module.exports = function (debugMode = false) {
  const
    debug = require('debug')('agm:db'),
    mongoose = require('mongoose'),
    env = require('../env'),
    utils = require('../utils');

  // Our custom middleware
  mongoose.Promise = global.Promise;  // Disable Promise depreciated warning

  const ops = {
    connectTimeoutMS: 40000,
    // TCP Connection timeout setting. Best = 2-3X time of longest operation (30 minutes)
    socketTimeoutMS: 30 * 60 * 1000,
    // The number of milliseconds to wait before initiating keepAlive on the TCP socket.  
    keepAlive: true, keepAliveInitialDelay: 30000,
    // Auto creation of indexes set to false for DEBUG and true only for dev for better performace in Production invironment
    autoIndex: !(env.PRODUCTION),
    maxPoolSize: env.DB_MAX_POOLSIZE, // Default 100 from MDB 5.0.x
    family: 4,
  }

  let _uri = `@${env.DB_HOSTS}/${env.DB_NAME}`;
  if (env.DB_HOSTS && !env.DB_HOSTS.includes(',')) {
    ops.directConnection = true;
  }

  if (env.DB_REPLSET)
    ops.replicaSet = env.DB_REPLSET

  if (env.DB_USE_TLS) {
    ops.tls = true; ops.sslValidate = true;
    ops.tlsCAFile = env.DB_TLS_CA_FILE;
    ops.tlsCertificateKeyFile = env.DB_TLS_CERT_FILE;
  }

  if (env.DB_USE_X509) {
    ops.authMechanism = 'MONGODB-X509';
    _uri = `mongodb://${encodeURIComponent(env.DB_USR)}` + _uri;
  } else {
    _uri = `mongodb://${env.DB_USR}:${env.DB_PWD}` + _uri;
    ops.authSource = env.DB_AUTH_SOURCE || env.DB_NAME
    ops.authMechanism = 'SCRAM-SHA-1'
  }

  mongoose.set('strictQuery', false);
  mongoose.set('strictPopulate', false);
  mongoose.set('bufferCommands', false); // Disable buffering globally

  mongoose.connect(_uri, ops);

  mongoose.connection.on('disconnected', () => { debug('-> MongoDB lost/closed connection'); });
  mongoose.connection.on('reconnect', () => { debug('-> MongoDB reconnected'); });
  mongoose.connection.on('connected', () => { debug('-> MongoDB connected'); });
  mongoose.connection.on('open', () => { debug('-> MongoDB open'); });
  mongoose.connection.on('reconnectFailed', () => { debug('-> MongoDB gave up reconnecting'); });
  mongoose.connection.on('error', console.error.bind(console, 'MongoDB connection errors:'));

  if (debugMode) mongoose.set('debug', true);

  function postExit() {
    if (mongoose.connection) {
      mongoose.connection.close(() => {
        debug('Mongoose default connection disconnected through app termination');
        process.exit(0);
      });
    } else {
      process.exit(0);
    }
  }
  // If the Node process ends, close the Mongoose connection 
  process.on('SIGINT', function () {  // Windows
    postExit();
  });

  return mongoose.connection;
}