MongoClient throw MongoError: server instance pool was destroyed - node.js

Question

I saw these posts on SO describing this error. Most of them was by the reason that JavaScript is async and mongoClient.close() called outside of callback. That's not my case, but I don't know what else can be the reason.
const MongoClient = require('mongodb').MongoClient;
const url = "mongodb://localhost:27017/";
const mongoClient = new MongoClient(url, {
useNewUrlParser: true
});
module.exports = class Mongo {
insertOne(article) {
mongoClient.connect((err, client) => {
const db = client.db('grabber');
db.collection("zr").insertOne(article, (err, res) => {
if (err) throw err;
mongoClient.close();
});
});
};
}

Answer 1

I observed that you open mongoClient.connect() in the insertOne() method, and also call mongoClient.close() within that method, with mongoClient as a global variable.
My hunch is that either:
There's another method that calls mongoClient that was closed by this method, or
You called the insertOne(article) twice
I can confirm that the second reason is the most likely one. Here's the code I tried:
const MongoClient = require('mongodb').MongoClient;
const url = "mongodb://localhost:27017/";
const mongoClient = new MongoClient(url, {
useNewUrlParser: true
});
class Mongo {
insertOne(article) {
mongoClient.connect((err, client) => {
const db = client.db('grabber');
db.collection("zr").insertOne(article, (err, res) => {
if (err) throw err;
mongoClient.close();
});
});
};
};
x = new Mongo()
setTimeout(function() { x.insertOne({'a': 1}); }, 1000);
setTimeout(function() { x.insertOne({'a': 2}); }, 2000);
The two setTimeout was there to ensure that the two insertOne() are called one after another. Result:
MongoError: server instance pool was destroyed
The way your code is currently structured, the node driver creates a new connection pool every time insertOne() is called. This is not optimal, and prevents the node driver to utilize connection pooling.
Instead of calling mongoClient.connect() inside insertOne(), call it globally outside of class Mongo. Pass the global connection object (the returned object from mongoClient.connect()) instead of the mongoClient object itself to your insertOne() method.