I've been trying to research this topic but haven't found anywhere that recommends where to install services such as Redis and ElasticSearch when migrating to a cloud framework.

I'm currently running a Symfony2 application on 2 static servers - one is running MySQL and the other is the public facing web server, which also has Redis and ElasticSearch running on it. Both of these servers are virtualised, but they're static in terms of not being able to replicate at present (various aspects are still dependent on the local filesystem).

The goal is to migrate to AWS and use auto-scaling to be able to spin up and kill web servers as required, but I'm not clear on what I should put on each EC2 instance. Should they be single-responsibility only? i.e. Set up individual instances for the web server(s), Redis, and ElasticSearch and most likely an RDS instance for MySQL and only set up auto-scaling on the web server(s)?

I don't foresee having to scale the ElasticSearch server anytime soon as it's only driving the search functionality, but it's possible that Redis may need to be replicated at some point - but should this be done manually? I'm not sure of how this could be done automatically as each instance needs to be configured to know about it's master/slave(s) as far as I know. I'd appreciate advice on this.

One more quick question while I'm here - how would I be able to deploy code changes when there are X web servers currently active? I'm using a Capifony deployment script (Symfony2 version of Capistrano), which I think can handle multiple servers easily enough by specifying an array of :domain addresses...but how can should this be handled when the number of web servers can vary?