I have a website primarily hosted on EC2 with the backends (MySql) replicated async to a Rackspace store. I am also planning to have a suite of web server, cache & db on standby at Rackspace in the eventuality that Amazon has an outage.
With this is perspective, is there a solution that will allow me to switch between EC2 and Rackspace depending on availability?
We use dnsmadeeasy for exact same thing. DNS monitoring and failover would help you with this. For mysql, you can setup 2 way or one way replication; two-way replication is better as you don't have to worry about replicating data back when you switch back. And everything can be completely automated failrly easily.
If you're just looking to have it on "stand-by" I suppose you can accomplish this well enough (depending on what your uptime requirements are) using DNS. Simply point the domain to a new IP when the change is required. But I know this is manual.
Barring that, enterprise level load balancers exist that can do the same, I've seen some in a few client environments, but never for failover, mostly it's geographical or performance-based decisions per user on which datacenter will service said user. That said, I know you can configure load balancing per host to work in many different ways on most devices, including active/passive (failover) so I'm sure the same can be done but in terms of what site instead of what host. I'm purposely avoiding recommending any one/some brands due to potential perception of conflict of interest at work.
An alternative would be to use a CDN, if the budget exists for it. I can't speak for others, but I've worked with Akamai for joint clients and they have a site failover service which you may find useful. Check out http://www.akamai.com/html/technology/products/site_failover.html for an example
You can prop up another box using Heartbeat as a load balancer. If you really want you could host redundant heartbeat boxes on Amazon and Rackspace, that would ensure that your load balancer is up even if one of the services goes down. You can easily configure Heartbeat to direct all of the traffic to Amazon unless the servers there are down.
You would need a combination of a ping service (or whatever you need to judge outage) and then an short lived DNS entries and a API to change DNS.
