Blockchain;
        contx = Context;
        Pro1.then(function(_db)
        {
            var dbo = _db.db('dbname');
            dbo.collection('collectionname').find().limit(1).skip(0).toArray(function(err,result) {
                if (err) throw err;
                console.log(result);
            });
        });
    },
    closeDB:async function()
    {
        Pro1.then(function(_db){
            _db.close();
        });
    }
};

const express = require('express')
const server = express()
const mongoClient = require('./MongoDB.js').client
const port = 3000
;(async () => {
    await mongoClient.connect()
    server.listen(port, () => console.log(`Server is listening on port ${port}!`))
})().catch(console.error)

You can use the Singleton Design Pattern to achive cross file usage of your MongoDB connection.

Init.mjs

/* ################ Controller ################ */
import ctrlLib from '../../controller/lib.mjs';

/* ################ MongoDB ################ */
import mongodb from 'mongodb';

/* ################ Logs ################ */
import log from 'fancy-log';
import chalk from 'chalk';


/** Init MongoDB connection */
export class Init {
  /**
   * Check if its first time usage of this class.
   If true set class instance to this that we always get same instance.
   * Then get MongoDB details from config.yml and set as global.
   * In the last step we return the class instance.
  */
  constructor() {
    if (Init.instance == null) Init.instance = this;

    const config = ctrlLib.getConfig();
    this.MongoURL = config.MongoDB.url;
    this.MongoName = config.MongoDB.dbname;
    ({MongoClient: this.MongoClient} = mongodb);

    return Init.instance;
  }; // constructor(){


  /** Connect to Database and return connection */
  async connect() {
    try {
      const client = await this.MongoClient.connect(
          this.MongoURL, {useNewUrlParser: true, useUnifiedTopology: true},
      );

      this.connection = {'db': client.db(this.MongoName), 'client': client};
      return this.connection;
    } // try {
    catch (e) {
      log( `${chalk.red.bold('❌ ERROR')} while try to connect to MongoDB DB
      ${chalk.white.bold('Error:\n')} ${e}` );
    } // catch (e) {
  }; // async connect() {


  /**
   * Return connection for cross file usage
   * @return {object}
  */
  getConnection() {return this.connection;};
}; // export class Init {

app.mjs

• Make sure to 1x time create your MongoDB connection anywhere inside of your project that you can use it later in other files.

/* ################ Services ################ */
import {Init} from './Init.mjs';

(async ()=>{
  await new Init().connect();
})().catch(e=>{log('app.mjs - Catch error: ' + e);});

anyOtherFile.mjs

/* ################ Services ################ */
import {Init} from './Init.mjs';

/** Subclass of Search which contains lib functions */
class Lib {
  /**
   * Find data by using search query and return result.
   * @param {string} collection - Name of collection
   * @param {object} query - Search query
  */
  async findOne(collection, query) {
    const connection = new Init().getConnection();
    return await connection.db.collection(collection).findOne(query);
  }; // async findOne() {
}; // class Lib {

I tried @go-oleg answer and it works pretty well. Inside getDb() , I make sure _db must be defined. And if not defined, I call the connectToServer() so that it will get defined again. After this I don't have to call connectToServer() in the app.js which makes my code clean.

let getDb = async() => {
    if(_db) {
        return _db
    } else {
        _db = await connectToServer()
        return _db
    }
}

And then, I simply call getDb() everywhere. Also, What I observed, It takes about 64ms on first call. After first call it takes about, 2-6ms everytime.

I answered here because i have less reputation to comment.

all after long effort my working by this operational method: Please follow this link this is also good solution: https://mrvautin.com/re-use-mongodb-database-connection-in-routes/