I'm involved in many open source development communities (like Drupal and Ansible), I'm the author of Ansible for DevOps and Ansible for Kubernetes, and I've been involved in the development of many large websites and applications.
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St. Louis, MO
Over in the Raspberry Pi subreddit, someone also pointed me to this excellent project, with even better visualization of the temperatures in the house: https://www.reddit.com/r/raspberry_pi/comments/2ujfj5/house_temperature…
There are a lot of really cool things you can do with Pis and environmental controls and monitoring!
It really depends on how strict a definition of cluster you mean; I intend it to mean a 'group of computers that work together to accomplish the goals of a single application'. In some places, that means all the computers work together in parallel processing a dataset. In others, that means there are different components of one large application that handle a request and serve a response (e.g. a webserver, split across multiple computers).
You have similar problems in either case—coordinating software and services across multiple servers, monitoring everything, making sure input gets routed the right way and output is successful.
Most admins I speak with use 'cluster' to refer to a number of different multi-machine scenarios, so that's how I'm using the term here.