How to connect to Mongodb reliably in a serverless setup? - node.js

8 out of ten times everything connects well. That said, I sometimes get a MongoClient must be connected before calling MongoClient.prototype.db error. How should I change my code so it works reliably (100%)?
I tried a code snippet from one of the creators of the Now Zeit platform.
My handler
const { send } = require('micro');
const { handleErrors } = require('../../../lib/errors');
const cors = require('../../../lib/cors')();
const qs = require('micro-query');
const mongo = require('../../../lib/mongo');
const { ObjectId } = require('mongodb');
const handler = async (req, res) => {
let { limit = 5 } = qs(req);
limit = parseInt(limit);
limit = limit > 10 ? 10 : limit;
const db = await mongo();
const games = await db
.collection('games_v3')
.aggregate([
{
$match: {
removed: { $ne: true }
}
},
{ $sample: { size: limit } }
])
.toArray();
send(res, 200, games);
};
module.exports = handleErrors(cors(handler));
My mongo script that reuses the connection in case the lambda is still warm:
// Based on: https://spectrum.chat/zeit/now/now-2-0-connect-to-database-on-every-function-invocation~e25b9e64-6271-4e15-822a-ddde047fa43d?m=MTU0NDkxODA3NDExMg==
const MongoClient = require('mongodb').MongoClient;
if (!process.env.MONGODB_URI) {
throw new Error('Missing env MONGODB_URI');
}
let client = null;
module.exports = function getDb(fn) {
if (client && !client.isConnected) {
client = null;
console.log('[mongo] client discard');
}
if (client === null) {
client = new MongoClient(process.env.MONGODB_URI, {
useNewUrlParser: true
});
console.log('[mongo] client init');
} else if (client.isConnected) {
console.log('[mongo] client connected, quick return');
return client.db(process.env.MONGO_DB_NAME);
}
return new Promise((resolve, reject) => {
client.connect(err => {
if (err) {
client = null;
console.error('[mongo] client err', err);
return reject(err);
}
console.log('[mongo] connected');
resolve(client.db(process.env.MONGO_DB_NAME));
});
});
};
I need my handler to be 100% reliable.

if (client && !client.isConnected) {
client = null;
console.log('[mongo] client discard');
}
This code can cause problems! Even though you're setting client to null, that client still exists, will continue connecting to mongo, will not be garbage collected, and its callback connection code will still run, but in its callback client will refer to the next client that's created that is not necessarily connected.
A common pattern for this kind of code is to only ever return a single promise from the getDB call:
let clientP = null;
function getDb(fn) {
if (clientP) return clientP;
clientP = new Promise((resolve, reject) => {
client = new MongoClient(process.env.MONGODB_URI, {
useNewUrlParser: true
});
client.connect(err => {
if (err) {
console.error('[mongo] client err', err);
return reject(err);
}
console.log('[mongo] connected');
resolve(client.db(process.env.MONGO_DB_NAME));
});
});
return clientP;
};

I had the same issue. In my case it was caused by calling getDb() before a previous getDb() call had returned. In this case, I believe that 'client.isConnected' returns true, even though it is still connecting.
This was caused by forgetting to put an 'await' before the getDb() call in one location. I tracked down which by outputting a callstack from getDb using:
console.log(new Error().stack);
I don't see the same issue in the sample code in the question, though it could be triggered by another bit of code that isn't shown.

I have written this article talking about serverless, lambda e db connections. There are some good concepts which could help you to find the root cause of your problem. There are also example and use cases of how to mitigate connection pool issues.
Just by looking your code I can tell it is missing this:
context.callbackWaitsForEmptyEventLoop = false;
Serverless: Dynamodb x Mongodb x Aurora serverless

Related

Mongodb native connections from nodejs return undefined databases list

I'm just starting to use Mongodb without mongoose (to get away from the schemas), and wanted to create a simple module with various exported functions to use in the rest of my app. I've pasted the code below.
The problem I'm having is that the databasesList.databases comes back as undefined, and I'm not sure why. There should be 2 databases on my cluster, and one collection in each database.
As a tangential question, I thought maybe I would check the collections instead (now commented out), but though I found this page (https://docs.mongodb.com/manual/reference/method/db.getCollectionNames/) the function getCollectionNames seems not to exist. Now I'm wondering if I'm using the wrong documentation and that is why my databases are coming back undefined.
const client = new MongoClient(uri)
const connection = client.connect( function (err, database) {
if (err) throw err;
else if (!database) console.log('Unknown error connecting to database');
else {
console.log('Connected to MongoDB database server');
}
});
module.exports = {
getDatabaseList: function() {
console.log('start ' + client);
databasesList = client.db().admin().listDatabases();
//collectionList = client.db().getCollectionNames();
//console.log("Collections: " + collectionList);
console.log("Databases: " + databasesList.databases);
//databasesList.databases.forEach(db => console.log(` - ${db.name}`));
}
}```
your code is correct Just need to change few things.
module.exports = {
getDatabaseList: async function() {
console.log('start ' + client);
databasesList = await client.db().admin().listDatabases();
//collectionList = await client.db().getCollectionNames();
//console.log("Collections: " + collectionList);
console.log("Databases: " + databasesList.databases);
databasesList.databases.forEach(db => console.log(` - ${db.name}`));
}
}
You have to declare async function and use await also.
The async and await keywords enable asynchronous, promise-based behaviour to be written in a cleaner style, avoiding the need to explicitly configure promise chains.
You can use this modular approach to build your database access code:
index.js: Run your database application code, like list database names, collection names and read from a collection.
const connect = require('./database');
const dbFunctions = require('./dbFunctions');
const start = async function() {
const connection = await connect();
console.log('Connected...');
const dbNames = await dbFunctions.getDbNames(connection);
console.log(await dbNames.databases.map(e => e.name));
const colls = await dbFunctions.getCollNames(connection, 'test');
console.log(await colls.map(e => e.name));
console.log(await dbFunctions.getDocs(connection, 'test', 'test'));
};
start();
database.js:: Create a connection object. This connection is used for all your database access code. In general, a single connection creates a connection pool and this can be used throughout a small application
const { MongoClient } = require('mongodb');
const url = 'mongodb://localhost:27017/';
const opts = { useUnifiedTopology: true };
async function connect() {
console.log('Connecting to db server...');
return await MongoClient.connect(url, opts );
}
module.exports = connect;
dbFunctions.js:: Various functions to access database details, collection details and query a specific collection.
module.exports = {
// return list of database names
getDbNames: async function(conn) {
return await conn.db().admin().listDatabases( { nameOnly: true } );
},
// return collections list as an array for a given database
getCollNames: async function(conn, db) {
return await conn.db(db).listCollections().toArray();
},
// return documents as an array for a given database and collection
getDocs: async function(conn, db, coll) {
return await conn.db(db).collection(coll).find().toArray();
}
}

MongoError: pool destroyed when fetching all data without conditions

I am new to mongoDb, as I am trying to query from different collection and in order to do that, when I am fetching data from category collection I mean when I am running select * from collection it is throwing error, MongoError: pool destroyed.
As per my understanding it is because of some find({}) is creating a pool and that is being destroyed.
The code which I am using inside model is below,
const MongoClient = require('mongodb').MongoClient;
const dbConfig = require('../configurations/database.config.js');
export const getAllCategoriesApi = (req, res, next) => {
return new Promise((resolve, reject ) => {
let finalCategory = []
const client = new MongoClient(dbConfig.url, { useNewUrlParser: true });
client.connect(err => {
const collection = client.db(dbConfig.db).collection("categories");
debugger
if (err) throw err;
let query = { CAT_PARENT: { $eq: '0' } };
collection.find(query).toArray(function(err, data) {
if(err) return next(err);
finalCategory.push(data);
resolve(finalCategory);
// db.close();
});
client.close();
});
});
}
When my finding here is when I am using
let query = { CAT_PARENT: { $eq: '0' } };
collection.find(query).toArray(function(err, data) {})
When I am using find(query) it is returning data but with {} or $gte/gt it is throwing Pool error.
The code which I have written in controller is below,
import { getAllCategoriesListApi } from '../models/fetchAllCategory';
const redis = require("redis");
const client = redis.createClient(process.env.REDIS_PORT);
export const getAllCategoriesListData = (req, res, next, query) => {
// Try fetching the result from Redis first in case we have it cached
return client.get(`allstorescategory:${query}`, (err, result) => {
// If that key exist in Redis store
if (false) {
res.send(result)
} else {
// Key does not exist in Redis store
getAllCategoriesListApi(req, res, next).then( function ( data ) {
const responseJSON = data;
// Save the Wikipedia API response in Redis store
client.setex(`allstorescategory:${query}`, 3600, JSON.stringify({ source: 'Redis Cache', responseJSON }));
res.send(responseJSON)
}).catch(function (err) {
console.log(err)
})
}
});
}
Can any one tell me what mistake I am doing here. How I can fix pool issue.
Thanking you in advance.
I assume that toArray is asynchronous (i.e. it invokes the callback passed in as results become available, i.e. read from the network).
If this is true the client.close(); call is going to get executed prior to results having been read, hence likely yielding your error.
The close call needs to be done after you have finished iterating the results.
Separately from this, you should probably not be creating the client instance in the request handler like this. Client instances are expensive to create (they must talk to all of the servers in the deployment before they can actually perform queries) and generally should be created per running process rather than per request.

MongoError: Topology is closed, please connect despite established database connection

I am writing a web application that uses asynchronous database requests as a part of the api. Currently, I have an async express route that awaits function returns from async functions. Both of these functions return booleans and both query the database. One works correctly, however the second one does not.
Here is the MongoClient setup:
const MongoClient = require('mongodb').MongoClient;
const uri = config.uri; // Contains custom url for accessing database
const client = new MongoClient(uri, { useUnifiedTopology: true}, { useNewUrlParser: true }, { connectTimeoutMS: 30000 }, { keepAlive: 1});
where config is from a file imported as.
const config = require("./config.js");
and functions properly.
Here is the express setup:
app.post("/signup", async function(request, response) {
log("POST request at /signup");
log("BEFORE UNIQUE USER");
const isUniqueUser = await validateUniqueUser(request.body.email, request.body.password);
log(isUniqueUser);
const status = {
status: null
};
if (isUniqueUser) {
log("AFTER UNIQUE USER");
let userCreated = await createPracticeProfile(request.body.email, request.body.password);
log("user created: " + userCreated);
if (userCreated) {
status.status = "user_created";
}
response.json(status);
} else {
response.json(status);
}
console.log("********************************end");
});
The console outputs:
BEFORE UNIQUE USER
true (which it should be)
AFTER UNIQUE USER
MongoError: Topology is closed.
user created: undefined
***...***end
Here is the function for validating that a user is unique:
/* VALIDATE_UNIQUE_USER
USE: ensure user does not have existing profile
PARAMS: email (string), password (string)
RETURN: isUniqueUser (bool)
*/
async function validateUniqueUser(email, password) {
// connect to database
const database = await client.connect().catch(err => {
log("ERROR while connecting to database at: validateUniqueUser");
console.log(err);
client.close();
});
// database connection failed
if (!database) {
return false;
}
// connection successful => find user
let user;
try {
user = await database.db("guitar-practice-suite").collection("users").findOne({email: email});
} catch(err) {
log("ERROR while finding user in database at: validateUniqueUser");
console.log(err);
client.close();
return false;
} finally {
client.close();
// user not found (unique)
if (user === null || user === undefined) {
return true;
}
return false;
}
}
Here is the function for inserting the user into the collections:
/* CREATE_PRACTICE_PROFILE
USE: insert a practice profile into the database
PARAMS: email (string), password (string)
RETURN: userCreated (bool)
*/
async function createPracticeProfile(email, password) {
// hash password
let hashedPassword;
try {
hashedPassword = await new Promise((resolve, reject) => {
bcrypt.hash(password, null, null, function(err, hash) {
if (err) {
reject(err);
}
resolve(hash)
});
});
} catch(err) {
log("ERROR while hashing password at: createPracticeProfile");
console.log(err);
return false;
}
// connect to database
const database = await client.connect().catch(err => {
log("ERROR while connecting to database at: validateUniqueUser");
console.log(err);
client.close();
});
// database connection failed
if (!database) {
return false;
}
// database connection successful => insert user into database
let insertUserToUsers;
let insertUserToExercises;
let insertUserToCustomExercises;
try {
insertUserToUsers = await database.db("guitar-practice-suite").collection("users").insertOne({email: email, password: hashedPassword});
insertUserToExercises = await database.db("guitar-practice-suite").collection("exercises").insertOne({email: email});
insertUserToCustomExercises = await database.db("guitar-practice-suite").collection("custom-exercises").insertOne({email: email, exercises: []});
} catch(err) {
log("ERROR while inserting user into database at: createPracticeProfile");
console.log(err);
client.close();
return false;
} finally {
client.close();
return insertUserToUsers && insertUserToExercises && insertUserToCustomExercises;
}
}
I've found the solution to the problem, but I'm not sure I understand the reasoning.
The client.close() in the finally block of the validateUniqueUser function. It was closing the connection before the connection in the createPracticeProfile function was finished inserting the user.
When that line is taken out, the function works.
The issue is client variable needs to be reinstantiated again,
const client = new MongoClient(uri, { useUnifiedTopology: true}, { useNewUrlParser: true }, { connectTimeoutMS: 30000 }, { keepAlive: 1});
Try putting this in start of createPracticeProfile, validateUniqueUser and other functions
I was getting the error
MongoError: Topology is closed
because of the authentication problem
MongoEror: Authentication failed
In my case, the problem was with the password of my database. My password only contained numerical digits.
I changed the password to all characters and both the errors were solved.
Configure your client connection like below example
var MongoClient = require('mongodb').MongoClient;
var Server = require('mongodb').Server;
var mongoClient = new MongoClient(new Server('localhost', 27017));
mongoClient.open(function(err, mongoClient) {
var db1 = mongoClient.db("mydb");
mongoClient.close();
});
In my case - connecting to AtlasDB using the MongoClient - I had to whitelist the IP i was accessing the cluster from
I think your mongodb service is stopped, to start it
Task Manager -> Services -> Mongodb -> RightClick -> Start
My code has been working fine for a long time and hasn't thrown this error before: MongoError: Topology is closed.
But due to the fact that my laptop was turned on for a long time and I was simultaneously developing other projects on it, while the main one was running in the terminal, mongo most likely did not close one of the connections to the database and opened another in parallel, creating some kind of collision.
In general, in my case, the usual restart of the computer helped and a similar error did not occur again.

How to manage Postgres connection in concurrent AWS lambda function?

Anybody who has experience building concurrent AWS Lambda Function with Postgres?
I have to build a lambda cron that will ingest thousands of invoices into a Postgres database. I have to call the ingestion lambda function concurrently for each invoices. The problem is, because the it is concurrent, each instance of the ingestion function will create a connection to the database. Which means, if I have a 1000 invoice to ingest, each invoice will invoke a lambda function, that will create 1000 database connection. This will exhaust the max connection that Postgres can handle. Some instance of the lambda function invoked will return an error saying that there are no more connection available.
Any tips you can give how to handle this problem?
Here are some snippets of my code:
ingestInvoiceList.js
var AWS = require('aws-sdk');
var sftp = require('ssh2-sftp-client');
var lambda = AWS.Lambda();
exports.handler = async (evenrt) => {
...
let folder_contents;
try {
// fetch list of Zip format invoices
folder_contents = await sftp.list(client_folder);
} catch (err) {
console.log(`[${client}]: ${err.toString()}`);
throw new Error(`[${client}]: ${err.toString()}`);
}
let invoiceCount = 0;
let funcName = 'ingestInvoice';
for (let item of folder_contents) {
if (item.type === '-') {
let payload = JSON.stringify({
invoice: item.name
});
let params = {
FunctionName: funcName,
Payload: payload,
InvocationType: 'Event'
};
//invo9ke ingest invoice concurrently
let result = await new Promise((resolve) => {
lambda.invoke(params, (err, data) => {
if (err) resolve(err);
else resolve(data);
});
});
console.log('result: ', result);
invoiceCount++;
}
}
...
}
ingestInvoice.js
var AWS = require('aws-sdk');
var sftp = require('ssh2-sftp-client');
var DBClient = require('db.js')l
var lambda = AWS.Lambda();
exports.handler = async (evenrt) => {
...
let invoice = event.invoice;
let client = 'client name';
let db = new DBClient();
try {
console.log(`[${client}]: Extracting documents from ${invoice}`);
try {
// get zip file from sftp server
await sftp.fastGet(invoice, '/tmp/tmp.zip', {});
} catch (err) {
throw err;
}
let zip;
try {
// extract the zip file...
zip = await new Promise((resolve, reject) => {
fs.readFile("/tmp/tmp.zip", async function (err, data) {
if (err) return reject(err);
let unzippedData;
try {
unzippedData = await JSZip.loadAsync(data);
} catch (err) {
return reject(err);
}
return resolve(unzippedData);
});
});
} catch (err) {
throw err;
}
let unibillRegEx = /unibill.+\.txt/g;
let files = [];
zip.forEach(async (path, entry) => {
if (unibillRegEx.exec(entry.name)) {
files['unibillObj'] = entry;
} else {
files['pdfObj'] = entry;
}
});
// await db.getClient().connect();
await db.setSchema(client);
console.log('Schema has been set.');
let unibillStr = await files.unibillObj.async('string');
console.log('ingesting ', files.unibillObj.name);
//Do ingestion queries here...
...
await uploadInvoiceDocsToS3(client, files);
} catch (err) {
console.error(err.stack);
throw err;
} finally {
try {
// console.log('Disconnecting from database...');
// await db.endClient();
console.log('Disconnecting from SFTP...');
await sftp.end();
} catch (err) {
console.log('ERROR: ' + err.toString());
throw err;
}
}
...
}
db.js
var { Pool } = require('pg');
module.exports = class DBClient {
constructor() {
this.pool = new Pool();
}
async setSchema(schema) {
await this.execQuery(`SET search_path TO ${schema}`);
}
async execQuery(sql) {
return await this.pool.query(sql);
}
}
Any answer would be appreciated, thank you!
I see two ways to handle this. Ultimately it depends on how fast you want to process this data.
Change the concurrency setting for you Lambda to a "Reserve Concurrency:
.
This will allow you to limit the number of concurrent Lambda's running (see this link for more details).
Change your code to queue the work to be done in an SQS queue. From there you would have to create another Lambda to be triggered by the queue and process it as needed. This Lambda could decide how much to pull off the queue at a time and it too would likely need to be limited on concurrency. But you could tune it to, for example, run for the maximum 15 minutes which may be enough to empty the queue and would not kill the DB. Or if you had, say, a max concurrency of 100 then you would process quickly without killing the DB.
First, you have to initialize your connection outside the handler, so each time your warm lambda will be executed it won't open a new one:
const db = new DBClient();
exports.handler = async (event) => {
...
await db.query(...)
...
}
If is node-pg there is a package that keep tracks of all the idle connections, kill them if necessary and retry in case of error or sorry, too many clients already:
https://github.com/MatteoGioioso/serverless-pg
Any other custom implemented retry mechanism with backoff will work as well.
There is also a one for MySQL as well: https://github.com/jeremydaly/serverless-mysql
These days a good solution to consider for this problem, on AWS, is RDS Proxy, which acts as a transparent proxy between your lambda(s) and database:
Amazon RDS Proxy allows applications to pool and share connections established with the database, improving database efficiency, application scalability, and security.

Structure of a synchronous application in Node.js and MongoDb

I need to build an application that does these things (in order):
on load:
01- connect to MongoDB 'db'
02- creates a collection 'cas'
03- check if a web page has updates, if yes go to step 04, if not go to step 07
04- do web scraping (using Cheerio) of the web site and get a $ variable like that $ = cheerio.load(body);
05- elaborate this object to get only informations I'm interested in and organize them in a jsons object like this one:
var jsons = [
{year: 2015, country: Germany, value: 51},
{year: 2015, country: Austria, value: 12},
{year: 2016, country: Germany, value: 84},
{year: 2016, country: Bulgaria, value: 104},
...
];
06- insert each of these elements ({year: 2015, country: Germany, value: 51}, ...) in the collection 'cas' of database 'db'
07- download the data (for example in a csv file)
08- create a web page for data visualization of these data using D3.js
09- disconnect from 'db'
If Node.js were synchronous, I could write something like this:
var url = 'http://...';
var jsons = [];
connectDb('db');
createCollection('db', 'cas');
if(checkForUpdates(url)) {
var $ = scrape(url);
jsons = elaborate($);
for(var i = 0; i < jsons.length; i++) {
saveDocumentOnDbIfNotExistsYet('db', 'cas', jsons[i]);
}
}
downloadCollectionToFile('db', 'cas', './output/casData.csv');
createBarChart('./output/casData.csv');
disconnectDb('db');
But Node.js is asynchronous so this code would not work properly.
I've read that I can use Promise to get the code to run in a certain order.
I read the documentation about the Promise and some sites that showed simple tutorials.
The structure of a Promise is:
// some code (A)
var promise = new Promise(function(resolve, reject) {
// some code (B)
});
promise.then(function() {
// some code (C)
});
promise.catch(function() {
// some code (D)
});
// some code (E)
If I understood correctly, in this case the execution (if Node.js were synchronous) would be equivalent to:
// some code (A)
// some code (E)
if(some code (B) not produce errors) {
// some code (C)
}
else {
// some code (D)
}
or (swap between code A and E, because they are asynchronous)
// some code (E)
// some code (A)
if(some code (B) not produce errors) {
// some code (C)
}
else {
// some code (D)
}
So now I wonder what is the right structure for my application.
I thought about:
var cheerio = require('cheerio');
var express = require('express');
var fs = require('fs');
var MongoClient = require('mongodb').MongoClient;
var dbUrl = 'mongodb://localhost:27017/';
var dbName = 'db';
var collectionName = 'cas';
const app = express(); // run using > node app.js
// connect to db
var connect = function(url) {
return new Promise(function(resolve, reject) {
MongoClient.connect(url + dbName, function(err, db) {
if(err) {
reject(err);
}
else {
console.log('Connected');
resolve(db);
}
});
});
}
// create collection
connect.then(function(db) {
db.createCollection(collectionName, function(err, res) {
if(err) {
throw err;
}
else {
console.log('Collection', collectionName, 'created!');
}
});
});
// connection error
connect.catch(function(err) {
console.log('Error during connection...');
throw err;
});
It's right? If yes, how can I proceed with other steps?
I can I improve my code?
EDIT 1
Following the example of Андрей Щербаков, I modified my code in this way:
app.js:
// my files
var db = require('./middlewares/db.js');
var url = 'mongodb://localhost:27017/';
var dbName = 'db';
var collectionName = 'cas';
const start = async function() {
const connect = await db.connectToMongoDb(url, dbName);
const cas = await connect.createYourCollection(collectionName);
const isPageHasUpdates = oneMoreFunction(); // i don't know how you gonna check it
if(isPageHasUpdates) {
await step 4;
await step 5;
await step 6;
}
await step 7
return something; // if you want
}
start()
.then(res => console.log(res)) // here you can use result of your start function if you return something or skip this then
.catch(err => console.log(err)); // do something with your error
middlewares/db.js:
var MongoClient = require('mongodb').MongoClient;
let dbInstance;
var methods = {};
methods.connectToMongoDb = function(url, dbName) {
if(dbInstance) {
return dbInstance;
}
else {
MongoClient.connect(url + dbName, function(err, db) {
if(!err) {
dbInstance = db;
return db;
}
});
}
}
methods.createYourCollection = function(collectionName) {
?.createCollection(collectionName, function(err, res) {
if(err) {
throw err;
}
});
}
module.exports = methods;
But I'm not sure I'm doing well.
How can I separate function in different files? For example I want to put all the function about db in file middlewares/db.js. But I have some problems in line ?.createCollection(collectionName, function(err, res).
If you are running node version 7.6 or higher, better way will be to use async await which works with promises.
So your code will look like
const start = async() => {
const connect = await connectToMongoDb(url);
const cas = await connect.createYourCollection();
const isPageHasUpdates = oneMoreFunction(); // i don't know how you gonna check it
if(isPageHasUpdates) {
await step 4;
await step 5;
await step 6;
}
await step 7
return something; // if you want
}
start()
.then(res => console.log(res)) // here you can use result of your start function if you return something or skip this then
.catch(err => console.log(err)); // do something with your error
Sure any function you are gonna await should be promisified as you did with your connect function( but if you are using https://www.npmjs.com/package/mongodb functions already promisified)
Update
The best way will be to use mongoose, but if you want to work with native mongodb you can write your mongodb like this https://pastebin.com/BHHc0uVN (just an example)
You can expand this example as you want.
You can create function createCollection
const createCollection = (connection, collectionName) => {
return connection.createCollection(collectionName); // actually i'm not sure that this function exists in mongodb driver
}
And usage will be:
const mongodbLib = require('./lib/mongodb'); //path to db.js file
mongodbLib.init()
.then(connection => mongodbLib.createCollection(connection, 'cas'))
.then(() => doSmthElse())
Or if you are sure that init is done(you can do it once before you main script like starting server or whatever you doing)
const mongodbLib = require('./lib/mongodb'); //path to db.js file
const connection = mongodbLib.getConnection();
Or if you want to simple work with collection like in step 6, add your cas collection(like user in example file). But this you can use when your init function is done as well.
So usage will be
const mongodbLib = require('./lib/mongodb');
const cas = mongodbLib.collections.cas;
cas().insertMany(docs)
.then()
.catch()

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