How to cache or reuse MongoDB database connection in Serverless (AWS Lambda) application

Question

I'm building a Serverless application using NodeJS and using MongoDB. I noticed that a lot of connections are created when handling a request. So I came up with a caching mechanism but it doesn't seem to be working. Here is my code.

Connection file

'use strict';

const connectDatabase = mongoClient => mongoClient.connect(process.env.MONGODB_STRING, { poolSize: 10 });

  const createConnection = (mongoClient, dbConnection) => {
    return new Promise(async (resolve, reject) => {
      try {
        if (dbConnection) {
          console.log('===== Cached Connection =====');
          resolve(dbConnection);
        }
        console.log('===== New Connection =====');
        let dbPool = await connectDatabase(mongoClient);
        resolve(dbPool);
      } catch (error) {
        reject(error);
      }
    });
  };

const closeDbConnection = conn => {
  if (conn) {
    return conn.close();
  }
};

module.exports = {
  connectDatabase: connectDatabase,
  createConnection: createConnection,
  closeDbConnection: closeDbConnection
};

Handler code

let admin = require('./lib/admin');
let helper = require('./lib/helper');
let connection = require('./lib/connection');
let mongo = require('mongodb');
let mongoClient = mongo.MongoClient;
let cachedDb = null;

let createUser = async (event, context, callback) => {
  context.callbackWaitsForEmptyEventLoop = false;
  cachedDb = await connection.createConnection(mongoClient, cachedDb);
  admin.createUser(cachedDb, helper.parseEvent(event), callback);
};

I'm using a global variable cachedDb to store the database connection but each and every time I make a request it logs ===== New Connection ===== any idea how to achieve this. Is there a better way to handle this?.

I think just check if (dbConnection) instead of if (dbConnection && dbConnection.isConnected(dbName)), what is the dbName, maybe it is a invalid options of isConnected — hoangdv
– hoangdv, Commented Nov 21, 2019 at 7:11
@hoangdv I did that too but dbConnection is alway null I don't know why — TRomesh
– TRomesh, Commented Nov 21, 2019 at 8:10

Bruno Smaldone · Accepted Answer · 2019-11-21 06:48:17Z

3

The approach I like using here is a singleton class, like this:

export default class MyMongoConnection {
    static instance;

    static getInstance() {
        if (!MyMongoConnection.instance) {
            MyMongoConnection.instance = MyMongoConnection.createInstance();
        }
        return MyMongoConnection.instance;
    }

    static createInstance() {
        // Do whatever you need to create the instance
    }
}

So you import MyMongoConnection anywhere and just ask for the instance using .getInstance(), if it exists, it reuses it.

Hope this helps.

answered Nov 21, 2019 at 6:48

Bruno Smaldone

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