如果你有Express.js，你可以使用express- MongoDB -db在请求之间缓存和共享MongoDB连接，而不需要一个池(因为接受的答案说这是共享连接的正确方式)。

如果没有，你可以查看它的源代码并在另一个框架中使用它。

npm i express mongoose

mongodb.js

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

mongoose.set('strictQuery', true);
mongoose.connect('mongodb://localhost:27017/db_name', {
    useNewUrlParser: true, 
    useUnifiedTopology: true
})
.then(() => console.log('MongoDB Connected...'))
.catch((err) => console.log(err))

app.listen(3000,()=>{ console.log("Started on port 3000 !!!") })

node mongodb.js

当Node.js应用程序启动时，打开一个新的连接，并重用现有的db连接对象:

/ server.js

import express from 'express';
import Promise from 'bluebird';
import logger from 'winston';
import { MongoClient } from 'mongodb';
import config from './config';
import usersRestApi from './api/users';

const app = express();

app.use('/api/users', usersRestApi);

app.get('/', (req, res) => {
  res.send('Hello World');
});

// Create a MongoDB connection pool and start the application
// after the database connection is ready
MongoClient.connect(config.database.url, { promiseLibrary: Promise }, (err, db) => {
  if (err) {
    logger.warn(`Failed to connect to the database. ${err.stack}`);
  }
  app.locals.db = db;
  app.listen(config.port, () => {
    logger.info(`Node.js app is listening at http://localhost:${config.port}`);
  });
});

/api/users.js

import { Router } from 'express';
import { ObjectID } from 'mongodb';

const router = new Router();

router.get('/:id', async (req, res, next) => {
  try {
    const db = req.app.locals.db;
    const id = new ObjectID(req.params.id);
    const user = await db.collection('user').findOne({ _id: id }, {
      email: 1,
      firstName: 1,
      lastName: 1
    });

    if (user) {
      user.id = req.params.id;
      res.send(user);
    } else {
      res.sendStatus(404);
    }
  } catch (err) {
    next(err);
  }
});

export default router;

来源:如何在Node.js/Express App中打开数据库连接

Manage mongo connection pools in a single self contained module. This approach provides two benefits. Firstly it keeps your code modular and easier to test. Secondly your not forced to mix your database connection up in your request object which is NOT the place for a database connection object. (Given the nature of JavaScript I would consider it highly dangerous to mix in anything to an object constructed by library code). So with that you only need to Consider a module that exports two methods. connect = () => Promise and get = () => dbConnectionObject.

使用这样的模块，您可以首先连接到数据库

// runs in boot.js or what ever file your application starts with
const db = require('./myAwesomeDbModule');
db.connect()
    .then(() => console.log('database connected'))
    .then(() => bootMyApplication())
    .catch((e) => {
        console.error(e);
        // Always hard exit on a database connection error
        process.exit(1);
    });

当你的应用程序运行时，当它需要一个DB连接时，你可以简单地调用get()。

const db = require('./myAwesomeDbModule');
db.get().find(...)... // I have excluded code here to keep the example  simple

如果你像下面这样设置你的db模块，你不仅可以确保你的应用程序不会启动，除非你有一个数据库连接，你也有一个全局的方式来访问你的数据库连接池，如果你没有连接，它会出错。

// myAwesomeDbModule.js
let connection = null;

module.exports.connect = () => new Promise((resolve, reject) => {
    MongoClient.connect(url, option, function(err, db) {
        if (err) { reject(err); return; };
        resolve(db);
        connection = db;
    });
});

module.exports.get = () => {
    if(!connection) {
        throw new Error('Call connect first!');
    }

    return connection;
}

使用下面的方法，您可以轻松地管理尽可能多的连接

var mongoose = require('mongoose');


//Set up default mongoose connection
const bankDB = ()=>{
    return  mongoose.createConnection('mongodb+srv://<username>:<passwprd>@mydemo.jk4nr.mongodb.net/<database>?retryWrites=true&w=majority',options);
    
}

bankDB().then(()=>console.log('Connected to mongoDB-Atlas bankApp...'))
       .catch((err)=>console.error('Could not connected to mongoDB',err));
       
//Set up second mongoose connection
const myDB = ()=>{
    return  mongoose.createConnection('mongodb+srv://<username>:<password>@mydemo.jk4nr.mongodb.net/<database>?retryWrites=true&w=majority',options);
   
}
myDB().then(()=>console.log('Connected to mongoDB-Atlas connection 2...'))
       .catch((err)=>console.error('Could not connected to mongoDB',err));

module.exports = { bankDB(), myDB() };

如果有人想在2021年用Typescript工作，这是我正在使用的:

import { MongoClient, Collection } from "mongodb";

const FILE_DB_HOST = process.env.FILE_DB_HOST as string;
const FILE_DB_DATABASE = process.env.FILE_DB_DATABASE as string;
const FILES_COLLECTION = process.env.FILES_COLLECTION as string;

if (!FILE_DB_HOST || !FILE_DB_DATABASE || !FILES_COLLECTION) {
  throw "Missing FILE_DB_HOST, FILE_DB_DATABASE, or FILES_COLLECTION environment variables.";
}

const client = new MongoClient(FILE_DB_HOST, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

class Mongoose {
  static FilesCollection: Collection;

  static async init() {
    const connection = await client.connect();
    const FileDB = connection.db(FILE_DB_DATABASE);
    Mongoose.FilesCollection = FileDB.collection(FILES_COLLECTION);
  }
}


Mongoose.init();

export default Mongoose;

我相信如果请求发生得太快(在蒙古.init()有时间完成之前)，将会抛出一个错误，因为Mongoose。FilesCollection将未定义。

import { Request, Response, NextFunction } from "express";
import Mongoose from "../../mongoose";

export default async function GetFile(req: Request, res: Response, next: NextFunction) {
  const files = Mongoose.FilesCollection;
  const file = await files.findOne({ fileName: "hello" });
  res.send(file);
}

例如，如果您调用文件。findOne({…})和猫鼬。FilesCollection未定义，那么您将得到一个错误。