db.js

var mongoose = require('mongoose');
var connectionStr = process.env.MONGO_DB_CONNECTION_STR;
if (!connectionStr) {
    connectionStr = 'mongodb://127.0.0.1:27017/dataex';
}

console.log('Connection string: ' + connectionStr);

const options = {
    autoIndex: false, // Don't build indexes
    reconnectTries: Number.MAX_VALUE, // Never stop trying to reconnect
    reconnectInterval: 500, // Reconnect every 500ms
    poolSize: 10, // Maintain up to 10 socket connections
    // If not connected, return errors immediately rather than waiting for reconnect
    bufferMaxEntries: 0
};

var connectWithRetry = function () {
    return mongoose.connect(connectionStr, options).then(
        () => {
            console.log('Connected to: ' + connectionStr);
        },
        err => {
            console.error('Failed to connect to mongo on startup - retrying in 5 sec', err);
            setTimeout(connectWithRetry, 5000);
        }
    );
};

connectWithRetry();