Error: server instance pool gets destroyed in nested db collection functions - node.js

Question

I have searched for the solution of the error specified in title.
MongoError: server instance pool was destroyed
I believe it is because misplacement of db.close(). But I am nesting dbo.collection and unable to get the exact solution of this error.
Firstly, I am fetching data (array of ids having status 0) from database and then I am concatenating (each app-id) them one by one with URL to get desired appUrl which will be used for crawling data one by one and then crawled data is meant to be stored into another collection of mongoDB. This process will repeat for each id in the array. But my code is having error of "server instance pool gets destroyed" before storing data into collection. I am doing misplacement of db.close() but I am unable to resolve this. Please help me resolving this error
Here is my code
///* global sitehead */
const request = require('request');
const cheerio = require('cheerio');
//const response = require('response');
const fs = require('fs');
const express = require('express');
const app = express();
var MongoClient = require('mongodb').MongoClient;
var url = "mongodb://localhost:27017/";
var dateTime = require('node-datetime');
MongoClient.connect(url, {useNewUrlParser: true}, function (err, db) {
if (err) {
throw err;
} else {
var dbo = db.db("WebCrawler");
var app_id;
var appUrl;
let arr = [];
dbo.collection("Unique_Apps").find({"Post_Status": 0}, {projection: {_id: 0, App_Id: 1}}).toArray(function (err, result)
{
// console.log(result);
if (err) {
throw err;
// console.log(err);
} else {
for (var i = 0; i < result.length; i++)
{
arr[i] = result[i];
}
arr.forEach((el) => {
app_id = el.App_Id;
//console.log(app_id);
appUrl = 'https://play.google.com/store/apps/details?id=' + app_id;
console.log(appUrl);
request(appUrl, function (error, response, html) {
if (!error && response.statusCode === 200) {
//START Crawling ###########
const $ = cheerio.load(html); //cheerio
const appTitle = $('.AHFaub');
const iconUrl = $('.T75of.sHb2Xb').attr("src");
const developedBy = $('.T32cc.UAO9ie').children().eq(0);
const category = $('.T32cc.UAO9ie').children().eq(1);
//store in database collection: "Single_App_Data_Post"
var curr_obj = {App_Id: app_id, App_Name: appTitle.text(),
Icon_Url: iconUrl, Price: "Free", Developed_By: developedBy.text(),
Category: category.text()
};
dbo.collection("Single_App_Data_Post").insertOne(curr_obj, function (err, res) {
console.log("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
if (err) {
throw err;
// console.log(err);
} else {
console.log("inserted....");
} //main else
});
dbo.collection("Unique_Apps").updateOne({App_Id: app_id}, {$set: {Post_Status: 0}}, function (err, res) {
if (err)
throw err;
console.log("1 document updated");
//dbo.close();
});
} else
{
throw error;
}
});
});
}
db.close();
});
} //else
}); //mongoClient connect db
Output

Answer 1

The following is a good start about how to turn callback into promises. Try to use it, execute the code block, by block, understand it and then add your updateOne/insertOne requests into it.
const request = require('request');
const cheerio = require('cheerio');
const fs = require('fs');
const express = require('express');
const app = express();
const MongoClient = require('mongodb').MongoClient;
const dateTime = require('node-datetime');
// Class used to handle the database basic interractions
class DB {
constructor() {
this.db = false;
this.url = "mongodb://localhost:27017/";
}
// Do connect to the database
connect() {
return new Promise((resolve, reject) => {
MongoClient.connect(this.url, {
useNewUrlParser: true,
}, (err, db) => {
if (err) {
console.log('error mongodb connect');
return reject(err);
}
this.db = db;
return resolve(db);
});
});
}
disconnect() {
db.close();
this.db = false;
}
getCollection(name) {
return this.db.db(name);
}
}
// Get the data from the database
function getAppsIds(dbObj) {
return new Promise((resolve, reject) => {
const dbo = dbObj.getCollection('WebCrawler');
dbo.collection('Unique_Apps').find({
'Post_Status': 0,
}, {
projection: {
_id: 0,
App_Id: 1,
}
}).toArray(function(err, result) {
if (err) {
return reject(err);
}
return resolve(result);
});
});
}
function requestPlayStore(idApp) {
return new Promise((resolve, reject) => {
const appUrl = `https://play.google.com/store/apps/details?id=${app_id}`;
request(appUrl, function(error, response, html) {
if (error || response.statusCode !== 200) {
return reject(error);
}
return resolve({
response,
html,
});
});
});
}
// Do treat one id app at a time
function treatOneIdApp(dbObj, idApp) {
return requestPlayStore(idApp)
.then(({
response,
html,
}) => {
// Perform your requests here updateOne and insertOne ...
});
}
const dbObj = new DB();
dbObj.connect()
.then(() => getAppsIds(dbObj))
.then(rets => Promise.all(rets.map(x => treatOneIdApp(dbObj, x.App_Id))))
.then(() => dbObj.disconnect())
.catch((err) => {
console.log(err);
});