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Static Pods

If you are running clustered Kubernetes and are using static pods to run a pod on every node, you should probably be using a DaemonSet!

Static pods are managed directly by kubelet daemon on a specific node, without API server observing it. It does not have associated any replication controller, kubelet daemon itself watches it and restarts it when it crashes. There is no health check though. Static pods are always bound to one kubelet daemon and always run on the same node with it.

Kubelet automatically creates so-called mirror pod on Kubernetes API server for each static pod, so the pods are visible there, but they cannot be controlled from the API server.

Static pod creation

Static pod can be created in two ways: either by using configuration file(s) or by HTTP.

Configuration files

The configuration files are just standard pod definition in json or yaml format in specific directory. Use kubelet --config=<the directory> to start kubelet daemon, which periodically scans the directory and creates/deletes static pods as yaml/json files appear/disappear there.

For example, this is how to start a simple web server as a static pod:

  1. Choose a node where we want to run the static pod. In this example, it’s my-node1.

    shell [joe@host ~] $ ssh my-node1

  2. Choose a directory, say /etc/kubelet.d and place a web server pod definition there, e.g. /etc/kubernetes.d/static-web.yaml:

    shell [root@my-node1 ~] $ mkdir /etc/kubernetes.d/ [root@my-node1 ~] $ cat <<EOF >/etc/kubernetes.d/static-web.yaml apiVersion: v1 kind: Pod metadata: name: static-web labels: role: myrole spec: containers: - name: web image: nginx ports: - name: web containerPort: 80 protocol: tcp EOF

  3. Configure your kubelet daemon on the node to use this directory by running it with --config=/etc/kubelet.d/ argument. On Fedora Fedora 21 with Kubernetes 0.17 edit /etc/kubernetes/kubelet to include this line:

    conf KUBELET_ARGS="--cluster-dns=10.254.0.10 --cluster-domain=kube.local --config=/etc/kubelet.d/"

    Instructions for other distributions or Kubernetes installations may vary.

  4. Restart kubelet. On Fedora 21, this is:

    shell [root@my-node1 ~] $ systemctl restart kubelet

Pods created via HTTP

Kubelet periodically downloads a file specified by --manifest-url=<URL> argument and interprets it as a json/yaml file with a pod definition. It works the same as --config=<directory>, i.e. it’s reloaded every now and then and changes are applied to running static pods (see below).

Behavior of static pods

When kubelet starts, it automatically starts all pods defined in directory specified in --config= or --manifest-url= arguments, i.e. our static-web. (It may take some time to pull nginx image, be patient…):

[joe@my-node1 ~] $ docker ps
CONTAINER ID IMAGE         COMMAND  CREATED        STATUS              NAMES
f6d05272b57e nginx:latest  "nginx"  8 minutes ago  Up 8 minutes        k8s_web.6f802af4_static-web-fk-node1_default_67e24ed9466ba55986d120c867395f3c_378e5f3c

If we look at our Kubernetes API server (running on host my-master), we see that a new mirror-pod was created there too:

[joe@host ~] $ ssh my-master
[joe@my-master ~] $ kubectl get pods
POD                     IP           CONTAINER(S)   IMAGE(S)    HOST                        LABELS       STATUS    CREATED         MESSAGE
static-web-my-node1     172.17.0.3                              my-node1/192.168.100.71     role=myrole  Running   11 minutes
                                     web            nginx                                                Running   11 minutes

Labels from the static pod are propagated into the mirror-pod and can be used as usual for filtering.

Notice we cannot delete the pod with the API server (e.g. via kubectl command), kubelet simply won’t remove it.

[joe@my-master ~] $ kubectl delete pod static-web-my-node1
pods/static-web-my-node1
[joe@my-master ~] $ kubectl get pods
POD                     IP           CONTAINER(S)   IMAGE(S)    HOST                        ...
static-web-my-node1     172.17.0.3                              my-node1/192.168.100.71     ...

Back to our my-node1 host, we can try to stop the container manually and see, that kubelet automatically restarts it in a while:

[joe@host ~] $ ssh my-node1
[joe@my-node1 ~] $ docker stop f6d05272b57e
[joe@my-node1 ~] $ sleep 20
[joe@my-node1 ~] $ docker ps
CONTAINER ID        IMAGE         COMMAND                CREATED       ...
5b920cbaf8b1        nginx:latest  "nginx -g 'daemon of   2 seconds ago ...

Dynamic addition and removal of static pods

Running kubelet periodically scans the configured directory (/etc/kubelet.d in our example) for changes and adds/removes pods as files appear/disappear in this directory.

[joe@my-node1 ~] $ mv /etc/kubernetes.d/static-web.yaml /tmp
[joe@my-node1 ~] $ sleep 20
[joe@my-node1 ~] $ docker ps
// no nginx container is running
[joe@my-node1 ~] $ mv /tmp/static-web.yaml  /etc/kubernetes.d/
[joe@my-node1 ~] $ sleep 20
[joe@my-node1 ~] $ docker ps
CONTAINER ID        IMAGE         COMMAND                CREATED           ...
e7a62e3427f1        nginx:latest  "nginx -g 'daemon of   27 seconds ago

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