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Fleet Unit Files for Kubernetes on CoreOS

These are a collection of fleet unit files that can be deployed to an existing CoreOS cluster. They still require a minimum amount of configuration. They come from snippets scattered throughout the kubernetes project as well as work from Kelsey Hightower.

Why Fleet instead of Cloud-Config?

Most of the Kubernetes examples have unit files set within the cloud-config section of CoreOS. This doesn't work well in practice. It's difficult to update the cloud-config of an entire CoreOS cluster, especially where cloud-drive is not available (i.e AWS). Using Fleet for deploying Kubernetes lets you focus on building a CoreOS cluster first (or using an existing one) and then deploy Kubernetes on top of it. It also lets you easily mess around with Kubernetes without having to deal with underlying nodes.

On a side note, if you just want to get up and running to experiment with Kubernetes, there are a variety of one-click solutions to launch a Kubernetes cluster. For people experimenting with complex topologies, where you want a variety of heterogenous nodes, manually deploying Kubernetes on top of an existing CoreOS cluster makes sense. Thus these unit files.

Assumptions

  • Assumes you have a running CoreOS cluster with etcd 2.0+.
  • Assumes you a setup-network-environment.service unit running on all nodes set via cloud-config. This can be copied from the Kubernetes repository.
  • Assumes all nodes running kubelet and proxy have an /etc/leader environment file with a ````LEADER_ENDPOINTset. TheLEADER_ENDPOINT``` can be a specific node running the api service or preferably some load balanced endpoint to a set of leader nodes running the API. (Because of the transient nature of CoreOS nodes, minimizing IP address is a plus).

Template Files

Each Kubernetes service is broken out into its own unit file, prefixed with kube- to make grouping in Fleet easier. Each unit file downloads its respective executable in ExecStartPre and executes it via ExecStart. There are a few things you'll want to configure in these unit files before launching.

If you're running CoreOS in production you probably have nodes assigned certain roles. For instance, you probably have a set of "core service" nodes running etcd while a set of "worker" nodes are running as proxys. The unit files use X-Fleet machine metadata to assign certain kubernetes functionality to specific nodes.

kube-apiserver

  • Runs the Kubernetes API.
  • Specified as a Global unit running on MachineMetadata=role=leader
  • Assumes Etcd2 is running on same node
  • Uses port 9090
  • Binds to all addresses
  • Sets service-cluster-ip-range to 172.22.0.0/16.

I was thrown for a loop with service-cluster-ip-range. This is different from your Flannel network. It is used by Kubernetes so service objects you specify can be routed alongside your pods. The kube-proxy service will dynamically configure your host's routing tables so services are accessible. I opted for the 172.22/16 private address space as 10.100/16 may already be used if you work in a large organization. If you have no idea what 10.100/16 or 172.22/16 means, I wrote a quick post on CIDR notation.

kube-controller-manager

  • Runs the Controller Manager service
  • Runs as a single instance with MachineOf=kube-apiserver.service
  • Assumes kube-apiserver is running on the same node on port 9090

kube-scheduler

  • Runs the Scheduler service
  • Runs as a single instance with MachineOf=kube-apiserver.service
  • Assumes kube-apiserver is running on the same node on port 9090

kube-kubelet

The kube-kubelet service assumes its not running on a node with an api server (as the api server is a "core" service while most pods will be generic worker nodes). It assumes there is an environment file at /etc/leader with a LEADER_ENDPOINT environment variable set. You can write to /etc/leader in your cloud-config when you bring up a node.

  • Runs the kubelet service
  • Runs as a global unit (no restrictions, you may want to change this)
  • Assumes there is a DEFAULT_IPV4 environment variable, set from a setup-network-environment.service unit from CoreOS's cloud-config.

kube-proxy

  • Like kubelet, assumes the presence of an /etc/leader environment file with a LEADER_ENDPOINT set.
  • Runs as a global unit (you may want to change this)