Doesn't it make more sense to scale based on New Relic response times? Why are you guys using # of users? Depending on the page they are requesting and how the requests are clustered that could produce vastly inferior results.
We first conceived of Dynosaur as a plugin-based autoscaler (with GA and New Relic plugins to start with), but we've found the times we really need to scale fast are the times we have a lot of traffic generated from press stories etc (like this from today, if you will excuse the shameless plug: http://dealbook.nytimes.com/2014/01/21/a-start-up-run-by-fri...) and using the analytics live API allows us to react a little quicker than if we waited for New Relic to tell us our response times are getting slow. So far, we're happy enough with just a Google Analytics plugin.
One possible improvement would be to scale differently based on different traffic / performance metrics across the site. I think New Relic or other performance instrumentation would be very useful for that.
Just a thought. I know you abstracted it out the way you did so as not to tie it to just GA, but if GA is your analytics platform of record, it could be worth pursuing. Cheers!
Scaling should occur on actual traffic/user throughput statistics not on response time. Response times can increase for a variety of reasons, of which only one of is increased traffic load. For example, response times can increase based on back-end database contention or thread contention.
Of course whichever configuration is chosen, it is important to load test your application so you understand its performance profile and that all contention points are understood.