How to scale your 1 server Rails application

Question

I have a rails application running on a single VPS that uses passenger, apache and MySQL. I am moving this to Amazon AWS with the following simple setup:

ELB > Web Server > MySQL

Lets say I am expecting a huge spike in daily users and want to start to scale this out on Amazon AWS using multiple instances. Where does a newbie start on this journey? Do I simply create an AMI from my production configured web server and get the ASG to launch these when required?

I understand that AWS increases the number of instances using auto scale groups as the load demands it, but do I need to architect anything differently in my Rails application for it to run at scale across multiple interfaces?

Answer 1

The problem with scaling horizontally is that it really depends on the application. There's no "just-add-water" ways to do it.

But there are some generic recipes you can follow in the beginning:

1. Extract MySQL server into a separate instance, which is capable of holding a higher load. Then create as many worker (ie app) instances that connect to the MySQL database as you need. You can keep doing so before your MySQL server gets saturated with requests, and can no longer keep up with the load.

2. When you're done with step 1, you can add MySQL replicas and setup a master-slave replication. This will leave you with a MySQL cluster, where one server can accept writes and all the others are read-only. After your set it up, change your application to send SELECT's to read-only replicas and INSERT/DELETE/UPDATE's to the writeable master server. This approach is based on the fact that most of the applications do reads way more often than writes. It can be not the case for you, but if it is, it'll keep your afloat pretty long. Right before you saturate MySQL master server write performance.

3. Once you've squeezed everything from step 2, you can go ahead and shard the data. This is now becoming more and more dependent on your application. But I will provide a blind example in order to convey the idea. Say, you have a user-centric application (eg a private photo-album, with no sharing capabilities), and each user has a name. In this case you can make two completely independent clusters, where the first one will serve users with names starting AM, and the second one will serve ones with NZ. It essentially makes the load twice as less, but complicates the whole architecture.

Though generic, these recipes can help you build a pretty solid application capable of serving millions of users daily before you're forced to bring up more exotic ways of scaling.

Hope this helps!