If you've ever used MongoDB with NextJS, you may recognise the following code from the official Next/Mongo example project template.
What you may not realise is that this example code will cause you a headache at some point.
import { MongoClient } from 'mongodb';
const uri = process.env.MONGODB_URI;
const options = {};
let client
let clientPromise
if (!process.env.MONGODB_URI) {
throw new Error('Please add your Mongo URI to .env.local')
}
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()
}
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 clientPromiseThe problem: Server restarts can break your site.
If the Mongo server is ready before the Next / Express site, everything will be just fine.
In a nutshell, if the server restarts and the Next app comes up before Mongo is ready, the connection attempt will fail and the promise will reject. This will semi-permanently break your database connection, disabling your site until the Next app restarted.
Why does this happen?
If you come from a PHP background (or similar) you'll be used to each request typically spawning a completely new instance of server-side code, including a new database connection. However with node-based servers, the singleton database connection persists across requests as node servers typically run as a daemon.
How can I fix it?
We can use a library called promise-retry which will detect the rejected promise, then reattempt the connection.
import { MongoClient } from 'mongodb';
import promiseRetry from 'promise-retry';
const uri = process.env.MONGODB_URI || '';
const options = {}
const promiseRetryOptions = {
retries: 10,
factor: 1, // sets the factor for exponential backoff
minTimeout: 1000,
maxTimeout: 2000
}
let client;
let clientPromise;
if (!process.env.MONGODB_URI) {
throw new Error('Please add your Mongo URI to .env.local')
}
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 = promiseRetry((retry, number) => {
console.log(`Mongo (dev) connection attempt number: ${number}`);
return client.connect().catch(retry)
}, promiseRetryOptions).then((promise) => {
console.log('Mongo (dev) successfully connected.');
return promise;
});
}
clientPromise = global._mongoClientPromise
} else {
// In production mode, it's best to not use a global variable.
client = new MongoClient(uri, options)
clientPromise = promiseRetry((retry, number) => {
console.log(`Mongo (prod) connection attempt number: ${number}`)
return client.connect().catch(retry)
}, promiseRetryOptions).then((promise) => {
console.log('Mongo (prod) successfully connected.');
return promise;
});
}
// Export a module-scoped MongoClient promise. By doing this in a
// separate module, the client can be shared across functions.
export default clientPromiseLuke Taylor is a software developer specialising in web. He mostly works with Next.js but has experience with a range of languages and frameworks. Based in London, UK.