Handling multiple MongoDB connections on Node.js

ClientsPool.js

'use strict'
/**
 * Module to create a Clients Pool
 * 
 */

/**
 * Dependencies
 */
let MongoClient = require('mongodb').MongoClient;
let crypto = require ('crypto');

/**
 * Clients Pool 
 * @param {number} capacity Pool capacity. Default = 100
 * @param {number} timeOutMS Time a connection is kept active in miliseconds. Default = 1000 * 60
 */
let ClientsPool = function (capacity, timeOutMS) {
    this.capacity = typeof (capacity) == 'number' ? capacity : 100;
    this.timeOutMS = typeof (timeOutMS) == 'number' ? timeOutMS : 1000 * 60;
    
    this.clients = {};
    this.size = 0;
    this.start();
}

/**
 * Method that creates a new connection and adds it to the pool if not existing,
 * or updates its expiring date if it does.
 * @param {string} mongoUri URI for the MongoDB instance
 * @param {string} username Username
 * @param {string} password Password
 * @param {Object} connectionOptions MongoClient connection options
 * @returns MongoClient
 * @throws MongerError if the pool has reached its maximum capacit
 */
ClientsPool.prototype.connect = async function (mongoUri, username, password, connectionOptions = {}) {
    // If no credentials, this is worthless. 
    if(!username || !password){ throw Error("Username and/or password not provided."); }

    // Key being used to access the active clients
    let key = crypto.createHash('sha1').update(`${username}:${password}`).digest('base64');
    let clientInstance = this.clients[key];
    let currentDate = Date.now();

    // If the client doesn't exists, try to create a new instance
    if(!clientInstance){
        // If has reached the maximum, gently reject the request
        if(this.size >= this.capacity){
            throw new Error("Server cannot handle more request. Please try again in a few minutes.");
        }
        // Increase the size of the pool
        this.size++;
        clientInstance = {
            expireDate: -1,
            client: null
        }
    }
  
    // If is new or has expired for some reason, create a new connection
    if( clientInstance.expireDate <= currentDate || clientInstance.client == null || !clientInstance.client.isConnected() ){        
        connectionOptions.auth = {
            "user": username,
            "password": password
        };
      
        clientInstance ={
            client: await MongoClient.connect(mongoUri, connectionOptions),
            expireDate: currentDate + this.timeOutMS
        }
    // If not expired yet, update its expireDate
    }else{
        clientInstance.expireDate = currentDate + this.timeOutMS;
    }

    this.clients[key] = clientInstance;
    return clientInstance.client;    
}

/**
 * Method that checks the current open connections and 
 * closes any that has already expired
 */
ClientsPool.prototype.checkClients = function(){
    let currentDate = Date.now();
    for(var key in this.clients){
        let clientInstance = this.clients[key];
        if(clientInstance && clientInstance.expireDate <= currentDate){
            clientInstance.client.close();
            delete this.clients[key];
            this.size--;
        }
    }
}

/**
 * Method that runs the @see{@link checkClients} method at intervals
 * equal half of the connection timeout time.
 */
ClientsPool.prototype.loop = function(){
    setInterval(function(){
        this.checkClients();
    }.bind(this), this.timeOutMS/2);
};

/**
 * Method to start/initiate the pool and its background process.
 */
ClientsPool.prototype.start = function(){
    this.checkClients();
    this.loop();
}

/**
 * Exporting the module
 */
module.exports = ClientsPool;