Federated Kubernetes with on-prem Clusters and Juju

Introduction

In this post we discus our efforts to setup a Federation of on-prem Kubernetes Clusters using CoreDNS. The Kubernetes Cluster version used is 1.6.2. We use Juju to deliver clusters on AWS, yet the clusters should be considered on-prem since they do not integrate with any of the cloud’s features. The steps described here are repeatable on any pool of resources you may have available (cloud or bare metal). Note that this is still a work in progress and should not be used on a production environment.

What is Federation

Cluster Federation (Fig. from http://blog.kubernetes.io/2016/07/cross-cluster-services.html)

Cluster federation allows you to “connect” your Kubernetes clusters and manage certain functionality from a central point. The need for central management arises when you have more than one clusters possibly spread across different locations. You may need a globally distributed infrastructure for performance, availability or even legal reasons. You might want to have isolated clusters satisfying the needs of different teams. Whatever the case may be, federation can assist in some of the ops tasks. You can find more on the benefits of federation in the official documentation.

Why Cluster Federation with Juju

Juju makes delivery of Kubernetes clusters dead simple! Juju builds an abstraction over the pool of resources you have and allows us to deliver Kubernetes to all the main public and private clouds (AWS, Azure, GCE, Openstack, Oracle etc) and bare metal physical clusters. Juju also enables you to experiment with small scale deployments stood up on your local machine using lxc containers.

Having the option to deploy a cluster with the ease Juju offers, federation comes as a natural next step. Deployed clusters should be considered on-prem clusters as they are exactly the same as if they were deployed on bare metal. This is simply because even if you deploy on a cloud Juju will provision machines (virtual machines in this case) and deploy Kubernetes as if it were a local on-prem deployment. You see, Juju is not magic after all!

There is a excellent blog post showing how you can place Juju clusters under a federation hosting cluster setup on GCE. Here we focus on a federation that has no dependencies on a specific cloud provider. To this end, we go with CoreDNS as a federation DNS provider.

Let’s Federate

Deploy clusters

Our setup is made of 3 clusters:

  • f1: will host the federation controller place
  • c1, c2: federated clusters

At the time of this writing (2nd of June 2017) cluster federation is in beta and is not yet integrated into the official Kubernetes charms. Therefore we will be using a patched version of charms and bundles (source of charms and cdk-addons, charms master and worker, and bundle).

We assume you already have Juju installed and a controller in place. If not have a look at the Juju installation instructions.

Let’s deploy these three clusters.

#> juju add-model f1
#> juju deploy cs:~kos.tsakalozos/bundle/federated-kubernetes-core-2
#> juju add-model c1
#> juju deploy cs:~kos.tsakalozos/bundle/federated-kubernetes-core-2
#> juju add-model c2
#> juju deploy cs:~kos.tsakalozos/bundle/federated-kubernetes-core-2

Here is what the bundle.yaml looks like: http://paste.ubuntu.com/24746868/

Let’s copy and merge the kubeconfig files.

#> juju switch f1
#> juju scp kubernetes-master/0:config ~/.kube/config
#> juju switch c1
#> juju scp kubernetes-master/0:config ~/.kube/config.c1
#> juju switch c2
#> juju scp kubernetes-master/0:config ~/.kube/config.c2

Using this script you can merge the configs

#> ./merge-configs.py config.c1 c1
#> ./merge-configs.py config.c2 c2

To enable CoreDNS on f1 you just need to:

#> juju switch f1
#> juju config kubernetes-master enable-coredns=True

Have a look at your three contexts:

#> kubectl config get-contexts
CURRENT NAME CLUSTER AUTHINFO NAMESPACE
f1 f1 ubuntu
c1 c1-cluster c1-user
c2 c2-cluster c2-user

Create the federation

At this point you are ready to deploy the federation control plane on f1. To do so you will need a call to kubefed init. Kubefed init will deploy the required services and it will also perform a health check ping to see everything is ready. During this last check kubefed init will try to reach a service on a port that is not open. At the time of this writing we have a patch waiting upstream to make the port configurable so you can open it before triggering the federation creation. If you do not want to wait for the patch to be merged you can call kubefed init with -v 9 to see what port the services were appointed to and expose them though Juju while keeping kubefed running. This will become clear shortly.

Create a coredns-provider.conf file with the etc endpoint of etcd of f1 and your zone. You can look at the Juju status to find out the IP of the etcd machine:

#> cat ./coredns-provider.conf 
[Global]
etcd-endpoints = http://54.211.106.117:2379
zones = juju-fed.com.

Call kubefed

#> juju switch f1
#> kubefed init federation — host-cluster-context=f1 — dns-provider=”coredns” — dns-zone-name=”juju-fed.com.” — dns-provider-config=coredns-provider.conf — etcd-persistent-storage=false — api-server-service-type=”NodePort” -v 9 — api-server-advertise-address=34.224.101.147 — image=gcr.io/google_containers/hyperkube-amd64:v1.6.2

Note here that the api-server-advertise-address=34.224.101.147 is the IP of the worker on your f1 cluster. You selected NodePort so the federation service is available from all the worker nodes under a random port.

Kubefed will end up trying to do a health check. You should see on your console something similar to this: “GET https://34.224.101.147:30229/healthz” . The port 30229 is randomly selected and will differ in your case. While keeping kubefed init running, go ahead and open this port from a second terminal:

#> juju run — application kubernetes-worker “open-port 30229”

The federation context should be present in the list of contexts you have.

#> kubectl config get-contexts
CURRENT NAME CLUSTER AUTHINFO NAMESPACE
f1 f1 ubuntu
federation federation federation
c1 c1-cluster c1-user
c2 c2-cluster c2-user

Now you are ready to have other clusters join the federation:

#> kubectl config use-context federation
#> kubefed join c1 — host-cluster-context=f1
#> kubefed join c2 — host-cluster-context=f1

Two clusters should soon appear ready:

#> kubectl get clusters
NAME STATUS AGE
c1 Ready 3m
c2 Ready 1m

Congratulations! You are all set, your federation is ready to use.

What works

Setup your default namespace:

#> kubectl — context=federation create ns default

Get this replica set and create it.

#> kubectl — context=federation create -f nginx-rs.yaml

See your replica set created:

#> kubectl — context=federation describe rs/nginx
Name: nginx
Namespace: default
Selector: app=nginx
Labels: app=nginx
type=demo
Annotations: <none>
Replicas: 1 current / 1 desired
Pods Status: error in fetching pods: the server could not find the requested resource
Pod Template:
Labels: app=nginx
Containers:
frontend:
Image: nginx
Port: 80/TCP
Environment: <none>
Mounts: <none>
Volumes: <none>
Events:
FirstSeen LastSeen Count From SubObjectPath Type Reason Message
— — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — -
26s 26s 1 federated-replicaset-controller Normal CreateInCluster Creating replicaset in cluster c2

Create a service.

#> kubectl — context=federation create -f beacon.yaml

Scale and open ports so the service is available

#> kubectl — context=federation scale rs/nginx — replicas 3

What does not work yet

Federation does not seem to handle NodePort services. To see that you can deploy a busybox and try to resolve the name of the service you deployed right above.

Use this yaml.

#> kubectl — context=c2 create -f bbox-4-test.yaml
#> kubectl — context=c2 exec -it busybox — nslookup beacon

This returns a local cluster IP which is rather unexpected.

There are some configuration options that did not have the effect I was expecting. First, we would want to expose the CoreDNS service as described in the docs. Assigning the same NodePort for both UDP and TCP had no effect; this is rather unfortunate since the issue is already addressed. The configuration described here and here (although it should be automatically applied I thought I would give it a try) did not seem to have any effect.

Open issues:

Conclusions

Federation on on-prem clusters has gone a long way and is getting significant improvements with every release. Some of the issues described here have already been addressed, I am waiting anxiously for the upcoming v1.7.0 release to see the fixes in action.

Do not hesitate to reach out to the sig-federation group, but most importantly I would be extremely happy and grateful if you pointed out some bug I have on my configs/code/thoughts ;)

References and Links

  1. Official federations docs, https://kubernetes.io/docs/concepts/cluster-administration/federation/
  2. Juju docs, https://jujucharms.com/
  3. Canonical Distribution of Kubernetes, https://jujucharms.com/canonical-kubernetes/
  4. Federation with host on GCE, https://medium.com/google-cloud/experimenting-with-cross-cloud-kubernetes-cluster-federation-dfa99f913d54
  5. CDK-addons source, https://github.com/juju-solutions/cdk-addons/tree/feature/coredns
  6. Kubernetes charms, https://github.com/juju-solutions/kubernetes/tree/feature/coredns
  7. Kubernetes master charm, https://jujucharms.com/u/kos.tsakalozos/kubernetes-master/31
  8. Kubernetes worker charm, https://jujucharms.com/u/kos.tsakalozos/kubernetes-worker/17
  9. Kubernetes bundle used for federation, https://jujucharms.com/u/kos.tsakalozos/federated-kubernetes-core/bundle/0
  10. Juju getting started, https://jujucharms.com/docs/stable/getting-started
  11. Patch kubefed init, https://github.com/kubernetes/kubernetes/pull/46283
  12. CoreDNS setup instructions https://kubernetes.io/docs/tasks/federation/set-up-coredns-provider-federation/#setup-coredns-server-in-nameserver-resolvconf-chain
  13. CoreDNS yaml file https://github.com/juju-solutions/cdk-addons/blob/feature/coredns/coredns/coredns-rbac.yaml#L95
  14. Nodeport UDP/TCP issue, https://github.com/kubernetes/kubernetes/issues/20092
  15. Federation DNS on-prem configuration https://kubernetes.io/docs/admin/federation/#kubernetes-15-passing-federations-flag-via-config-map-to-kube-dns
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