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What even is a kubelet?

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Tags: kubernetes kubelet
Clipped on: 2015-11-11

What even is a kubelet?

Aug 27, 2015

Kubernetes is Google’s open source, container-focused cluster management thing. I see it as their attempt to tell everyone how they think containers and clusters fit together. The Kubernetes documentation is quite good, but it’s divided up in a way that makes it great as a reference. I want to understand both the concepts Kubernetes introduces, and the components that make up a Kubernetes cluster, and I want to learn by doing. I’m planning to build up a cluster from scratch, documenting the moving parts and concepts as I go.

I’ll start of with a look at the kubelet, which is the lowest level component in Kubernetes. It’s responsible for what’s running on an individual machine. You can think of it as a process watcher like supervisord, but focused on running containers. It has one job: given a set of containers to run, make sure they are all running.

Kubelets run pods

The unit of execution that Kubernetes works with is the pod. A pod is a collection of containers that share some resources: they have a single IP, and can share volumes. For example, a web server pod could have a container for the server itself, and a container that tails the logs and ships them off to your logging or metrics infrastructure.

Pods are defined by a JSON or YAML file called a pod manifest. A simple one with one container looks like this:

apiVersion: v1
kind: Pod
  name: nginx
  - name: nginx
    image: nginx
    - containerPort: 80

The container’s image is a Docker image name. The containerPort exposes that port from the nginx container so we can connect to the nginx server at the pod’s IP. By default, the entrypoint defined in the image is what will run; in the nginx image, that’s the nginx server.

Let’s add a log truncator container to this pod. This will take care of the nginx access log, truncating it every 10 seconds—who needs those anyway? To do this, we’ll need nginx to write its logs to a volume that can be shared to the log truncator. We’ll set this volume up as an emptyDir volume: it will start off as an empty directory when the pod starts, and be cleaned up when the pod exits, but will persist across restarts of the component containers.

Here’s the updated pod manifest:

apiVersion: v1
kind: Pod
  name: nginx
  - name: nginx
    image: nginx
    - containerPort: 80
    - mountPath: /var/log/nginx
      name: nginx-logs
  - name: log-truncator
    image: busybox
    - /bin/sh
    args: [-c, 'while true; do cat /dev/null > /logdir/access.log; sleep 10; done']
    - mountPath: /logdir
      name: nginx-logs
  - name: nginx-logs
    emptyDir: {}

We’ve added an emptyDir volume named nginx-logs. nginx writes its logs at /var/log/nginx, so we mount that volume at that location in the nginx container. For the log-truncator container, we’re using the busybox image. It’s a tiny Linux command line environment, which provides everything we need for a robust log truncator. Inside that container, we’ve mounted the nginx-logs volume at /logdir. We set its command and args up to run a shell loop that truncates the log file every 10 seconds.

Now we’ve got this paragon of production infrastructure configured, it’s time to run it!

Running a pod

There are a few ways the kubelet finds pods to run:

  • a directory it polls for new pod manifests to run
  • a URL it polls and downloads pod manifests from
  • from the Kubernetes API server

The first of these is definitely the simplest: to run a pod, we just put a manifest file in the watched directory. Every 20 seconds, the kubelet checks for changes in the directory, and adjusts what it’s running based on what it finds. This means both launching pods that are added, as well as killing ones that are removed.

The kubelet is such a low level component with such limited responsibilities that we can actually use it independently of Kubernetes—all we have to do is not tell it about a Kubernetes API server. The kubelet supports Docker and rkt as continer runtimes. The default is Docker, and that’s what we’ll use in the examples here. You’ll need a machine with Docker installed and running to try this out.

First off, let’s get the kubelet binary from Google.

$ wget
$ chmod +x kubelet

If you run ./kubelet --help, you’ll get an overwhelming list of options. For what we’re about to do, we only need one of them though: the --config option. This is the directory that the kubelet will watch for pod manifests to run. We’ll create a directory for this, and then start the kubelet. You might need to run it under sudo so that it can talk to the docker daemon.

$ mkdir manifests
$ ./kubelet --config=$PWD/manifests

Now let’s stick the example nginx pod manifest from above in an nginx.yaml file, and then drop it in the manifests directory. After a short wait, the kubelet will notice the file and fire up the pod.

We can check the list of running containers with docker ps:

$ docker ps
CONTAINER ID        IMAGE                                  COMMAND                CREATED             STATUS              PORTS               NAMES
f1a27680e401        busybox:latest                         "/bin/sh -c 'while t   6 seconds ago       Up 5 seconds                            k8s_log-truncator.72cfff7a_nginx-kx_default_419bc51e985b6bb5e53ea305e2c1e737_401a4c94   
c5e357fc981a        nginx:latest                           "nginx -g 'daemon of   6 seconds ago       Up 6 seconds                            k8s_nginx.515d0778_nginx-kx_default_419bc51e985b6bb5e53ea305e2c1e737_cd02602b           
b2692643c372   "/pause"               6 seconds ago       Up 6 seconds                            k8s_POD.ef28e851_nginx-kx_default_419bc51e985b6bb5e53ea305e2c1e737_836cadc7             

There are three containers running: the nginx and log-truncator containers we defined, as well as the pod infrastructure container.1 The infrastructure container is where the kubelet puts all the resources that are shared across containers in the pod. This includes the IP, as well as any volumes we’ve defined. We can poke around with docker inspect to see how they’re configured and hooked up to each other:

$ docker inspect --format '{{ .NetworkSettings.IPAddress  }}' f1a27680e401

$ docker inspect --format '{{ .NetworkSettings.IPAddress  }}' c5e357fc981a

$ docker inspect --format '{{ .NetworkSettings.IPAddress  }}' b2692643c372

The nginx and log trunctator containers have no IP, but the infrastructure container does. Taking a closer look at at the containers we defined, we can see their NetworkMode is set to use the infrastructure container’s network:

$ docker inspect --format '{{ .HostConfig.NetworkMode  }}' c5e357fc981a
$ docker inspect --format '{{ .HostConfig.NetworkMode  }}' f1a27680e401

Since we exposed port 80 from the nginx container with containerPort, we can connect to the nginx server at port 80 at the pod’s IP:

$ curl --stderr /dev/null | head -4
<!DOCTYPE html>
<title>Welcome to nginx!</title>

It really is running! And just to check the log truncator is doing what we expect, let’s watch the log file and make some requests with

$ docker exec -tty f1a27680e401 watch cat /logdir/access.log

while we make a few requests. The log lines accumulate for a bit, but then they all disappear: the truncator doing its job!

Kubelet introspection

The kubelet also has an internal HTTP server. We won’t go into it in detail, except to say that it serves a read-only view at port 10255. There’s a health check endpoint at /healthz:

$ curl http://localhost:10255/healthz

There are also a few status endpoints. For example, you can get a list of running pods at /pods:

$ curl --stderr /dev/null http://localhost:10255/pods | jq . | head
  "kind": "PodList",
  "apiVersion": "v1",
  "metadata": {},
  "items": [
      "metadata": {
        "name": "nginx-kx",
        "namespace": "default",
        "selfLink": "/api/v1/pods/namespaces/nginx-kx/default",

You can also get specs of the machine the kubelet is running on at /spec/:

$ curl --stderr /dev/null  http://localhost:10255/spec/ | jq . | head
  "num_cores": 4,
  "cpu_frequency_khz": 2700000,
  "memory_capacity": 4051689472,
  "machine_id": "9eacc5220f4b41e0a22972d8a47ccbe1",
  "system_uuid": "818B908B-D053-CB11-BC8B-EEA826EBA090",
  "boot_id": "a95a337d-6b54-4359-9a02-d50fb7377dd1",
  "filesystems": [
      "device": "/dev/mapper/kx--vg-root",

Tearing things down

Finally, we can clean up after ourselves. Just deleting the nginx pod manifest will result in the kubelet stopping the containers.

$ rm $PWD/manifests/nginx.yaml
$ sleep 20  # wait for the kublet to spot the removed manifest
$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
$ curl --stderr /dev/null http://localhost:10255/pods

All gone!

We’ve seen that while the kubelet is a part of Kubernetes, at heart it’s a container-oriented process watcher. You can use it in isolation to manage containers running on a single host. In fact, the Kubernetes getting started guides for Docker run the kubelet under Docker and use the kubelet to manage the Kubernetes master components. In a later post, we’ll do something similar!

  1. The pause command that the infrastructure container runs is a 129 byte ELF binary that just calls the pause system call, and exits when a signal is received. This keeps the infrastructure container around until the kubelet brings it down. It’s pretty cool, check the source!