Standalone Kubelet Tutorial

This tutorial will guide you through running the Kubernetes Kubelet in standalone mode on Container Linux. You will also deploy an application using a static pod, test it, then upgrade the application.

Rationale

In some cases you just want to run one or more compute instances without the need for the entire Kubernetes feature set. There are many options for managing containers on a single compute instance including docker compose, or some configuration management tool like ansible or chef, however the Kubernetes Kubelet running in standalone mode may be the better option. In standalone mode the Kubelet allows you to manage containers using pod manifests, which brings the benefits of running tightly coupled application as a single unit while leveraging a subset of advanced features including init containers, CNI networking, and built-in health checks.

Running the Kubelet in standalone mode provides a nice on-ramp to a full Kubernetes cluster; if the time comes your pod manifests can be reused.

Compute Instance

Create the standalone-kubelet compute instance:

gcloud compute instances create standalone-kubelet \
  --async \
  --boot-disk-size 200 \
  --can-ip-forward \
  --image-family coreos-stable \
  --image-project coreos-cloud \
  --machine-type n1-standard-1 \
  --tags standalone-kubelet

Allow HTTP traffic to the standalone-kubelet instance:

gcloud compute firewall-rules create allow-standalone-kubelet \
  --allow tcp:80 \
  --target-tags standalone-kubelet

Install the Standalone Kubelet

SSH into the standalone-kubelet compute instance:

gcloud compute ssh standalone-kubelet

Download the Kubelet systemd unit file:

wget -q --show-progress --https-only --timestamping \
  https://raw.githubusercontent.com/kelseyhightower/standalone-kubelet-tutorial/master/kubelet.service

Move the kubelet.service unit file to the systemd configuration directory:

sudo mv kubelet.service /etc/systemd/system/

Start the kubelet service:

sudo systemctl daemon-reload

sudo systemctl enable kubelet

sudo systemctl start kubelet

Verification

It will take a few minutes for the kubelet container to download and initialize. Verify the kubelet is running:

sudo systemctl status kubelet

Static Pods

In this section you will deploy an application that responds to HTTP requests with its running config and version. The application configuration will be initialized before the HTTP service starts by using an init container. Once the pod has started the application configuration will be updated every 30 seconds by a configuration sidecar.

SSH into the standalone-kubelet compute instance:

gcloud compute ssh standalone-kubelet

Verify no containers are running:

sudo docker ps

Verify no container images are installed:

sudo docker images

Create the kubelet manifests directory:

sudo mkdir -p /etc/kubernetes/manifests

Download the app-v0.1.0.yaml pod manifest:

wget -q --show-progress --https-only --timestamping \
  https://raw.githubusercontent.com/kelseyhightower/standalone-kubelet-tutorial/master/pods/app-v0.1.0.yaml

Move the app-v0.1.0.yaml pod manifest to the kubelet manifest directory:

sudo mv app-v0.1.0.yaml /etc/kubernetes/manifests/app.yaml

Notice the app-v0.1.0.yaml pod manifest is being renamed to app.yaml. This prevents our application from being deployed twice. Each pod must have a unique metadata.name.

List the installed container images:

sudo docker images

REPOSITORY                             TAG                 IMAGE ID            CREATED             SIZE
gcr.io/hightowerlabs/app               0.1.0               c7d7002a0776        23 minutes ago      6.325 MB
gcr.io/hightowerlabs/configurator      0.1.0               164e54187008        4 hours ago         2.346 MB
gcr.io/google_containers/pause-amd64   3.0                 99e59f495ffa        16 months ago       746.9 kB

List the running containers:

docker ps

You should see three containers running which represent the app pod. Docker does not understand pods so the containers are listed as individual containers following the Kubernetes naming convention.

Verification

At this point the app pod is up and running on port 80 in the host namespace. Make a HTTP request to the app pod using the localhost address:

curl http://127.0.0.1

version: 0.1.0
hostname: standalone-kubelet.c.hightowerlabs.internal
key: 1506065606

Wait about 30 seconds and make another HTTP request to the app pod:

curl http://127.0.0.1

version: 0.1.0
hostname: standalone-kubelet.c.hightowerlabs.internal
key: 1506065636

Notice the key field has changed. This configuration setting is being updated by the configurator sidecar container running in the app pod.

Testing Remote Access

The app pod is listening on 0.0.0.0:80 in the host network and is accessible via the external IP of the standalone-kubelet compute instance.

Get the external IP of the standalone-kubelet instance:

EXTERNAL_IP=$(gcloud compute instances describe standalone-kubelet \
  --format 'value(networkInterfaces[0].accessConfigs[0].natIP)')

Make a HTTP request to the app pod using the standalone-kubelet external IP address:

curl http://${EXTERNAL_IP}

version: 0.1.0
hostname: standalone-kubelet.c.hightowerlabs.internal
key: 1506065696

Updating Static Pods

gcloud compute ssh standalone-kubelet

Download the app-v0.2.0.yaml pod manifest:

wget -q --show-progress --https-only --timestamping \
  https://raw.githubusercontent.com/kelseyhightower/standalone-kubelet-tutorial/master/pods/app-v0.2.0.yaml

Move the app-v0.2.0.yaml pod manifest to the kubelet manifest directory:

sudo mv app-v0.2.0.yaml /etc/kubernetes/manifests/app.yaml

Notice the app-v0.2.0.yaml is being renamed to app.yaml. This overwrites the current pod manifest and will force the kubelet upgrade the app pod.

List the installed container images:

docker images

REPOSITORY                             TAG                 IMAGE ID            CREATED             SIZE
gcr.io/hightowerlabs/app               0.1.0               c7d7002a0776        30 minutes ago      6.325 MB
gcr.io/hightowerlabs/app               0.2.0               3028d4a68eb1        31 minutes ago      6.325 MB
gcr.io/hightowerlabs/configurator      0.1.0               164e54187008        4 hours ago         2.346 MB
gcr.io/google_containers/pause-amd64   3.0                 99e59f495ffa        16 months ago       746.9 kB

Notice the gcr.io/hightowerlabs/app:0.2.0 image has been added to the local repository.

Verification

At this point app version 0.2.0 is up and running. Make a HTTP request to the app pod using the localhost address:

curl http://127.0.0.1

version: 0.2.0
hostname: standalone-kubelet.c.hightowerlabs.internal
key: 1506065786

Cleanup

gcloud -q compute instances delete standalone-kubelet

gcloud -q compute firewall-rules delete allow-standalone-kubelet