Recently I've been experimenting with various cloud management tools like RightScale, Scalr, custom scripts for managing a variety of servers, each hosting several roles (app, db, load balancer, job queues, etc).

The one thing I find lacking in most solutions is a way to do rolling deployments, i.e. running deployments sequentially across a number of servers with the same role. For instance, I dont want to build all of my webservers at the same time, as that will almost definitely result in some down time or 500s for my customers. I'd rather have one or two servers build at a time, while other servers are still available to handle requests.

The other alternative is obviously to launch new servers that automatically update themselves on boot, but this isn't as cost effective, and most likely requires more time for the build to complete (it's faster to build on an existing server than to launch a new server and kill old ones).

We've all heard of the big companies having the famous "push to build" button (companies like Twilio, Etsy, etc.) but it seems that they all have custom implementations of this. I'm not talking about a simple ssh-loop, clusterssh, or even an mcollective - I preferably want something with a nice simple interface that allows me to specify something like a RightScript or a Scalr script to run on a set of servers with a specific role, and it builds them sequentially.

Does any one know of easy ways to get this done, or is this a candidate for a new open source project?