8 Dec 2015
In my last post we learnt about CoreOS and took a look at the steps needed to install it on your laptop (using VirtualBox). We also learnt CoreOS doesn’t ship a package manager. Instead it comes with Docker pre-installed. So, for every service that you need (e.g. web server, database, cache, and so on) you can just create and use a Docker container for it.
So, what’s the deal with self-sufficient containers?
Containers are self-sufficient by default, right? Well, this depends on what you call self-sufficient. Of course, containers are self-sufficient in that they don’t depend on any software running outside of their logical boundaries. But they need kernel support and computing resources from the host computer.
Let’s see it like this: containers are generally set up on a network with several nodes, with each node running one or more containers. Each container may be self-sufficient, but the node on which the container is supposed to run is not! Nodes may crash or run slow or get disconnected off the network. And in such cases, you need to find out the offline node and re-route the traffic meant for that node to other nodes.
But how do we do that?
Enter CoreOS and three particularly crucial components: systemd, fleet, and etcd.
In this post, we’ll take a look at these three components and learn how they’re used by CoreOS to help you create self-sufficient containers.