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 the API server observing it. It does not have an associated replication controller, and kubelet daemon itself watches it and restarts it when it crashes. There is no health check. Static pods are always bound to one kubelet daemon and always run on the same node with it.
Kubelet automatically tries to create a mirror pod on the Kubernetes API server for each static pod. This means that the pods are visible on the API server but cannot be controlled from there.
Static pod can be created in two ways: either by using configuration file(s) or by HTTP.
The configuration files are just standard pod definitions in json or yaml format in a specific directory. Use
kubelet --pod-manifest-path=<the directory> to start kubelet daemon, which periodically scans the directory and creates/deletes static pods as yaml/json files appear/disappear there.
Note that kubelet will ignore files starting with dots when scanning the specified directory.
For example, this is how to start a simple web server as a static pod:
Choose a node where we want to run the static pod. In this example, it’s
[joe@host ~] $ ssh my-node1
Choose a directory, say
/etc/kubelet.d and place a web server pod definition there, e.g.
[root@my-node1 ~] $ mkdir /etc/kubelet.d/ [root@my-node1 ~] $ cat <<EOF >/etc/kubelet.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
Configure your kubelet daemon on the node to use this directory by running it with
On Fedora edit
/etc/kubernetes/kubelet to include this line:
KUBELET_ARGS="--cluster-dns=10.254.0.10 --cluster-domain=kube.local --pod-manifest-path=/etc/kubelet.d/"
Instructions for other distributions or Kubernetes installations may vary.
Restart kubelet. On Fedora, this is:
[root@my-node1 ~] $ systemctl restart kubelet
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
--pod-manifest-path=<directory>, i.e. it’s reloaded every now and then and changes are applied to running static pods (see below).
When kubelet starts, it automatically starts all pods defined in directory specified in
--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 PORTS 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 NAME READY STATUS RESTARTS AGE static-web-my-node1 1/1 Running 0 2m
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.
Note: Make sure the kubelet has permission to create the mirror pod in the API server. If not, the creation request is rejected by the API server. See PodSecurityPolicy.
[joe@my-master ~] $ kubectl delete pod static-web-my-node1 pod "static-web-my-node1" deleted [joe@my-master ~] $ kubectl get pods NAME READY STATUS RESTARTS AGE static-web-my-node1 1/1 Running 0 12s
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 ...
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/kubelet.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/kubelet.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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