Update to use the latest MongoDB best practices to limit connection pooling issues. (#28350)

* Update to use the latest MongoDB best practices.

* lint-fix

Co-authored-by: Lee Robinson <me@leerob.io>
Co-authored-by: jj@jjsweb.site <jj@jjsweb.site>

Author: 3 people

examples/with-mongodb/.env.local.example

@@ -1,2 +1 @@
-MONGODB_URI=
-MONGODB_DB=
+MONGODB_URI=

examples/with-mongodb/README.md

@@ -11,7 +11,7 @@ If you want to learn more about MongoDB, visit the following pages:
 
 Once you have access to the environment variables you'll need, deploy the example using [Vercel](https://vercel.com?utm_source=github&utm_medium=readme&utm_campaign=next-example):
 
-[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/git/external?repository-url=https://github.com/vercel/next.js/tree/canary/examples/with-mongodb&project-name=with-mongodb&repository-name=with-mongodb&env=MONGODB_URI,MONGODB_DB&envDescription=Required%20to%20connect%20the%20app%20with%20MongoDB)
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/git/external?repository-url=https://github.com/vercel/next.js/tree/canary/examples/with-mongodb&project-name=with-mongodb&repository-name=with-mongodb&env=MONGODB_URI&envDescription=Required%20to%20connect%20the%20app%20with%20MongoDB)
 
 ## How to use
 
@@ -40,7 +40,6 @@ cp .env.local.example .env.local
 Set each variable on `.env.local`:
 
 - `MONGODB_URI` - Your MongoDB connection string. If you are using [MongoDB Atlas](https://mongodb.com/atlas) you can find this by clicking the "Connect" button for your cluster.
-- `MONGODB_DB` - The name of the MongoDB database you want to use.
 
 ### Run Next.js in development mode
 
@@ -56,7 +55,7 @@ yarn dev
 
 Your app should be up and running on [http://localhost:3000](http://localhost:3000)! If it doesn't work, post on [GitHub discussions](https://github.com/vercel/next.js/discussions).
 
-You will either see a message stating "You are connected to MongoDB" or "You are NOT connected to MongoDB". Ensure that you have provided the correct `MONGODB_URI` and `MONGODB_DB` environment variables.
+You will either see a message stating "You are connected to MongoDB" or "You are NOT connected to MongoDB". Ensure that you have provided the correct `MONGODB_URI` environment variable.
 
 When you are successfully connected, you can refer to the [MongoDB Node.js Driver docs](https://mongodb.github.io/node-mongodb-native/3.4/tutorials/collections/) for further instructions on how to query your database.
 

examples/with-mongodb/lib/mongodb.js

@@ -1,49 +1,32 @@
 import { MongoClient } from 'mongodb'
 
-const MONGODB_URI = process.env.MONGODB_URI
-const MONGODB_DB = process.env.MONGODB_DB
-
-if (!MONGODB_URI) {
-  throw new Error(
-    'Please define the MONGODB_URI environment variable inside .env.local'
-  )
-}
-
-if (!MONGODB_DB) {
-  throw new Error(
-    'Please define the MONGODB_DB environment variable inside .env.local'
-  )
+const uri = process.env.MONGODB_URI
+const options = {
+  useUnifiedTopology: true,
+  useNewUrlParser: true,
 }
 
-/**
- * Global is used here to maintain a cached connection across hot reloads
- * in development. This prevents connections growing exponentially
- * during API Route usage.
- */
-let cached = global.mongo
+let client
+let clientPromise
 
-if (!cached) {
-  cached = global.mongo = { conn: null, promise: null }
+if (!process.env.MONGODB_URI) {
+  throw new Error('Please add your Mongo URI to .env.local')
 }
 
-export async function connectToDatabase() {
-  if (cached.conn) {
-    return cached.conn
+if (process.env.NODE_ENV === 'development') {
+  // In development mode, use a global variable so that the value
+  // is preserved across module reloads caused by HMR (Hot Module Replacement).
+  if (!global._mongoClientPromise) {
+    client = new MongoClient(uri, options)
+    global._mongoClientPromise = client.connect()
   }
-
-  if (!cached.promise) {
-    const opts = {
-      useNewUrlParser: true,
-      useUnifiedTopology: true,
-    }
-
-    cached.promise = MongoClient.connect(MONGODB_URI, opts).then((client) => {
-      return {
-        client,
-        db: client.db(MONGODB_DB),
-      }
-    })
-  }
-  cached.conn = await cached.promise
-  return cached.conn
+  clientPromise = global._mongoClientPromise
+} else {
+  // In production mode, it's best to not use a global variable.
+  client = new MongoClient(uri, options)
+  clientPromise = client.connect()
 }
+
+// Export a module-scoped MongoClient promise. By doing this in a
+// separate module, the client can be shared across functions.
+export default clientPromise

examples/with-mongodb/pages/index.js

@@ -1,5 +1,5 @@
 import Head from 'next/head'
-import { connectToDatabase } from '../lib/mongodb'
+import clientPromise from '../lib/mongodb'
 
 export default function Home({ isConnected }) {
   return (
@@ -223,7 +223,13 @@ export default function Home({ isConnected }) {
 }
 
 export async function getServerSideProps(context) {
-  const { client } = await connectToDatabase()
+  const client = await clientPromise
+
+  // client.db() will be the default database passed in the MONGODB_URI
+  // You can change the database by calling the client.db() function and specifying a database like:
+  // const db = client.db("myDatabase");
+  // Then you can execute queries against your database like so:
+  // db.find({}) or any of the MongoDB Node Driver commands
 
   const isConnected = await client.isConnected()