Autoscaling solutions for Digital Ocean - are there existing solutions?

Take a look at Apache Brooklyn project. It supports many providers and can actually create/destroy vm instances based on many parameters (cluster load average mean, load balancer request rate, etc).

you can simply use DO API. we have some example configs here: https://github.com/magenx/Digitalocean-Auto-Scaling-Droplets-Auto-Scale-DIY

the idea is to check the load on master droplet and start new droplet if load is too high. configure it, rsync files and inject ip into load balancer.

This is a very interesting question. One I’ve spent considerable time working with for a lot of my projects as well. This really comes down to deciding what languages and tools you want to invest in, or which languages you are already familiar with to work on.

There are a couple components to any software solution that can do this. The ability to get feedback when your system is under- or over-loaded, and the logic/code for deciding when to build out or break down. The build-out and break-down part benefits from automation tools and the feedback part can leverage automation tools or monitoring solutions.

Check out the links to articles for saltstack, chef, puppet, ansible, vagrant and monitoring or search for any other tools you may already be acquainted with as they may already provide modules or extensions that can help get this type of solution off the ground.

Many of these already support getting information from the systems that are managed which can cover the feedback side of auto-scaling or you can use any type of monitoring software ( and interface with your automation tools ) that works for you. Some already have components for integrating with our API through modules to make things even easier as well.

The key thing to note here is that it will take quite a bit of an investment to work up configurations, find what works with the feedback for deciding which/when services get broken down or built up, and interface all the components in a reliable way. I typically leverage existing tools for this, but if you start something small that gets the job done, you could build it up to something beautiful and powerful.